All about Science and Our Earth

All about Mountain

2009-09-13T18:39:00.000-07:00

It is a lot harder to climb a mountain than to hike across a plain. You might need special shoes to help your feet grip the ground on a mountain or ropes to help pull you up in steep places. A plain is flat. A mountain rises up high above the ground.

The Highest Mountain on Each Continent Continent Mountain Location feet meters

The Highest Mountain on Each Continent
Continent	Mountain	Location	feet	meters
Asia	Mount Everest	China-Nepal	29,035	8,850
South America	Aconcagua	Argentina	22,834	6,960
North America	Mount McKinley	United States	20,320	6,194
Africa	Kilimanjaro	Tanzania	19,341	5,895
Europe	Mount Elbrus	Russia	18,510	5,642
Antarctica	Vinson Massif	Antarctica	16,066	4,897
Australia and the Pacific Islands	Mount Wilhelm	Papua New Guinea	14,793	4,509

HOW DO MOUNTAINS DIFFER FROM HILLS?

You would have a longer climb to the top of a mountain than to the top of a hill. Mountains are higher than hills. The tallest mountain in the world is Mount Everest between Nepal and Tibet. The top of Mount Everest is 29,035 feet (8,850 meters) above the level of the sea. That’s more than five miles high!

Most hills are rounded. A mountain comes to a peak at the top. A mountain looks a little like a pyramid. A mountain has a wide base, or bottom, and a narrow top.

Some mountains stand by themselves. Most mountains are in groups called ranges. Mount Everest is in a range called the Himalayas. The Rockies and the Appalachians are mountain ranges in North America. Low places between mountains in a range are called valleys.

WHERE DO MOUNTAINS COME FROM?

Geologists (scientists who study the Earth) think that some mountains come from movements in Earth’s crust. The crust is the rocky outside layer of Earth. Geologists think that Earth’s crust is made up of gigantic pieces called plates that move around very slowly.

Sometimes the moving plates crunch together. The edges of the crunching plates wrinkle up to make mountains. Plates crunching together and wrinkling up made the Himalayas and the Rocky Mountains.

Sometimes one side of a plate lifts up like a big block of rock. You can see these kinds of mountains in the southwestern United States.

Some mountains form when hot, melted rock oozes up from deep inside Earth. The melted rock is called magma. Mountains made from magma are volcanoes. Magma that comes out of the top of a volcano is called lava. Lava cools, turns solid, and builds up to make the hard rock that forms a mountain. Mount Rainier near Seattle, Washington, is a volcano.

Some mountains are carved out by erosion. Wind and water wear away soil and soft rock. The hard rock stays and becomes mountains. The Ozark Mountains in Arkansas and Missouri were made by erosion.

All about Ocean

2009-09-13T18:32:00.000-07:00

When most people think about Earth, they think about land. You’ve probably spent your entire life on land. If you look at a globe, however, you can see that there is not as much land as water. Most of the Earth is covered by water, and most of that water is salty. Almost all of the salt water is connected into one huge ocean. The ocean covers almost two-thirds of the Earth.

The World’s Oceans
Ocean	square miles	square kilometers
Pacific Ocean	63,980,000	165,700,000
Atlantic Ocean	31,810,000	82,400,000
Indian Ocean	28,360,000	73,440,000
Arctic Ocean	5,430,000	14,060,000

ONE OCEAN OR MANY?

There is only one world ocean. Big pieces of land called continents divide the world ocean into parts, but all of the parts are connected. The four major parts are called the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.

Parts of some oceans are called seas. The Caribbean Sea and the Mediterranean Sea are part of the Atlantic Ocean. The Coral Sea is part of the Pacific Ocean.

HOW DEEP IS THE OCEAN?

The ocean is not very deep at the beach. Water along seashores might not even cover your toes. You can wade out into the ocean. You can stand on the bottom near a beach.

Farther out, the ocean is very deep. The deepest parts are in the Pacific Ocean. A place called the Mariana Trench is almost 7 miles (11 kilometers) deep!

Some parts of the ocean floor are flat. Other parts have underwater mountains. The Mid-Atlantic Ridge is a mountain range under the Atlantic Ocean. Melted rock called magma oozes up from cracks along the Mid-Atlantic Ridge. The magma cools and makes new rock.

IS OCEAN WATER WARM OR COLD?

The ocean is freezing cold in some places and very warm in others. The Arctic Ocean near the North Pole and the ocean in the south around Antarctica are freezing cold. You can see icebergs floating in these oceans.

Ocean water near the equator is always warm. The equator is an imaginary line around the middle of Earth. You can swim in the ocean near the equator all year long.

CURRENTS IN THE OCEAN

The ocean has “rivers” of cold or warm water running through it. These “rivers” are called currents. One of the best-known currents is the Gulf Stream. The Gulf Stream carries warm ocean water from the equator up to the north part of the Atlantic Ocean.

Warm and cold ocean water is always moving around the world. In some places cold water comes up from deep in the ocean. Warm water sinks down. Wind blows ocean water at the surface. Waves also move ocean water. Changes in ocean currents can make weather changes on land. Sometimes the changes cause mild winters in some places and lots of rainfall in other places.

WHAT LIVES IN THE OCEAN?

The ocean is full of living creatures. The tiniest ocean creatures are plankton. Plankton drift on the surface. The biggest ocean animal is the blue whale. The blue whale can be 80 feet (24 meters) long. It is the biggest animal on Earth.

Many kinds of fish and shellfish live in the ocean. Your tuna sandwich comes from the tuna, a fish that swims in the ocean. Crabs, lobsters, and shrimp crawl around on the ocean floor. Squid and octopus dart about with their long arms and tentacles.

Different creatures live at different ocean depths. Very strange creatures live deep in the ocean where there is no sunlight. Some deep-ocean fish glow in the dark. Some of them are blind.

PROTECTING THE OCEAN

Oceanographers are scientists who study the ocean. They worry about ocean pollution. Thousands of ships carry oil and other goods across the ocean. Oil spills from ships can pollute the ocean. People build factories close to the ocean shore. Chemicals from factories can pollute the oceans. Chemical bug killers from farms can also drain into the ocean.

People get many things from the ocean. Fish and other food come from the ocean. It is important to keep ocean water clean.

space travel

2009-09-09T23:41:00.000-07:00

You’re strapped into a seat. You hear a loud roar. Rocket engines fire and lift you into the sky. The rocket goes faster and faster, pushing you harder and harder against your seat. Suddenly, everything gets quiet. The engines stop. You take off your seat belt and start to float around. You are almost weightless. This is what space travel feels like.

GETTING INTO SPACE

Since ancient times, people have dreamed of leaving Earth and exploring other worlds. But gravity holds everything on the ground. Gravity is a pulling force between two objects. Earth is a very large object, and its gravity is strong. Even airplanes that fly thousands of feet above Earth can’t leave our atmosphere and go into space. Scientists and engineers had to make a force much greater than gravity to travel to outer space.

Finally, the dream came true. Engineers built rockets powerful enough to lift a rocket into space. In 1957, scientists from the Union of Soviet Socialist Republics (USSR) sent the first artificial satellite, Sputnik 1, into space. The United States soon sent an artificial satellite, Explorer 1, into space as well. The Space Age had begun.

The first spacecraft just orbited (went around) Earth. There were no humans on these spacecraft. Then scientists sent robot spacecraft to the Moon. The spaceships carried cameras that took pictures of the Moon’s surface. Some robot spacecraft even landed on the Moon.

The first person went into space in 1961. Soviet cosmonauts and American astronauts made several trips into orbit around Earth. The next goal was to send people to the Moon.

HEADING FOR THE MOON

After the Space Age began, engineers worked hard to figure out how to send people to the Moon. They made controls for steering spacecraft. They made spacesuits to allow astronauts to breathe and keep them safe from heat, cold, and harmful rays.

Engineers made special rocket ships for taking astronauts to the Moon. They named this series of spacecraft Apollo. An Apollo spacecraft held three astronauts. It also carried a smaller landing ship that looked sort of like a spider.

LANDING ON THE MOON

In July 1969, three astronauts in Apollo 11 headed for the Moon. On July 20, they made history. Astronauts Neil Armstrong and Buzz Aldrin climbed into the landing module, which was named Eagle. They went down to the Moon. The spiderlike legs of the lander dug into the Moon’s surface. Armstrong radioed back to Earth, “The Eagle has landed.” When he stepped on the Moon, Armstrong said, “That’s one small step for man, one giant leap for mankind.”

Astronauts in other Apollo spacecraft landed on the Moon five more times. Astronauts in spacesuits walked around on the Moon. They rode around in a kind of car called a lunar rover. All the astronauts brought back moon rocks and soil for scientists to study.

VISITING OTHER PLANETS?

Astronauts haven’t yet visited another planet. Robot spacecraft, also known as probes, have journeyed to all the planets except Pluto. These space probes carried cameras and took pictures of the planets. They studied gases around the planets. They can send these pictures and other information back to Earth by using special radio equipment.

Some of the space probes fly quickly past other planets. Voyagers 1 and 2 took off in 1977 to fly by Jupiter, Saturn, Uranus, and Neptune. After zooming by these planets, the Voyager probes headed out of the solar system. They continued to explore the space between the stars.

Some probes go into orbit around a planet. Some also drop landers on these planets. Several Soviet probes dropped landers on Venus. An American probe dropped a lander on Jupiter. More probes and landers continue to be sent, designed to explore other planets.

EXPLORING MARS

Of all the planets in the solar system, Mars is the most similar to Earth. Beginning in the 1960s, many robot spacecraft visited Mars. The Viking mission in 1975 was the first probe to safely land on Mars. A camera sent pictures of the surface to Earth. A robot arm scooped up soil.

The Mars Pathfinder spacecraft landed on Mars in 1997. It had a robot wagon called the Sojourner rover. The rover moved around on Mars, taking pictures and studying rocks. Other probes, landers, and rovers went on to study Mars.

Some people dream of astronauts someday landing on Mars. Other people say it would be best to send more robots. If astronauts do go to other planets, Mars would be the first one they visit.

THE SPACE SHUTTLE

In the 1970s, the United States developed a new kind of spacecraft called a shuttle. The shuttle blasts into space on big rockets. Unlike previous spacecraft, which used lander ships and could only be used once, the shuttle can land on Earth like an airplane and be used again. The first shuttle flew into space in 1981.

The shuttle has a big area called a cargo bay to hold large equipment. Astronauts on space shuttles launch satellites from the cargo bay. Some satellites study Earth from space, while others relay phone calls and other communications. Astronauts can also launch space telescopes from the shuttle.

LIVING IN SPACE

Space travel is hard on people’s bodies. Spending long amounts of time in space makes bones and muscles weak. It is hard to eat in space. It is hard to sleep and take showers.

Scientists use space stations to study how people can live and work in space. Space stations orbit around Earth. The Soviets sent up several space stations. The first, Salyut 1, was launched in 1971. The first U.S. space station, Skylab, was launched in 1973.

The most famous Soviet space station was Mir, which orbited Earth from 1986 to 2001. Astronauts from many different countries visited Mir. Many of them performed experiments on the space station. They learned many things about living and working in space.

In the late 1990s many nations worked together to build an International Space Station. The space shuttle carried parts for the station into space. Astronauts put the pieces together. The International Space Station was scheduled to be completed by 2006. The goal is to have people living and working in the space station all the time. Someday, maybe everyone who wants to will be able to travel into space.

Knowledge about Maps

2009-09-09T05:42:00.000-07:00

Have you ever drawn a picture of your street, town, or neighborhood, showing landmarks like trees, homes, and stores? If you have then you’re already a mapmaker. Or, to use a fancier word, you’re a cartographer!

WHAT ARE MAPS AND GLOBES FOR?

Maps show the locations of places on a sheet of paper. Globes are a type of map. But instead of being flat, globes are ball-shaped. This makes them more like Earth, the Moon, or other planets they represent.

Maps help show where one location is when compared with another. They can help you get from place to place. They can also provide other types of information. How high is a mountain range? How deep does a certain spot in the ocean go? Are there mineral deposits in a particular region? In what sections of the world do the most people live? These are all questions that the right kind of map can answer.

TYPES OF MAPS

The most common maps are called general maps. They usually show a mix of geographic and political features. Geographic features are things in nature, such as mountains, lakes, and rivers. Political features, in comparison, are decided on by people. They include lines showing where one country or state ends and other ones begin.

Thematic maps are another type of map. Thematic maps do not give as many types of data as general maps. Instead, they give more details about a specific topic. A thematic map, for example, may provide specifics about an area’s climate or natural resources. It could show you what languages the people living there speak, or even how much coffee they drink!

Charts are another category of maps. These special maps help sea captains and airplane pilots navigate. Why are their maps different from others? Captains out on the ocean and pilots up in the air rely on landmarks and symbols not usually needed by others. Ships, for example, need to know where the shallow places in the ocean are and which way the water currents move.

WHAT A RELIEF!

If you run your finger along most maps, you’ll find a smooth surface. However, if you have a map or globe that’s done in relief you might feel bumps representing high places like mountains, and dips representing low places like valleys.

Some relief maps use clay or plastic molding to make the bumps and dips. Others rely on colors or shapes to represent areas that are higher up or lower down. A special kind of map called a stereogram requires 3-D glasses. These glasses trick your eyes into seeing a flat page’s surface as though it were shown in three dimensions.

HOW TO READ A MAP

Almost all maps contain certain aids to help you read them. North is toward the top of most maps. But to make sure, look for an arrow on the map showing one or more cardinal directions (north, south, east, and west).

A key or legend is also found on just about all maps. A legend is a list of the symbols used on the map, along with each symbol’s meaning. A legend helps you figure out what the dots, circles, colors, and squiggly lines on a map stand for. A dot might, for example, represent a city, museum, group of people, or something else entirely.

A scale allows you to determine the size of the area shown on a particular map. Does a centimeter or an inch stand for a mile, a kilometer, or a million miles? The scale helps you compare the short distances on the map with real-life distances.

Lines of latitude and longitude also help you to read a map. Lines of latitude are imaginary lines that run around Earth from east to west. Lines of longitude are imaginary lines that circle Earth from north to south. Lines of longitude pass through the North Pole and the South Pole. Together lines of latitude and longitude create a grid on maps that allows you to describe precisely any spot on Earth.

Crystals

2009-09-06T00:57:00.000-07:00

Look closely at some table salt through a magnifying glass. You’ll see that the bits of salt are made up of tiny cubes. Each cube is a salt crystal. The salt crystals within the particles can be different sizes, but they always have this shape.

TINY PARTICLES IN PATTERNS

A crystal contains identical particles that are arranged in a particular pattern such as a cube, rectangle, or hexagon. As a crystal grows in size, this pattern is repeated over and over.

Salt is made up of the elements sodium and chlorine. Extremely tiny particles of sodium and chlorine, called atoms, form a repeating cubic pattern in a crystal of table salt. The more times the pattern is repeated, the bigger the crystal that forms.

HOW DO CRYSTALS FORM?

Crystals form when some liquids turn into solids. A liquid may freeze into a crystal. Snow, for example, is made of tiny crystals of frozen water. Crystals can also be left behind when a liquid dries out. When seawater in a rock pool dries out, tiny crystals of salt remain.

Most of the rocks and minerals in Earth’s crust are crystals. Some crystals were formed from melted rock when it cooled and became solid. Others were left behind by the waters of a sea, lake, or river that dried up long ago.

HOW DO WE USE CRYSTALS?

Many crystals are beautiful. Diamonds, rubies, and emeralds are crystals that are made into attractive jewelry. Crystals also have many practical uses. Quartz crystals are used in clocks, radios, and sonar, the system that allows ships and submarines to see things underwater. Quartz crystals can also be pressed or heated to make electricity.

All about AIR

2009-09-06T00:44:00.000-07:00

Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows.

Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.

WHAT IS AIR?

Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon. Air also contains smaller amounts of hydrogen, carbon dioxide, water vapor, helium, and other gases. Oxygen is the most important gas for animals. Animals must breathe oxygen in order to live.

Carbon dioxide is the most important gas for plants. Plants use carbon dioxide and sunlight to make food. Plants give off oxygen. Animals turn the oxygen back into carbon dioxide when they breathe.

TAKING AIR WITH YOU

You can go to places where there is no air. There is no air underwater, but you can dive underwater. You can stay underwater a short time just by holding your breath. Air tanks let you stay underwater for a long time. Scuba divers wear tanks on their backs. The tanks are filled with gases that make up air. The divers breathe the gases through hoses.

There is less and less air the higher up you go. People gasp for breath at the tops of tall mountains. Airplanes must carry air. Once the airplane gets up high, air is pumped into the cabin where passengers sit. Astronauts have to take all the air they need with them—there’s no air in space!

African Art and Architecture

2009-08-11T04:51:00.000-07:00

African Art and Architecture, works of art and architecture created on the African continent south of the Sahara. The immense Sahara acts as a natural barrier, separating African cultures to the north from those to the south. Although there has always been some intermingling of peoples on the two sides of the Sahara, differences in history and culture are pronounced. This article primarily discusses the art created south of the Sahara, a region known as sub-Saharan Africa. For information on the art of northern Africa, see Islamic Art and Architecture; Egyptian Art and Architecture; and Coptic Art and Architecture.

The history of African art and architecture spans a vast period, beginning as early as 25,500 bc and continuing to the present. Among the earliest surviving examples of African art are images of animals painted on rock slabs found in caves in Namibia. Animal images painted on or cut into rocks and canyon walls in the Sahara date from 6000 to 4000 bc. Later Saharan rock art depicts ritual activities, herding, and food preparation. The earliest known African sculptures (500 bc to ad 200) are sculpted clay heads and human figures from central Nigeria. Many surviving examples of African art date from the 14th to the 17th century. However, most of the African art known today is relatively recent, from the 19th century or later. Very little earlier African art has survived, primarily because it was made largely of perishable materials such as wood, cloth, and plant fibers, and because it typically met with intensive use in ceremonies and in daily life. Scholars of African art base suppositions about earlier art mainly on art of the last two centuries, but they can only guess at the earlier traditions from which the recent art developed.

African art does not constitute a single tradition. Africa is an enormous continent with hundreds of cultures that have their own languages, religious beliefs, political systems, and ways of doing things. Each culture produces its own distinctive art and architecture, with variations in materials, intentions, and results. Whereas some cultures excel in carving wood, others are known for casting objects in metal. In one culture a decorated pot might be used for cooling water, while in another culture a similar pot is used in ritual ceremonies.

The major types of art produced in Africa are masks, statues, furniture, textiles, pottery, baskets, beadwork, and metalwork. Most objects that are sculpted or shaped—masks and statues, for example—are created chiefly by men and depict human or animal forms. Where two-dimensional art exists, as in textile design or painted decoration on houses, it is generally produced by women.

Operating system

2009-07-23T00:21:00.001-07:00

Operating System (OS), in computer science, the basic software that controls a computer. The operating system has three major functions: It coordinates and manipulates computer hardware, such as computer memory, printers, disks, keyboard, mouse, and monitor; it organizes files on a variety of storage media, such as floppy disk, hard drive, compact disc, digital video disc, and tape; and it manages hardware errors and the loss of data. HOW AN OS WORKS Operating systems control different computer processes, such as running a spreadsheet program or accessing information from the computer's memory. One important process is interpreting commands, enabling the user to communicate with the computer. Some command interpreters are text oriented, requiring commands to be typed in or to be selected via function keys on a keyboard. Other command interpreters use graphics and let the user communicate by pointing and clicking on an icon, an on-screen picture that represents a specific command. Beginners generally find graphically oriented interpreters easier to use, but many experienced computer users prefer text-oriented command interpreters. Operating systems are either single-tasking or multitasking. The more primitive single-tasking operating systems can run only one process at a time. For instance, when the computer is printing a document, it cannot start another process or respond to new commands until the printing is completed. All modern operating systems are multitasking and can run several processes simultaneously. In most computers, however, there is only one central processing unit (CPU; the computational and control unit of the computer), so a multitasking OS creates the illusion of several processes running simultaneously on the CPU. The most common mechanism used to create this illusion is time-slice multitasking, whereby each process is run individually for a fixed period of time. If the process is not completed within the allotted time, it is suspended and another process is run. This exchanging of processes is called context switching. The OS performs the “bookkeeping” that preserves a suspended process. It also has a mechanism, called a scheduler, that determines which process will be run next. The scheduler runs short processes quickly to minimize perceptible delay. The processes appear to run simultaneously because the user's sense of time is much slower than the processing speed of the computer. Operating systems can use a technique known as virtual memory to run processes that require more main memory than is actually available. To implement this technique, space on the hard drive is used to mimic the extra memory needed. Accessing the hard drive is more time-consuming than accessing main memory, however, so performance of the computer slows. III. CURRENT OPERATING SYSTEMS Operating systems commonly found on personal computers include UNIX, Macintosh OS, and Windows. UNIX, developed in 1969 at AT&T Bell Laboratories, is a popular operating system among academic computer users. Its popularity is due in large part to the growth of the interconnected computer network known as the Internet. Software for the Internet was initially designed for computers that ran UNIX. Variations of UNIX include SunOS (distributed by SUN Microsystems, Inc.), Xenix (distributed by Microsoft Corporation), and Linux (available for download free of charge and distributed commercially by companies such as Red Hat, Inc.). UNIX and its clones support multitasking and multiple users. Its file system provides a simple means of organizing disk files and lets users control access to their files. The commands in UNIX are not readily apparent, however, and mastering the system is difficult. Consequently, although UNIX is popular for professionals, it is not the operating system of choice for the general public. Instead, windowing systems with graphical interfaces, such as Windows and the Macintosh OS, which make computer technology more accessible, are widely used in personal computers (PCs). However, graphical systems generally have the disadvantage of requiring more hardware—such as faster CPUs, more memory, and higher-quality monitors—than do command-oriented operating systems. IV. FUTURE TECHNOLOGIES Operating systems continue to evolve. A recently developed type of OS called a distributed operating system is designed for a connected, but independent, collection of computers that share resources such as hard drives. In a distributed OS, a process can run on any computer in the network (presumably a computer that is idle) to increase that process's performance. All basic OS functions—such as maintaining file systems, ensuring reasonable behavior, and recovering data in the event of a partial failure—become more complex in distributed systems. Research is also being conducted that would replace the keyboard with a means of using voice or handwriting for input. Currently these types of input are imprecise because people pronounce and write words very differently, making it difficult for a computer to recognize the same input from different users. However, advances in this field have led to systems that can recognize a small number of words spoken by a variety of people. In addition, software has been developed that can be taught to recognize an individual's handwriting.

Marshall Warren Nirenberg

2009-07-04T23:06:00.001-07:00

Marshall Warren Nirenberg, born in 1927, American biochemist and Nobel laureate, who is credited with experiments that made possible the solving of the genetic code. Born in New York City and educated at the University of Florida in Gainesville, Nirenberg earned a Ph.D. degree in biochemistry from the University of Michigan in 1957. He was a postdoctoral fellow of the American Cancer Society and joined the National Institutes of Health in Bethesda, Maryland, in 1957, where he performed research on the genetic code, protein synthesis, and nucleic acids. In 1962 he became director of the biochemical genetics section of the National Heart Institute. Nirenberg shared the 1968 Nobel Prize in physiology or medicine with the American biochemist Robert William Holley and the Indian-born geneticist Har Gobind Khorana; the three conducted separate pioneering research on how deoxyribonucleic acid (DNA; see Nucleic Acids) determines the structure of proteins. The three scientists demonstrated that certain triplet combinations of the four possible bases in DNA correspond to specific amino acids. A series of these special triplets in a DNA sequence instructs a cell on how to combine the appropriate amino acids to obtain a protein.

Émile Durkheim

2009-07-04T21:12:00.002-07:00

Émile Durkheim (1858-1917), French social theorist, who was one of the pioneers in the development of modern sociology.

Durkheim was born in Épinal, France, a descendant of a distinguished line of rabbinical scholars. He graduated from the École Normale Supérieure in Paris in 1882 and then taught law and philosophy. In 1887 he began teaching sociology, first at the University of Bordeaux and later at the University of Paris.

Durkheim believed that scientific methods should be applied to the study of society. He proposed that groups had characteristics that were more than, or different from, the sum of the individuals' characteristics or behaviors. He was also concerned with the basis of social stability—the common values shared by a society, such as morality and religion. In his view, these values, or the collective conscience, are the cohesive bonds that hold the social order together. A breakdown of these values, he believed, leads to a loss of social stability and to individual feelings of anxiety and dissatisfaction. He explained suicide as a result of an individual's lack of integration in society. Durkheim discussed the correlation in Suicide: A Study in Sociology (1897; translated 1951). In his studies and writings he made much use of anthropological materials, especially those dealing with aboriginal societies, to support his theories. Among his other books are The Division of Labor in Society (1893; translated 1933), The Rules of Sociological Method (1895; translated 1938), and The Elementary Forms of Religious Life (1912; translated 1915).

Anthropology

2009-07-04T19:19:00.000-07:00

Anthropology, the study of all aspects of human life and culture. Anthropology examines such topics as how people live, what they think, what they produce, and how they interact with their environments. Anthropologists try to understand the full range of human diversity as well as what all people share in common.

Anthropologists ask such basic questions as: When, where, and how did humans evolve? How do people adapt to different environments? How have societies developed and changed from the ancient past to the present? Answers to these questions can help us understand what it means to be human. They can also help us to learn ways to meet the present-day needs of people all over the world and to plan how we might live in the future

Michael Jackson

2009-07-04T19:03:00.000-07:00

Michael Jackson, born in 1958, American singer, dancer, and songwriter. Jackson is one of the bestselling popular music artists in history, but beginning in the 1990s he became better known for his eccentric behavior and strange personal life.
Michael Joseph Jackson was born in Gary, Indiana. At the age of five he joined his brothers' singing group, which was dubbed the Jackson 5 (later renamed the Jacksons). Michael's dancing ability as well as his singing skills quickly made him the group's leader. Under the guidance of music producer Berry Gordy, founder of the Motown Records label, the group became very successful. Among the Jackson 5’s most popular songs were “I Want You Back” (1969), “ABC” (1970), and “I’ll Be There” (1970).

Jackson's first solo album, Got to Be There (1971), established him as an independent performer. Leaving the family group, Jackson played the Scarecrow in the musical film The Wiz (1978) and recorded the hit album Off the Wall (1979), which included a number of songs he had written. His 1982 album Thriller, produced by Quincy Jones, earned an unprecedented eight Grammy Awards and sold more than 40 million copies, making it the bestselling album in history up to that time. The music videos of the singles “Beat It” (for which he received a Grammy), “Thriller,” and “Billie Jean” from this album made Jackson a popular performer on MTV (see Music Television). The videos popularized one of Jackson’s singular dance moves, known as the moonwalk.

Jackson rejoined his brothers for the album Victory (1984) and a subsequent six-month tour. He also cowrote the song “We Are the World” (1985), which was performed by a group of more than 40 famous musicians. All profits from the single and video of this song were donated to programs targeting world hunger. Jackson's album Bad (1987) produced a number of hit singles—including the title song and “Man in the Mirror”—and his 1991 album Dangerous and its single “Remember the Time” were also bestsellers

In 1995 Jackson’s double album HIStory was released. Half of the album is a compilation of the most successful songs from Thriller, Bad, and Dangerous, while the other half is a collection of original compositions. In 1996 Jackson won a Grammy Award for the music video “Scream” (1995), created with his sister Janet (see Janet Jackson). That same year he divorced Lisa Marie Presley, the late Elvis Presley's daughter, whom he had wed in 1994. Jackson eventually remarried and fathered two children before divorcing a second time in 1999.

Jackson’s career went into decline after he faced allegations in 1993 that he had molested a 13-year-old boy. Although he was never arrested, in 1994 Jackson settled a multimillion-dollar civil lawsuit with the boy’s family without admitting to any wrongdoing. The boy refused to testify against Jackson in a criminal trial, and the investigation into the allegations became inactive. Following this incident the California legislature revised state law to require victims of child molestation to testify in criminal proceedings.

Jackson released the album Invincible in 2001, but the collection of new material did not sell nearly as well as his previous albums. The same year he was elected to the Rock and Roll Hall of Fame (the Jackson 5 had been elected in 1997).

In 2003 Jackson was arrested on multiple counts of child molestation, giving alcohol to a minor, extortion, and false imprisonment. The boy who accused Jackson of molestation testified during the trial, as did his mother, and testimony about alleged previous incidents of molestation by Jackson was also presented. Jackson’s defense attorneys countered by casting doubt on the credibility of the accuser and his family. After a long, heavily publicized trial, a jury acquitted Jackson of all charges in June 2005. See also Rhythm-and-Blues Music; Popular Music.

A WEB OF COMPUTERS

2009-07-04T18:47:00.000-07:00

All communication on the Web is carried out among a set of computers that are interconnected by a computer network. Web technology can be used across an intranet (a network within a company or organization) or across the global Internet. As with all communications among computers, computers that comprise the Web employ two types of software: client and server.

To make information available, a computer runs a server program. To obtain and display information from a server, a computer user runs a client program. The client contacts a server to request information; the server responds by sending a copy of the requested information. To ensure that the exchange is meaningful, the client and server programs must follow a communication protocol, a set of rules that the two programs use to talk to one another. Like a language, a protocol specifies both the form and meaning of each possible message.

In principle, any computer can run a client or a server. In practice, however, large, powerful computers are usually chosen to run server software, and small personal computers (PCs) are sufficient to run client software. Powerful computers are chosen for server software because they must be able to handle requests for information from millions of people and do so quickly so that users who request information from the server will not experience long delays. PCs, however, are used by a single person to request a Web page. After a user makes a request, the user waits for the information to be displayed. Thus, the client program running on a user's computer only needs to handle one activity at a time. A server, however, must handle simultaneous requests from many clients, possibly millions.

The difference between the Web and the Internet is similar to the difference between a trucking service and a highway system. The Internet corresponds to a highway that allows traffic to flow between computers, and the Web corresponds to a service that uses the highway to move information from one computer to another. Confusion about the difference between the Web and the Internet has arisen because the Web has become extremely popular and currently accounts for the majority of Internet traffic. However, other services also use the Internet to carry their traffic. For example, the Internet's electronic mail service permits users to send and receive textual messages, and the file transfer service allows a user to transfer a copy of a file from one computer to another.

Although many services use the Internet to carry data from one computer to another, each service follows a separate set of rules that define the messages used in the exchange. The Web uses the HyperText Transfer Protocol (HTTP), electronic mail uses the Simple Mail Transfer Protocol (SMTP), and file transfer uses the File Transfer Protocol (FTP). The application programs that users run to access the Internet often blur the distinction among these services. For example, an application program that can send e-mail also allows a user to transfer the contents of a file, and an application program used to access the Web also allows the user to process e-mail.

World Wide Web

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World Wide Web (WWW), computer-based network of information resources that combines text and multimedia. The information on the World Wide Web can be accessed and searched through the Internet, a global computer network. The World Wide Web is often referred to simply as “the Web

The Web started to become a popular resource after 1993 when the first widely distributed browser provided a convenient way to access a variety of information on the Internet. The Web uses multimedia, which means that information can be displayed in a wide variety of formats. Users can read text, view pictures, watch animation, listen to sounds, and even explore interactive virtual environments on the Web. A user can move seamlessly from a document or Web page stored on the computer to a document or Web page stored on another computer

The Web offers a place where companies, universities and other institutions, and individuals can display information about their products, services, facilities, or research, or their private lives. Only a small percentage of information on the Web is restricted to subscribers or other authorized users. The majority of Web pages are available to anyone who can access a computer that connects to the Internet. The Web has become a marketplace for many companies selling products or services, and a forum for people to exchange opinions and information. Museums, libraries, government agencies, and schools post information on the Web to make it available to others.

Natural Satellite

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Natural Satellite, in astronomy, a celestial body that orbits a larger celestial body. The larger body is referred to as the satellite’s primary. Natural satellites that orbit planets are often called moons.

The best-known natural satellite is Earth's Moon. The Moon is unusually large relative to the size of its primary (Earth); in fact, it is significantly larger than the planet Pluto. The Moon’s surface, like the surfaces of most of the natural satellites in the solar system, is heavily cratered and geologically inactive.

Neither Mercury nor Venus has any natural satellites, but Mars has two small moons: Phobos and Deimos. Jupiter has more than 30 natural satellites, four of which are quite large: Io, Ganymede, Callisto, and Europa. Active volcanoes cover Io, and scientists believe that oceans of water may hide beneath the icy crusts of Ganymede, Callisto, and Europa. All four of these moons are larger than Pluto, and Ganymede is larger than the planet Mercury as well. Saturn has more than 20 natural satellites, the largest of which is Titan. Titan is bigger than Mercury, and is the only moon with a thick atmosphere. Enceladus, one of Saturn’s smaller moons, has an unusually bright, geologically young surface apparently composed of ice. Uranus has more than 20 moons, none of which are nearly as large as Earth’s Moon. Miranda, one of Uranus’s smaller moons, shows signs of terrific upheavals on its surface. Neptune’s largest natural satellite, Triton, is slightly larger than Pluto. Its surface appears to be continually reshaped by the freezing and thawing of nitrogen. Pluto’s single moon, Charon, is half as large as Pluto itself. Some astronomers consider the pair a double planet.

The motion of most of the solar system's natural satellites about their planets is direct: west to east, in the same direction as the rotation of their planets. Several small satellites of the large outer planets, however, revolve in the retrograde direction: east to west, opposite the direction of the rotation of their planets. These retrograde satellites tend to orbit far from their primaries and were probably captured by the planets' gravitational fields some time after the formation of the solar system.

Encarta has separate articles on most of the natural satellites that have been studied in detail. In addition, overviews of planets’ systems of moons appear in the planet articles.

Relativity

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theory, developed in the early 20th century, which originally attempted to account for certain anomalies in the concept of relative motion, but which in its ramifications has developed into one of the most important basic concepts in physical science (see Physics). The theory of relativity, developed primarily by German American physicist Albert Einstein, is the basis for later demonstration by physicists of the essential unity of matter and energy, of space and time, and of the forces of gravity and acceleration

The theory

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A The Big Bang Theory

The big bang theory describes a hot explosion of energy and matter at the time the universe came into existence. This theory explains why the universe is expanding. Recent versions of the theory also explain why the universe seems so uniform in all directions and at all places.

The work of Edwin Hubble, which showed that the universe is expanding, led cosmologists to begin tracking the history of the universe. The dominant idea is that the universe would have been hotter and denser billions of years ago. In the 1940s Russian American physicist George Gamow and his students, American physicists Ralph Alpher and Robert Herman, developed the idea of a hot explosion of matter and energy at the time of the origin of the universe. (This theory of an explosion at the beginning of the universe was given the originally derisive name “big bang” by British astronomer Fred Hoyle in 1950.) Current calculations place the age of the universe at about 13.7 billion years. Gamow and his students realized that some of the chemical elements in the universe today were forged in the hot early stage of the universe’s existence. They also hypothesized that some radiation that remains from the big bang explosion may still be circulating in the universe, though this idea was forgotten for some time.

Current methods of particle physics allow the universe to be traced back to a tiny fraction of a second—1 × 10-43 seconds—after the big bang explosion initiated the expansion of the universe. To understand the behavior of the universe before that point cosmologists would need a theory that merges quantum mechanics and general relativity. Scientists do not actually study the big bang itself, but infer its existence from the universe’s expansion.

In the 1950s American astronomer William Fowler and British astronomers Fred Hoyle, Geoffrey Burbidge, and Margaret Burbidge worked out a series of calculations that showed that the lightest of the chemical elements (those of lowest atomic weight) were formed in the early universe shortly after the big bang. These light elements include ordinary hydrogen, hydrogen’s isotope deuterium, and helium. Heavier elements, according to those calculations, were formed later. Scientists now know that the elements heavier than helium and lighter than iron were formed in nuclear processes in stars, and the heaviest elements (those heavier than iron) were formed in supernova explosions.

B Steady-State Theory

In the 1940s British scientists Hermann Bondi, Thomas Gold, and Fred Hoyle were philosophically opposed to the requirements that the big bang theory put forth for the extreme conditions in the early universe. The big bang theory was framed in terms of what they called the cosmological principle—that the universe is homogeneous (the same in all locations) and isotropic (looks the same in all directions) on a large scale. Bondi, Gold, and Hoyle suggested an additional postulate, which they called the perfect cosmological principle. This principle stated that the universe is not only homogeneous and isotropic but also looks the same at all times. Since the universe is expanding, though, one might think that the density of the universe would decrease. Such a decrease would be a change that would not fit with the perfect cosmological principle. Bondi, Gold, and Hoyle thus suggested that matter could be continuously created out of nothing to maintain the density over time. The rate at which matter would have to be created was much too low to be observationally testable, however. They called this theory the steady-state theory.

C Big Bang vs. Steady-State

The only evidence necessary for supporters of the big bang theory to prove that this theory was more acceptable than the steady-state theory was to show that the universe changed over time. Just such a change was found in 1963 when Dutch American astronomer Maarten Schmidt identified quasars while working at the Palomar Observatory in California. As seen from Earth, quasars are bluish astronomical objects that resemble stars. Astronomers believe that quasars are the cores of certain types of galaxies. Quasars are all quite far from Earth, which means they must have originated during the early formation of the universe. They are distant from Earth in both time and space. The lack of quasars near Earth (and therefore nearer in time to Earth) shows that the universe has been evolving. This finding dealt a serious blow to steady-state cosmology.

D Discovery of Cosmic Background Radiation

In 1965 a piece of evidence was found that almost all scientists agree conclusively rules out the steady-state theory of the universe. At that time, American physicists Arno Penzias and Robert W. Wilson, working at the Bell Laboratories in New Jersey (now part of Lucent Technologies), discovered faint isotropic radio waves. American astronomers James Peebles, David Roll, David Wilkinson, and Robert Dicke at Princeton University had recently predicted that just such radiation would have been emitted as a result of the hot, dense early universe predicted by the big bang theory. These scientists were themselves preparing a radio telescope to search for this radiation. (Scientists only later recalled that Gamow and colleagues had earlier predicted such radiation.) This cosmic background radiation is now widely accepted as proof of the big bang theory. The existence of cosmic background radiation is the third pillar of modern cosmology. The other two pillars are: (1) the uniform expansion of the universe and (2) the match between calculations of the amounts of the lightest chemical elements that would be formed in the first few minutes after a big bang and observations of these elements’ actual relative abundance in space.

III MODERN COSMOLOGY

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Modern cosmologists base their theories on astronomical observations, physical concepts such as quantum mechanics, and an element of imagination and philosophy. Cosmologists have moved beyond trying to find Earth’s place in the universe to explaining the origins, nature, and fate of the universe. The current “standard model” of the origin of the universe, called the big bang theory, proposes that a major event, not unlike a huge explosion, set free all the matter and energy in the universe and started its expansion. Theories of the evolution and fate of the universe go on to describe a universe that has been expanding and cooling since the big bang. Early versions of the theory held that the universe would keep expanding forever or eventually collapse back to its initial state, an extremely dense object that contains all of the matter in the universe. When the big bang theory was developed in the mid-20th century, some cosmologists found the idea of a sudden beginning of the universe philosophically unacceptable. They proposed the steady-state theory, which said that the universe has always looked more-or-less the same as it does now and that it does not change over time. The steady-state theory could not explain the background radiation, though, and essentially all cosmologists have abandoned it.

Discovering the Structure of the Universe

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In 1917 American scientist Harlow Shapley measured the distance to several groups of stars known as globular clusters. He measured these distances by using a method developed in 1912 by American astronomer Henrietta Leavitt. Leavitt’s method relates distance to variations in brightness of Cepheid variables, a class of stars that vary periodically in brightness. Shapley’s distance measurements showed that the clusters were centered around a point far from the Sun. The arrangement of the clusters was presumed to reflect the overall shape of the galaxy, so Shapley realized that the Sun was not in the center of the galaxy. Just as Copernicus’s observations revealed that Earth was not at the center of the universe, Shapley’s observations revealed that the Sun was not at the center of the galaxy. Cosmologists now realize that Earth and the Sun do not occupy any special position in the universe. Starting in about 1913, new large telescopes and advances in photography and spectroscopy, the study of the particular colors making up a beam of light, allowed astronomers to observe and begin measuring a reddening of the light from distant galaxies. These redshifts are similar to those caused by the see Doppler effect. The Doppler effect is observed when an object emitting radiation moves with respect to the observer of that radiation. If the object is moving toward the observer, each wave of radiation originates from a place that is a little bit closer to the observer than the previous wave’s point of origin, so the distance between successive wave peaks, called wavelength, is shorter than usual. If the object is moving away from the observer, the wavelength is longer than usual. The wavelength change is proportional to the speed at which the object is moving relative to the observer. In visible light, a shift to longer wavelengths is equivalent to a shift toward the red end of the visible spectrum. Therefore, cosmologists refer to shifts in the color of light coming from galaxies that are moving away from Earth as redshifts. The faster a galaxy is moving away, the more red its light will appear. By measuring the redshifts of distant galaxies, astronomers began to understand how the universe was evolving. In 1915 German American physicist Albert Einstein, who was working in Switzerland, advanced a theory of gravitation known as the general theory of relativity. His theory involves a four-dimensional space-time continuum that bends in the presence of massive objects. This bending causes light and other objects that are moving near these massive objects to follow a curved path, just as a golfer's ball curves on a warped putting green. In this way, Einstein explained gravity. His theory showed that Newton’s theory of gravitation was a special case, valid in conditions normal to Earth but not in very strong gravitational fields or in other extreme conditions. Einstein’s theory also made several predictions that were not part of Newton's theory. When these predictions were verified, Einstein's theory was accepted. Einstein's equations were very complicated, though, and it was other scientists who eventually found widely accepted solutions to Einstein’s equations. Most of cosmology today is based on the set of solutions found in the 1920s by Russian mathematician Alexander Friedmann. Dutch astronomer Willem de Sitter and Belgian astronomer Georges Lemaître also developed cosmological models based on solutions to Einstein’s equations. In the early 1920s, astronomers debated about whether the spiral structures seen in the sky, called spiral nebulae, were galaxies like our own Milky Way Galaxy or smaller objects in the Milky Way. Measuring the distances to these galaxies depended on the Leavitt-Shapley method of observing Cepheid variable stars. In 1924 American astronomer Edwin Hubble was able to detect Cepheid variables in other galaxies and show that the galaxies were beyond our own. These findings indicated that the spiral structures were probably galaxies separate from the Milky Way. In 1929 Hubble had measured enough spectra of galaxies to realize that the galaxies’ light, except for that of the few nearest galaxies, was all shifted toward the red end of the visible spectrum. This shift increased the more distant the galaxies were. Cosmologists soon interpreted these redshifts as akin to Doppler shifts, which meant that the galaxies were moving away from Earth. The redshift, and therefore the speed of the galaxy, was greater for more distant galaxies. Galaxies in different directions at equivalent distances from Earth, however, had equivalent redshifts. This constant relationship between distance and speed led cosmologists to believe that the universe is expanding uniformly. The uniform relationship between velocity of expansion and distance from Earth is known as Hubble's law. The redshifts are not true Doppler shifts but rather result from the expansion of space, which carries the galaxies along with it.

Newton and Beyond

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Later in the 17th century, British astronomer Edmond Halley presented British physicist Isaac Newton with a query about the shape of planetary orbits. Newton responded with his three laws of motion (see Mechanics: Newton’s Three Laws of Motion). Newton also developed the idea of universal gravitation, realizing that the same force that makes an apple fall to Earth also keeps the Moon constantly falling toward Earth, although in the Moon’s case Earth continually moves out of the way, resulting in the Moon orbiting the planet. Newton's calculations were eventually expanded into his greatest book, Philosophiae Naturalis Principia Mathematica, which was published in 1687. In the Principia, Newton derived a wide range of theoretical results about planetary orbits and advanced the law of universal gravity. Newton's laws were the foundation of cosmological thought until the 20th century. Newton’s laws, however, left some questions unanswered. Beginning in the 17th century, scientists wondered why the sky was dark at night if space is indeed infinite (an idea proposed in ancient Greece and still accepted by most cosmologists today) and stars are distributed throughout that infinite space. An infinite amount of starlight should make the sky very bright at night. This cosmological question came to be called Olbers’s paradox after the German astronomer Heinrich Olbers, who wrote about the paradox in the 1820s. The paradox was not solved until the 20th century. In the 19th century, counts of the numbers of stars appearing in different directions in the sky left astronomers with the incorrect idea that Earth and the Sun were approximately in the center of the universe. This conclusion did not take into account the modern idea that dust in our Milky Way Galaxy prevented astronomers from seeing very far in any direction.

Sun-Centered Universe

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The ideas of Ptolemy were accepted in an age when standards of scientific accuracy and proof had not yet been developed. Even when Polish astronomer Nicolaus Copernicus developed his model of a Sun-centered universe, published in 1543, he based his ideas on philosophy instead of new observations. Copernicus’s theory was simpler and therefore more sound scientifically than the idea of an Earth-centered universe. A Sun-centered universe neatly explained why Mars appears to move backward across the sky: Because Earth is closer to the Sun, Earth moves faster than Mars. When Mars is ahead of or relatively far behind Earth, Mars appears to move across Earth’s night sky in the usual west-to-east direction. As Earth overtakes Mars, Mars’s motion seems to stop, then begin an east-to-west motion that stops and reverses when Earth moves far enough away again. Copernicus’s model also explained the daily and yearly motion of the Sun and stars in Earth’s sky. Scientists were slow to accept Copernicus’s model of the universe, but followers grew in number throughout the 16th century. By the mid-17th century, most scientists in western Europe accepted the Copernican universe. In the 16th century, Danish astronomer Tycho Brahe made the most scientific and accurate observations of the universe to that time. Brahe discovered discrepancies between astronomical predictions and the actual events, and built a set of large instruments that enabled him to record the positions of the planets and stars with unprecedented accuracy. He moved to Prague, and, after his death, his observations were taken over by German astronomer Johannes Kepler. Kepler discovered that the planets orbited around the Sun in ellipses (elongated circles) with the Sun a bit off-center at one focus. This discovery was Kepler’s first law, and he developed two more laws about how the speeds and periods of the planets changed (see Kepler’s Laws). The first two laws were published in 1609 and the third was published in 1619. The Italian scientist Galileo Galilei lived and worked during the same time period as Kepler. Galileo was the first astronomer to use a telescope to observe the sky and to recognize what he saw there. He saw that the Moon had craters, that Venus went through a full set of phases like the Moon, and that Jupiter had satellites, or moons, of its own. His discoveries, published in 1610, marked the scientific end of the cosmological systems of Ptolemy and Aristotle, though it took some time for his findings to be generally accepted.

Cosmology

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I INTRODUCTION Cosmology, study of the universe as a whole, including its distant past and its future. Cosmologists study the universe observationally—by looking at the universe—and theoretically—by using physical laws and theories to predict how the universe should behave. Cosmology is a branch of astronomy, but the observational and theoretical techniques used by cosmologists involve a wide range of other sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which used to mean the study of the origin of the universe but now usually refers only to the study of the origin of the solar system. II EVOLUTION OF COSMOLOGICAL THEORIES Humans have been examining and wondering about the sky for many millennia. As scientific discoveries have been made, ideas about the origin of the universe have changed and are still changing. A Ancient Cosmologies As far back as 1100 bc, Mesopotamian astronomers drew constellations, or formations of stars perceived to form shapes. Some of today’s constellation names date back to that time. Mesopotamian and Babylonian cultures mapped the motion of the planets across the sky by observing how they moved against the background of stars. Until the 16th century, most people (including early astronomers) considered Earth to be at the center of the universe. Greek philosopher Aristotle proposed a cosmology in about 350 bc that held for thousands of years. Aristotle theorized that the Sun, the Moon, and the planets all revolved around Earth on a set of celestial spheres. These celestial spheres were made of the quintessence—a perfect, unchanging, transparent element. According to Aristotle, the outermost sphere was made of the stars, which appear to be fixed in position. Early astronomers called the stars “fixed stars” to differentiate between stars and planets. The spheres inside the sphere of the fixed stars held the planets, which astronomers called the “wandering stars.” The Sun and Moon occupied the two innermost spheres. Four elements (earth, air, fire, and water) less pure than the quintessence made up everything below the innermost sphere of the Moon. In about 250 bc, Greek astronomer Aristarchus of Sámos became the first known person to assert that Earth moved around the Sun, but Aristotle’s model of the universe prevailed for almost 1,800 years after that assertion. Early astronomers called the planets wandering stars because they move against the background of the stars. Astronomers noted that the planets sometimes moved ahead with respect to the stars but sometimes reversed themselves, making retrograde loops. In about ad 140, Greek scientist Ptolemy explained the retrograde motion as the result of a set of small circles, called epicycles, on which the planets moved. Ptolemy hypothesized that the epicycles moved on larger circles called deferents and that the combination of these motions caused the dominant forward motion and the occasional retrograde loops.

Constellation (astronomy)

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Constellation (astronomy), in astronomy, any of 88 imagined groupings of bright stars that appear on the celestial sphere (see Ecliptic) and that are named after religious or mythological figures, animals, or objects. The term also refers to the delimited areas on the celestial sphere that contain the named groups of stars. The oldest known drawings of constellations are motifs on seals, vases, and gaming boards from the Sumerians, indicating that constellations may have been developed as early as 4000 bc. The constellation Aquarius was named by the Sumerians after their god of heaven An, who pours the waters of immortality upon the earth. The division of the zodiac into 12 equal signs was known around 450 BC by the Babylonians. The northern constellations known today are little different from those known by the Chaldeans and the ancient Egyptians, Greeks, and Romans. Homer and Hesiod mentioned constellations, and the Greek poet Aratus of Soli (circa 315-c. 245 bc) gave a verse description of 44 constellations in his Phaenomena. The Alexandrian astronomer and mathematician Ptolemy, in his Almagest, described 48 constellations, of which 47 are known today by the same name. In the past many other peoples have grouped stars in constellations, although their arrangements usually did not correspond to those of the ancients. Some Chinese constellations, however, resemble those of the ancients, indicating the possibility of a common origin. At the end of the 16th century the first explorers of the South Seas mapped the southern sky, which was largely unknown to the ancients. New constellations were added by a Dutch navigator, Pieter Dirckz Keyser, who participated in the exploration of the East Indies in 1595. Subsequently, other southern constellations were added by the German astronomer Johann Bayer, who published the first extensive star atlas in the Western world, the Uranometria; by Johannes Hevelius; and by the French astronomer Nicolas Louis Lacaille. Many others proposed new constellations, but astronomers finally settled on a list of 88. The boundaries of constellations, however, remained a matter of discussion until 1930, when definitive boundaries were fixed by the International Astronomical Union. The genitive forms of the names of constellations, preceded by a Greek letter, are used to designate about 1300 bright stars; this system was introduced by Johann Bayer. The famous star Algol in the constellation Perseus, for example, is called Beta Persei. The accompanying table lists the constellations on which separate articles appear in this encyclopedia.

black hole

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Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black.

The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity—that is, a dimensionless object of infinite density.

Black holes are thought to form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their energy production is no longer available to resist contraction of the core to ever-higher densities. Two new types of pressure, electron and neutron pressure, arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the star is more than about five times as massive as the Sun, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.

In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material) circling the center of the galaxy M87 with an acceleration that indicated the presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black holes in the centers of dozens of galaxies and had found that the masses of the black holes were correlated with the masses of the parent galaxies. More massive galaxies tend to have more massive black holes at their centers. Learning more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars

The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. For black holes of sufficiently small mass it is possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping (see X Ray: Pair Production). The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.

The American astronomer Kip Thorne of California Institute of Technology in Pasadena, California, has evaluated the chance that black holes can collapse to form "wormholes," connections between otherwise distant parts of the universe. He concludes that an unknown form of "exotic matter" would be necessary for such wormholes to survive.

History of big bang

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The overall framework of the big bang theory came out of solutions to Einstein’s general relativity field equations and remains unchanged, but various details of the theory are still being modified today. Einstein himself initially believed that the universe was static. When his equations seemed to imply that the universe was expanding or contracting, Einstein added a constant term to cancel out the expansion or contraction of the universe. When the expansion of the universe was later discovered, Einstein stated that introducing this “cosmological constant” had been a mistake.

After Einstein’s work of 1917, several scientists, including the abbé Georges Lemaître in Belgium, Willem de Sitter in Holland, and Alexander Friedmann in Russia, succeeded in finding solutions to Einstein’s field equations. The universes described by the different solutions varied. De Sitter’s model had no matter in it. This model is actually not a bad approximation since the average density of the universe is extremely low. Lemaître’s universe expanded from a “primeval atom.” Friedmann’s universe also expanded from a very dense clump of matter, but did not involve the cosmological constant. These models explained how the universe behaved shortly after its creation, but there was still no satisfactory explanation for the beginning of the universe. In the 1940s George Gamow was joined by his students Ralph Alpher and Robert Herman in working out details of Friedmann’s solutions to Einstein’s theory. They expanded on Gamow’s idea that the universe expanded from a primordial state of matter called ylem consisting of protons, neutrons, and electrons in a sea of radiation. They theorized the universe was very hot at the time of the big bang (the point at which the universe explosively expanded from its primordial state), since elements heavier than hydrogen can be formed only at a high temperature. Alpher and Hermann predicted that radiation from the big bang should still exist. Cosmic background radiation roughly corresponding to the temperature predicted by Gamow’s team was detected in the 1960s, further supporting the big bang theory, though the work of Alpher, Herman, and Gamow had been forgotten.

Big Bang

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Big Bang Theory, currently accepted explanation of the beginning of the universe. The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 13.7 billion years ago, and the universe has since been expanding and cooling.

The theory is based on the mathematical equations, known as the field equations, of the general theory of relativity set forth in 1915 by Albert Einstein. In 1922 Russian physicist Alexander Friedmann provided a set of solutions to the field equations. These solutions have served as the framework for much of the current theoretical work on the big bang theory. American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was universally shifted toward the red end of the spectrum (see Redshift). Once “tired light” theories—that light slowly loses energy naturally, becoming more red over time—were dismissed, this shift proved that the galaxies were moving away from each other. Hubble found that galaxies farther away were moving away proportionally faster, showing that the universe is expanding uniformly. However, the universe’s initial state was still unknown.

In the 1940s Russian-American physicist George Gamow worked out a theory that fit with Friedmann’s solutions in which the universe expanded from a hot, dense state. In 1950 British astronomer Fred Hoyle, in support of his own opposing steady-state theory, referred to Gamow’s theory as a mere “big bang,” but the name stuck. Indeed, a contest in the 1990s by Sky & Telescope magazine to find a better (perhaps more dignified) name did not produce one.

Earth’s Relative Motion

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Earth moves in two basic ways: It turns in place, and it revolves around the Sun. Earth turns around its axis, an imaginary line that runs down its center through its North and South poles. The Moon also revolves around Earth. All of these motions produce day and night, the seasons, the phases of the Moon, and solar and lunar eclipses.

EARHT'S NIGHT SKY

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Up to about 3,000 stars are visible at a time from Earth with the unaided eye, far away from city lights, on a clear night. A view at night may also show several planets and perhaps a comet or a meteor shower. Increasingly, human-made light pollution is making the sky less dark, limiting the number of visible astronomical objects. During the daytime the Sun shines brightly. The Moon and bright planets are sometimes visible early or late in the day but are rarely seen at midday.

Analysis and Theory

2009-05-03T06:30:00.000-07:00

Whether astronomers take data from a ground-based telescope or have data radioed to them from space, they must then analyze the data. Usually the data are handled with the aid of a computer, which can carry out various manipulations the astronomer requests. For example, some of the individual picture elements, or pixels, of a CCD may be slightly more sensitive than others. Consequently, astronomers sometimes take images of blank sky to measure which pixels appear brighter. They can then take these variations into account when interpreting the actual celestial images. Astronomers may write their own computer programs to analyze data or, as is increasingly the case, use certain standard computer programs developed at national observatories or elsewhere.

Often an astronomer uses observations to test a specific theory. Sometimes, a new experimental capability allows astronomers to study a new part of the electromagnetic spectrum or to see objects in greater detail or through special filters. If the observations do not verify the predictions of a theory, the theory must be discarded or, if possible, modified.
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Questions and Answers About Astronomy

2009-05-03T06:08:00.000-07:00

Astronomer Jay M. Pasachoff, author of A Field Guide to the Stars and Planets (Fourth edition, 2001), is one of the world’s leading experts on the universe and the celestial bodies it contains. Here, Pasachoff answers a wide range of intriguing questions, including what could be done if an asteroid threatened to collide with the Earth, what are brown dwarfs and wormholes, is there life on Mars or on extrasolar planets, and can you ever escape Earth’s gravity.

Q: About how large does an asteroid need to be before its gravity makes it round?

A: The largest known asteroid, 1 Ceres, is about 1,000 km (600 mi) in diameter. Only the six largest asteroids are larger than 300 km (200 mi) in diameter, and they are all round. The dividing line below which asteroids do not become round is somewhere between 100 and 300 km (60 and 200 mi), depending on the circumstances of their formation and cooling. The most studied asteroid is 433 Eros. The Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft, created and run by the National Aeronautics and Space Administration (NASA) and the Johns Hopkins University Applied Physics Laboratory, orbited Eros for about a year, starting on February 14, 2000. Eros is oblong, about 33 by 13 by 13 km (21 by 8 by 8 mi), and images of it are posted on the Web.

Several space missions are now discovering asteroids at an astonishing rate. During the year 2000, 10,000 new asteroids were discovered-–as many as had been discovered during the preceding 200 years.

Q: If a giant asteroid were headed toward Earth, could people do anything to stop it?

A: We need more information about asteroids before we can tackle this task. If we were to put a bomb on an asteroid, we don’t know whether the asteroid would swerve slightly, missing Earth, or just break into bits, with each of the bits still coming straight at us. For this reason, astronomers would like to make a survey of the sky to detect all the asteroids that might intersect Earth’s orbit, and to understand the composition of asteroids well enough to know how solid they are.

Our current understanding of these near-Earth objects indicates that we can expect a big collision every few hundred thousand years, and a really devastating one every few million years. Astronomers think that over 1,000 of these near-Earth objects are more than 1 km (0.6 mi) across. More and more scientists agree that a collision with an asteroid or comet 65 million years ago killed off the dinosaurs and many other species. In fact, mammals like us benefited from that event, since mammals survived while the dinosaurs didn’t. Statistically, scientists think there is a 1 percent chance that a much smaller object, around 300 m (1,000 ft) across, will hit Earth sometime in the next century. An asteroid of that size would still make a devastating crater on land or create a tidal wave if it fell in the ocean.

The spacecraft NEAR Shoemaker (Near-Earth Asteroid Rendezvous, also named for planetary geologist Eugene Shoemaker) went into orbit around the asteroid Eros on February 14, 2000, and it has sent back incredibly detailed images. NEAR Shoemaker will get closer and closer to Eros, eventually landing on the asteroid. Eros seems to be solid, while the asteroid Mathilde, which NEAR Shoemaker passed in 1997, seems to be more of a rubble pile, with only half the density of Eros.

Q: I’ve heard that astronomy absorbs tremendous amounts of money, but I cannot find exact figures. Can you tell me how much is spent yearly? And is it too much for just pondering the mysteries of the universe? Wouldn’t it be wiser to spend a fraction of this money for more mundane purposes? I like seeing the pretty pictures from the Hubble telescope, but I wonder if it is worthwhile.

A: Actually, astronomy absorbs relatively little money compared with other government functions. Over the years, it has been demonstrated that investing money in basic research such as astronomy has a return of several times the investment. Astronomy attracts some of the world’s brightest people to scientific problems and leads to discoveries of physical laws that are important to everybody, although the results of such discoveries are not usually immediately apparent.

Astronomic research that could have important impacts in the near future includes studies that may improve our understanding of Earth’s atmosphere and studies of the runaway greenhouse effect on Venus.

In the United States the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA) are the primary sources of funding for astronomy. The NSF astronomy budget is about $100 million per year. The population of the United States is more than 250 million, so that is 40 cents per person per year.

NASA’s budget is harder to categorize since about one-third of it is devoted to crewed space flight—largely for political reasons or for general exploration rather than for scientific research. Indeed, many scientists decry the current emphasis on the International Space Station because of the limited amount of science that will be carried out on it. However, NASA conducts resource mapping and other studies of Earth that have proved very valuable.

Still, even if we say that about $2.5 billion of NASA’s $15 billion per year budget is related to space sciences, with a smaller fraction devoted to astronomy, that is only around $10 per person. The astronomy part is about $500 million, roughly $2 per person.

Note that for $10 per person, you aren’t going to solve major problems on Earth. You aren’t going to solve poverty or make medical advances that will revolutionize the world, or even provide health insurance for individuals. It seems more worthwhile to invest that level of money in basic scientific research that has the promise of making breakthroughs that will bring new health and prosperity to people in the future. Our country spends about 40 times as much on social programs as it does on space.

Incidentally, the Hubble Space Telescope puts out pretty pictures to show the people that it is working, but its major scientific work isn’t in those pretty pictures. It has spectrographs and special filters that allow details to be investigated. Most people like the pictures, but please don’t be misled to think that pictures are Hubble’s main scientific work.

Q: How rare is the aurora borealis? What causes it?

A: The aurora borealis isn’t rare if you live near one of the Earth’s magnetic poles. The north magnetic pole currently is in the Queen Elizabeth Islands of Canada’s Northwest Territories. Views from space show an auroral oval most of the time.

The aurora borealis and its sister in the Southern Hemisphere, the aurora australis, are caused by particles from the Sun hitting the air molecules in the Earth’s atmosphere. These particles give energy to the air molecules and make them glow. Different molecules glow in different colors.

When a coronal mass ejection or a solar flare sends a large number of high-energy particles into the auroral oval, the oval expands. Such events take place more often near the maximum of the solar-activity cycle (which is most commonly regarded as the sunspot cycle). This 11-year cycle is already or will soon be past its peak, which will probably be shown to have occurred in 2000 and 2001.

Solar coronal mass ejections and flares occur frequently in the declining phase of the cycle, so we have hope of seeing auroras at latitudes closer to the equator than is usual. Such an event can cause an aurora that is visible throughout the whole continental United States. If you live in Alaska, you can see an aurora most of the time when the night sky is clear. Phenomena that result from the interaction of the Sun and Earth—like the aurora—are called space weather.

NASA has several satellites in space that image the aurora regularly, including Polar and IMAGE (Imager for Magnetopause-to-Aurora Global Exploration). The IMAX movie SolarMax (2000) shows auroras and other Earth-Sun phenomena on a huge screen.

Q: What was there in the universe before the big bang?

A: Astronomers’ observations indicate that the universe is expanding, with every cluster of galaxies moving away from every other cluster. Projecting their motion backward in time, the clusters of galaxies would be closer and closer together, and the universe’s density would be very, very high. We can project back in time only as far as this original explosion, which we call the big bang. (The name was given derisively in the 1940s, but it caught on and is now used as the formal name.)

Astronomers measure time from the big bang. As far as we know, time started and space originated in the big bang. So there was no “before the big bang.” The universe simply did not exist.

This explanation may become clearer if we examine a similar example: On the Earth’s surface, the North Pole marks latitude 90 degrees. If you go north along any line of longitude toward the North Pole and then keep going, you don’t go above latitude 90 degrees. Once you reach 90 degrees (the North Pole), you start going south. So the question “what is north of the North Pole” doesn’t have a meaningful answer in the same way that “what was there in the universe before the big bang” doesn’t have a meaningful answer.

Q: What is the main advantage of being on-site for an eclipse? Can’t you create an artificial eclipse anywhere by placing a dark black-out disk over the Sun’s face?

A: Well, you can’t place a dark disk over the Sun’s face and see the corona. The corona is just fainter than the blue sky, and it isn’t visible. If you go to a very high mountain, such as Haleakala in Hawaii, you can make such a coronagraph, although it requires intricate optics. And even then you don’t see as much of the corona as well as you do from an eclipse.

Even from space, where there is a coronagraph on the Solar and Heliospheric Observatory (SOHO) spacecraft, the coronagraphs can’t make as good an eclipse as we see from the ground. SOHO hides (occults) not only the Sun but also all the corona for one solar radius above the ordinary solar disk. This is the interval that we see best from a ground-based eclipse site.

It is pretty to see the Moon cover the Sun in the sky, but the actual eclipse is pretty small in angle—you can cover it with your thumb at the end of your outstretched arm. The real glory of a total solar eclipse for ordinary watchers (aside from the science that professionals are interested in) is the whole set of atmospheric changes. It is dramatic to be outdoors when the sky grows dark in the middle of the day. Shadows strangely sharpen and the quality of the light changes, making it eerie in a way that is hard to pin down. Then the shadow of the Moon rushes at you at a speed of thousands of miles per hour and within seconds, day is transformed into night. Nobody who is outdoors during these abrupt changes remains unmoved.

Q: In theory if you go into a black hole, when you come out the other side, would you be in a different time? Because if a black hole sucks in light then wouldn’t it suck in time?

A: In principle, you could be in a different time—IF you could go through a black hole and come out somewhere else (the other end would be a location known as a white hole, which is a hypothetical region in space from which stars, light, and other forms of energy explosively emerge). If this were possible, you would be going through a wormhole, a hypothetical passage in space-time connecting widely separated parts of the universe. It is not clear if a wormhole can stay open sufficiently long for anyone to go through it.

With regard to your second question, a black hole doesn’t “suck in light.” If light has the bad luck to go inside, it can’t get out. But the black hole’s effect on space outside itself is the same as that of any equivalent amount of mass.

Q: What are brown dwarfs?

A: Brown dwarfs are failed stars—balls of gas that didn’t become hot enough for fusion of hydrogen to start inside. A star has to have a mass of about 7 percent of the Sun’s mass for its core to get hot enough for fusion to begin. We think that starlike objects with less than that critical mass become brown dwarfs.

Since they aren’t shining brightly, brown dwarfs are very hard to detect. Indeed, until the 1990s, there had been no accepted brown dwarf discoveries. Recently, new large telescopes and satellites sensitive to the infrared radiation that brown dwarfs give off have led to dozens of discoveries.

Some brown dwarfs are as cool as 2000°C (3600°F) at their surfaces. Some gases, such as lithium, which is destroyed at hotter temperatures, survive at the surfaces of brown dwarfs. The presence of such gases convinces astronomers that they are indeed observing this type of star.

For decades, astronomers have divided stars into seven spectral types: O, B, A, F, G, K, and M. O stars are the hottest and M stars are the coolest. Now we have added two cooler types, L and T, to include brown dwarfs. People remember the earlier sequence from the first letters of each word in the sentence “Oh, be a fine girl, kiss me.” Try to think of mnemonics that include not only the original types but also types L and T!

Q: Which star is closest to Earth, other than the Sun?

A: We measure distances to the closest stars using the concept of parallax, which relies on the fact that the background behind an object looks different depending on the angle from which the object is viewed. For example, when the passenger in a car looks at the speedometer needle it appears to indicate a slightly lower speed than the one the driver sees. This is because the driver is looking straight ahead at the speedometer and the passenger is looking at it from an angle, and the needle is projected against the background numbers.

You can check this effect by holding your thumb in front of your face. If you hold your thumb only a few inches from your nose and then look with first one eye and then the other, you can see that your thumb seems to jump a lot across the background. But if you extend your arm and look again with each eye, your thumb seems to jump less.

In astronomy, we project the angle of the nearest stars against a background of much more distant stars. If we look in January, and then look again when Earth has moved halfway around its orbit in June, we find that the nearest stars are measurably displaced compared with farther stars.

Using parallax, astronomers have determined that the nearest star is part of a triple-star system known as Alpha Centauri. Alpha Centauri is so named because it is the brightest system in the constellation Centaurus (alpha is the first letter in the Greek alphabet). Centaurus is too far south in the sky to be visible from most of North America and Europe. One of the three stars in Alpha Centauri, known as Proxima Centauri (from the Greek word for “close”), is the nearest star to us. Still, it is four light-years away. That is, its light travels for four years, at the tremendously great speed of light, before it reaches us. In contrast, light from the Sun reaches us in eight minutes, and light from the Moon reaches us in about one second.

Q: When the Moon is near the horizon, it often takes a red or orange color. Is this caused by the refraction or reflection of light or neither? And why is this “color change” so short-lived?

A: The Moon looks reddish or orange near sunset for the same reason there is a reddish sunset glow on the horizon. And this reason is connected to why the sky is blue.

Sunlight bounces (scientists say “scatters”) off air molecules, and the shorter the wavelength (that is, the bluer the light) the more effectively it bounces. So when the Sun or Moon is overhead, its light comes to us directly but off to the side a bit. Blue light bounces toward us more effectively than red light.

But when the Sun is on the horizon, its light has to pass through a greater length of atmosphere to reach us. The blue light scatters all along the path, making the sky blue for people in that direction. By the time the sunlight reaches us, most of the blue is gone and we are left with relatively more red. The same effect occurs for moonlight, but it is so much fainter than the sunlight that the blue-sky effect is gone. Still, the light that reaches us near the horizon is relatively red because the blue light has been scattered away.

That kind of scattering is called “Rayleigh scattering,” after the great physicist Lord Rayleigh of the 19th century.

Q: Why do the constellations change over time?

A: The visibility of constellations changes over the year, although specific constellations are visible at the same time every year. Constellations also change shape over tens of thousands of years.

First, why do we see different constellations at different times of the year? Earth orbits the Sun once a year. Sunlight that hits Earth’s atmosphere scatters around and makes the sky blue, preventing people on the ground from seeing the stars during the day. So people can only see constellations when they are situated on the side of Earth that faces away from the Sun. At different times of the year, Earth is on different sides of the Sun, so we see different parts of the sky and different constellations. Since our view of the sky changes by 360 degrees in 12 months, the view changes by 30 degrees in 1 month; 30 degrees is about three fists’ width held at the end of your outstretched arm. So each month, stars that you saw last month have moved about 30 degrees.

Why do constellations change over long periods of time? Because stars move through space. We can measure how fast they are coming toward us or going away from us by studying their spectrum, using a phenomenon known as the Doppler effect. We can measure how fast they are moving from side to side only for the closest stars, which have noticeable “proper motions” with respect to other stars. If the proper motion is large enough, then the star will move noticeably if you wait long enough. Stars are so far away, however, that they have to move a long way for us to see any difference. The star with the largest proper motion is known as Barnard’s star; it moves the diameter of the Moon in the sky every 180 years. Unfortunately, Barnard’s star is too faint to see with the naked eye. No star visible to the naked eye moves noticeably in a human lifetime, so no constellation will change noticeably in a human lifetime.

Q: My astronomy professor said that due to the Sun’s cyclical orbit around this arm of the Milky Way, the Earth’s perspective regarding the Sun’s apparent transit through the heavens has changed since 2,000 years ago when the “science” of astrology was formulated. As a result of this quarter-turn shift, the Sun is no longer moving through the same constellations of the zodiac as it was before, and calculations are off by one constellation. Thus, instead of being a Cancer, am I actually a Gemini?

A: Yes, it is true the “signs” you see in the newspaper are not the constellations where the Sun actually is at those times of year, because of the precession of the equinoxes. And the Sun goes through parts of 13 constellations in the course of the year.

Furthermore, the idea that there are 12 “signs of the zodiac” is completely arbitrary—other cultures had different numbers of constellations filling the same space. There is simply nothing to astrology, and I hope that readers of this reply will spend their time on the fascinating topics in the science of astronomy instead of wasting time on the pseudoscience of astrology.

Q: The Discovery Channel recently aired a program called “Planet Storm.” One of the space weather phenomena the show depicted was a coronal mass ejection (CME). While solar flares and sunspots are infrequent events on Earth, nothing of the magnitude of a CME (which could deplete 100 percent of our atmosphere) has ever occurred. In the dramatization, a CME of this level penetrates and vaporizes the ozone layer, creating a brilliant aurora borealis effect for people to enjoy in the short run, only to wake up the next day to be killed off almost instantly by fatal ultraviolet (UV) rays.

Can a CME like this occur and what precautions can be taken? Are we fooling ourselves about this deadly potential as we did with Kiddy Cocoons in the 1950s and 60s?

A: Coronal mass ejections occur daily, and solar physicists spend a lot of time talking about them. Whereas decades ago they thought that giant solar flares were the main Sun-Earth link, they now realize that coronal mass ejections are a more regular link. Whether coronal mass ejections are linked to flares is very much debated; some are, but they may not be cause and effect.

The importance of coronal mass ejections has been shown especially by the movies made by the Large Angle Spectrographic Coronagraph (LASCO) instrument on the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint project of NASA and the European Space Agency.

Usually, LASCO shows CMEs shooting off to the side of the Sun, but every once in a while it sees a “halo event,” when material seems to surround the “occulting disk” that hides the bright ordinary solar disk. These halo events no doubt mean that the CME is pointing at the Earth. Particles from the CME arrive hours or days later. CMEs have been known to disrupt terrestrial communications and, probably, to damage spacecraft.

CMEs have varying strengths. I know no reason why the Sun’s magnetic field can’t rearrange itself to shoot off a much more powerful CME than normal. It is statistically unlikely that such a powerful CME, as depicted in your question, would arise. But if it should happen, I see no way that we earthlings could prevent it or deal with it. There are no precautions we can take, short of moving to another planet—which is at least centuries in our future for more than a handful of individuals.

Q: How does Earth’s distance from the Sun affect the seasons?

A: Basically it doesn’t. Earth’s orbit, although elliptical, is so close to round (only 1.7 percentage points from being perfectly round) that Earth’s distance from the Sun does not vary enough to affect the seasons. Indeed, Earth is closest to the Sun each year on January 4, when it is winter and cold in the Northern Hemisphere.

The seasons are caused by Earth’s tilt on its axis. Earth’s tilt is constant with respect to the stars, and as Earth goes around the Sun each year sometimes the Northern Hemisphere is tilted toward the Sun and sometimes it is tilted away. The Southern Hemisphere, of course, is tilted the opposite way. When one hemisphere is tilted toward the Sun, it is summer there and winter in the opposite hemisphere.

Earth’s tilt affects the seasons because a beam of sunlight 1 m (3 ft) in diameter warms a patch of ground 1 m across if the beam shines directly down on Earth’s surface. But when the beam hits a part of Earth’s surface that is tilted, that same beam has to warm a larger patch of ground. Thus the energy in the beam is diffused, and each square centimeter of Earth’s surface receives less energy per second.

Q: If the Moon is receding from the Earth at approximately five centimeters per lunar year, is the Earth going toward the Sun or receding from the Sun? If so, by what distance per Earth year?

A: The Moon is receding from the Earth because of its tidal interaction, which is the result of the gravitational pull of the Moon on the Earth. But the planets have very little mass compared with the Sun, so there is no measurable tidal effect of planets on the Sun.

I am unaware of any yearly change in the Earth’s orbit around the Sun. In the distant future, the Sun will lose enough mass that Earth’s orbit will stabilize at a greater distance than it is now. But that is still billions of years away. If Earth’s orbit were to double in size over 3 billion years, that would mean an increase of 150 million km (93,210,000 mi), or approximately 50 m (164 ft) each year.

Although the Earth is thought to be on a track to recede in the future, some planets spiral in to some stars. In some recently discovered planetary systems, giant planets orbit their parent stars in days (instead of the ten years it takes Jupiter to orbit our Sun). That means these planets orbit very closely. They may have formed farther out and gradually moved close in, according to theories now being developed. There has also been a report of elements detected (spectroscopically) on a star that could well have resulted from a planet crashing into the star.

Q: What is the safest way to observe a solar eclipse?

A: You shouldn’t stare at the Sun, except during the total phase of a total eclipse. During the total phase, the solar surface is completely covered by the Moon. When the Moon covers the Sun only partially during an eclipse, some of the solar surface remains visible, so you cannot safely look at it directly. This part of an eclipse is known as the partial phase; some eclipses are never more than partial when seen from Earth. The Sun is also partially eclipsed before and after the total phase of a total eclipse.

Whenever the solar surface is visible—during a partial eclipse or the partial phase of a total eclipse—you must take special precautions to see the eclipse. Special solar filters are inexpensive; they cut out more than 99.99 percent of the Sun’s light. Another option is to make a pinhole camera. Just punch a hole the size of a pencil in a piece of paper, cardboard, or aluminum foil. Then hold that hole up to the Sun, next to your shoulder, and look away from the Sun at the image formed on the floor or a wall. Since you are looking away from the Sun, there is no hazard.

If you are lucky enough to be in a location where the Sun is totally eclipsed, then the solar surface will be entirely covered. At that time, the solar corona comes into view. The corona is a million times fainter than the solar surface. It is about the same brightness as the full Moon and is equally safe to look at.

So, look at the Sun directly ONLY when the solar surface is completely covered during a total eclipse. Use special filters or a pinhole camera whenever the Sun is partially eclipsed. There are no special rays that come out of the Sun during an eclipse; the hazard is that you are more tempted to stare.

Q: How far away do you need to be to escape Earth’s gravity?

A: As long as you remain in the universe, you can never actually escape Earth’s gravity. Isaac Newton discovered the law of universal gravity, which shows that the strength of gravity declines according to the square of the distance between objects. Thus when you go twice as far away from an object, you feel one-fourth the strength of its gravitational force. But each object in the universe retains some gravitational pull, however minuscule.

If you go nine-tenths of the distance toward the Moon, the Moon’s gravity becomes stronger than Earth’s gravity. If you go one-hundredth of the distance toward the Sun, the Sun’s gravity becomes stronger than Earth’s gravity.

French mathematician Joseph Louis Lagrange (1736-1813) worked out a set of solutions to describe how the gravitational attraction between two large objects and their orbital velocities balance each other such that a small body placed in the orbital plane of the larger bodies will remain balanced there. He found five such points at which smaller objects remain balanced.

For example, there is a point partway to the Moon at which Earth’s gravity and the Moon’s gravity are equal. If you were to pass that point, you would fall toward the Moon instead of toward Earth. This point, known as L1, is closer to the Moon than to Earth since Earth has more mass than the Moon.

Similarly, there is an L1 point partway to the Sun at which Earth’s gravitational pull and the Sun’s gravitational pull on an object are equal. This point is relatively close to Earth. The joint European Space Agency/NASA spacecraft called the Solar and Heliospheric Observatory (SOHO) is located at L1, hovering so the Sun remains in steady view for scientific study, and needs little fuel to remain in place.

Other Lagrangian points in a line with two massive objects include L2, which is on the far side of the smaller of the two objects. L3 is on the far side of the larger object, and is the same distance from it as the smaller body.

Other Lagrangian points, L4 and L5, exist one-third of the way around an orbit. For example, the Trojan asteroids are in Jupiter’s orbit around the Sun, 60 degrees (one-sixth of a circle) ahead of and behind Jupiter. Many people have proposed installing an inhabited space station at the L5 Lagrangian point.

Q: Not knowing your religious beliefs I would like to ask you: As you explore the universe through a telescope do you find the existence of God to be more, or less, believable as you discover the universe’s qualities? Why or why not?

A: My belief in God is independent of what I learn about the universe. The universe seems to follow a set of laws, such as the laws of gravity and orbits and Maxwell’s equations for electromagnetism. Whether you choose to think that God made those laws or that they arose naturally is up to you and is a matter of religious belief. I see no need to mix science and religion.

Q: What are geostationary and geosynchronous orbits, and what is the difference between them?

A: There is no difference. The word used depends on whether you are looking up at a satellite or down from a satellite. A satellite only a few hundred kilometers above Earth’s surface orbits in about 90 minutes. The higher the satellite, the longer it takes to orbit. If the satellite is about 6.5 Earth radii high, or about 40,000 km (about 25,000 mi) above Earth’s center, it orbits in 24 hours.

Earth also spins once every 24 hours, however, so a satellite with a 24-hour orbit is in synch with Earth’s rotation. We call such an orbit geosynchronous. To an observer on the ground looking up, the satellite seems stationary overhead, so the orbit may also be called geostationary. The satellite has to be orbiting above the equator to appear still in the sky, so the band of orbits over the equator has a lot of satellites in it.

Geosynchronous/geostationary satellites are especially useful in relaying television and other communications signals. A radio telescope or satellite dish on Earth can remain fixed and still always point at the satellite.

Q: How hot is the Sun?

A: The Sun is a ball of gas, heated at its center by nuclear fusion. In its center, hydrogen atoms merge one at a time to make helium, which has almost four times the mass of hydrogen. The mass difference is converted into energy, in an amount that follows Albert Einstein’s famous formula E = mc2, where E is energy, m is the mass converted, and c is the speed of light. A tiny bit of mass converts to a tremendous amount of energy, and as a result the Sun is extremely hot.

The Sun is a little over 5500°C (10,000°F) at its visible surface, but it is much hotter inside. Astronomers can tell the temperature of the Sun’s surface in several ways. One way is that it is brightest in the yellow-green part of the spectrum, a characteristic of gas at 6000°C (11,000°F). Another way is to study the spectral lines caused by atoms in the Sun’s outer atmosphere, which are also characteristic of that temperature.

We can’t see the inside of the Sun directly, but calculations show that its temperature is about 15 million degrees C (about 27 million degrees F). There are two ways to find out about the center of the Sun. One is helioseismology, another name for solar seismology. In helioseismology, astronomers look at vibrations on the Sun’s surface to find out about its inside, just as geologists on Earth find out about the Earth’s inside by studying seismic waves on the planet’s surface. A second way is to study solar neutrinos, elementary particles that come straight out through the Sun once they are formed in fusion.

Q: How many people have walked on the Moon? Can you tell me something about each of them?

A: Six National Aeronautics and Space Administration (NASA) Apollo missions reached the Moon from 1969 to 1972. Many people at that time hoped that the Apollo program would lead to a permanent station on the Moon, and it is very disappointing to realize that nobody has been to the Moon in about 30 years.

Each lunar-landing Apollo mission (which began with Apollo 11) carried three people into orbit around the Moon. One of the astronauts then remained in the command module, while two others used the lunar module to descend to the Moon’s surface.

The first Moon landing was made by Neil Armstrong and Buzz Aldrin in Apollo 11’s lunar module, named Eagle, on July 20, 1969. Armstrong’s first words on the Moon were “That’s one small step for man, one giant leap for mankind.” There has been much discussion over the years as to why he didn’t say “for a man,” and there has been speculation that the word “a” was merely swallowed by a radio glitch, but the consensus seems to be that Armstrong just said “step for man,” perhaps out of nervousness. Armstrong became a professor of engineering and shunned public appearances, although he was arguably the most famous person in the world. Aldrin has recently coauthored a novel, The Return, which is a murder mystery involving the commercialization of space tourism.

Pete Conrad and Alan Bean landed on the Moon in Apollo 12’s lunar module, Intrepid, in November 1969. Conrad quipped, “That may have been a small one for Neil, but it’s a long one for me.” Conrad, always a daredevil, died in a motorcycle accident in 1999. Bean, an artist, has painted space scenes.

Apollo 13 suffered an explosion en route to the Moon. The astronauts were able to return safely to Earth, but they missed their chance to walk on the Moon.

Apollo 14’s lunar module, Antares, shuttled Alan Shepard and Ed Mitchell to the Moon’s surface in February 1971. The long hiatus was the result of the commission of inquiry over the cause of the Apollo 13 explosion. Shepard was chief of the Astronaut Office until he retired from NASA and later went into business. He died in 1998.

Apollo 15’s lunar module, Falcon, carried Dave Scott and Jim Irwin to the lunar surface in July 1971. They were able to go farther on the Moon than earlier astronauts because they had a vehicle, the Lunar Rover. With it, they were able to cover 27.3 km (17 mi). Scott had the idea of demonstrating the physics experiment of dropping a feather and a hammer in the Moon’s airlessness. Confirming Galileo’s experiment (which traditionally involved dropping weights off the Leaning Tower of Pisa), the feather and the hammer landed simultaneously, something shown in video clips to generations of science students since. Irwin retired from NASA to form a religious organization. He has since died.

John Young and Charles Duke went to the lunar surface in April 1972 in Apollo 16’s lunar module, Orion. They landed in the highlands, a rougher and therefore more dangerous region to explore than the smoother areas that NASA had chosen for the earlier flights. Young later became chief of NASA’s Astronaut Office. Duke retired from the astronaut corps to go into business; he also has a religious ministry.

Apollo 17’s lunar module, Challenger, carried Gene Cernan and Jack Schmitt to the Moon’s surface in December 1972. Schmitt was the only Ph.D. scientist and the only trained geologist to walk on the Moon. He later became a United States senator. Cernan went into business and is currently president of Cernan Energy Group, an energy and aerospace consulting company.

Originally NASA planned for longer missions to eventually take place, once the Apollo program got the bugs out. These longer missions were intended to accomplish more scientific investigations. Scientists looked for Apollo 18 to 20 for those scientific opportunities, but these missions were cancelled for financial reasons.

A few spacecraft went back to the Moon in the 1990s, notably the Clementine and Lunar Prospector missions in 1994 and 1998, respectively. A few uncrewed missions are scheduled for the coming decade, but there are presently no plans to send humans back to the Moon.

Q: How many stars are there?

A: In Antoine de Saint-Exupery’s wonderful book The Little Prince, a businessman sits on his tiny asteroid counting the stars, because he thinks he owns them. We can’t own the stars, and there are more than we can count.

Dust in space hides many stars from us, so we couldn’t count all the stars even if we tried. But astronomers can measure the amount of mass in our galaxy. It turns out that there is about a few hundred billion times more mass in our galaxy than our Sun has, and we know that much of that mass is in the form of stars. So we might estimate that there are a few hundred billion stars in our galaxy, assuming that, on average, each star has a mass that is roughly equal to the mass of the Sun.

Actually, the Sun’s mass is higher than that of an average star. This would indicate that there are even more stars in our galaxy. On the other hand, we are increasingly finding that much of the mass in our galaxy is not in the form of stars. Since it isn’t shining, we say this mass is dark matter. There is probably more mass in the form of dark matter in our galaxy than there is in the form of stars; this would tend to lower our estimate of the number of stars in our galaxy. At any rate, a few hundred billion stars is a reasonable estimate for our galaxy.

Our galaxy is only one of billions of galaxies, however, so there are truly a tremendous number of stars in the universe.

Q: Why does the Hubble Space Telescope take sharper pictures than larger telescopes on the ground?

A: The larger a telescope’s mirror, the finer the detail it can distinguish, according to the laws of optics. But ground-based telescopes look up through the atmosphere, and the atmosphere shimmers and shakes (we call it twinkling), blurring the stars.

One good way of getting around this limitation is to put the telescope in space, as NASA did with the Hubble Space Telescope. This telescope can see about ten times more clearly than typical ground-based telescopes, but it is only 94 in (240 cm) across—not very large for a telescope these days. So it can’t collect a lot of light at any given time.

Since the Hubble telescope was launched in 1990, astronomers have made progress in getting better images from ground-based telescopes. Just making sure that there aren’t sources of heat near a telescope or in the telescope dome, and adding fans to the telescope dome to make sure that no hot air is rising up through the dome slit, has helped improve image quality at many telescopes.

In addition, astronomers are rapidly developing a new technique known as active optics. In active optics, a mirror is made thin enough that it can be deformed several times a second by robotically controlled pins pushing on its back. The deformations compensate for the blurring caused by Earth’s atmosphere. Active optics is providing images that in some cases match those from Hubble. Hubble can point anywhere in the sky, however, while active optics can match Hubble’s high resolution only near certain bright objects, where the blurring can be taken out by deforming the mirror. Since we already know how these bright object should appear, we can adjust the mirror to compensate for atmospheric distortion. Other objects nearby in the sky also appear clearer. Another new technique uses laser beams shined up in the sky to make artificial bright objects anywhere we need to point the telescope.

Q: Could there be life in an ocean beneath Europa’s icy surface?

A: Jupiter’s moon Europa has been observed by the Voyager spacecraft and now, in more detail, by the Galileo spacecraft in orbit around Jupiter. Based on the observations of these spacecraft, scientists think that there is an ocean beneath Europa’s surface. Whether life exists in that ocean is a more difficult question.

Europa is similar in size to our Moon. Cracks in its surface look like cracks in eggshells, and some of Galileo’s close-up views show areas that look like ice floes, refrozen in a jumbled manner. The surface may be cracked because tides caused by other moons of Jupiter push and pull, flexing Europa. We don’t know how thick the ice might be—perhaps only 10 km (6 mi) or perhaps as thick as 100 km (60 mi).

New evidence from Galileo concerning the strength and direction of Europa’s magnetic field indicates even more strongly than does the presence of jumbled surface ice that a salty ocean lies beneath. Some observations in the infrared part of the spectrum indicate that there may be salt deposits on the ice.

But could there be life? We just don’t know whether life can arise by itself there or anywhere else. There are those who believe that life arises whenever the right chemical mix and energy input occurs. Others believe that life has come from afar—the panspermia theory—and that we have been seeded from other solar systems. If so, Jupiter’s moons could be included. Or it could be that life in our solar system began on Earth or even on Mars and spread to the other inner planets as well as to Europa. We will have to wait for on-site exploration of Europa to know. NASA is in the early stages of planning a Europa lander that will have the ability to drill through the ice.

Q: Is there life on Mars?

A: This question is one of the most interesting of all time. We are planning to send more spacecraft to Mars to find out. NASA’s Viking spacecraft reached Mars in 1976, landed on the surface, and carried out experiments to search for life. All the results were negative.

Mars Global Surveyor, another NASA spacecraft, is now in orbit around Mars and is sending back incredibly detailed pictures. The pictures show many signs that water has flowed on Mars. With the older, less detailed images, scientists thought they saw signs of streambeds with tributaries, indicating that rivers flowed on Mars. The latest images verify that the water that created these features is gone, but we don’t know how recently the latest features were formed.

Based on the knowledge of Mars’s composition gained from spacecraft and landers, scientists conclude that some meteorites found on Earth actually originated on Mars. Large asteroid impacts may have blasted these rocks into space, and the rocks then eventually fell to Earth. Detailed examination of one of these Mars meteorites under powerful microscopes shows tiny structures that some scientists think are fossils of primitive life on Mars. Most scientists disagree with those conclusions and think the shapes formed naturally, but debate continues.

NASA plans to launch a pair of automated rovers to Mars in 2003. These rovers will land on the surface by first using a parachute to slow their descent, then bouncing in giant airbags, much as the Mars Pathfinder did in 1997. They will be able to roam much farther across the surface of Mars than Pathfinder’s rover, Sojourner, did. The United Kingdom is also launching a spacecraft to Mars in 2003, carrying the Beagle 2 rover. Beagle 2 will include a laboratory to search for life.

Q: Is automated spaceflight more efficient and economical than manned spaceflight?

A: That is the conclusion that NASA has reached, and as computer controls and robotic systems get even more sophisticated, automated spaceflight is where we seem to be heading. Since the Apollo missions sent 12 people to the Moon’s surface from 1969 to 1972, we haven’t sent anybody out beyond Earth’s orbit. The missions to the planets—such as the Vikings to Mars, the Voyagers to Jupiter, Saturn, Uranus, and Neptune, and more recently Galileo to Jupiter and Cassini to Saturn—have all been robotic. Most scientists think they are more efficient and economical.

So what is the role of people in space? It is very controversial if people are really necessary or if they are merely inspirational. NASA is now spending incredible sums of money on the International Space Station, and most scientists think the benefits to be gained are not worth the expenditure. But if a rising tide raises all boats, then the space station’s role in keeping NASA’s budget up may indirectly benefit science.

Q: Why is there more matter than antimatter in the universe?

A: A Nobel Prize-winning experiment by James Cronin of the University of Chicago and Val Fitch of Princeton University showed that a certain unusual type of subatomic particle decayed slightly more often in one way than another. Scientists think that about 13 billion years ago, before the universe was one second old, this uneven decay caused a slight imbalance between the number of particles of matter and the number of particles of antimatter. Most of the matter and antimatter particles hit each other and annihilated, destroying each other in a burst of energy. They made photons of energy, similar to light. The remaining small excess of matter makes up all of the matter in the universe today. This process explains why there are 100 million times more photons in the universe than there are particles of matter: Due to uneven decay there were originally 100,000,001 particles of matter for each 100,000,000 particles of antimatter.

The type of imbalance found by Cronin and Fitch is even more obvious in a type of interaction involving a subatomic particle known as a B meson. Particle physicists (physicists who use atom smashers to study subatomic particles) are building machines that will create billions of these B mesons to study. The machines are known as B factories, because of the large number of identical B mesons they will create. The results of these experiments should soon tell us more about the imbalance between matter and antimatter and its origins in the earliest moments of the universe.

Q: Why does the Moon appear larger when it is close to the horizon?

A: The effect is psychological and does not relate to the actual size of the Moon. Apparently, as verified in an experiment widely reported in 2000, people misjudge the distance to objects when the objects are near the horizon. The human brain interprets the Moon to be relatively far and therefore large in angle when the Moon is near the horizon. Recently I was driving in a car on a winding road in which foreground trees sometimes appeared close to the bottom of the Moon in the sky and sometimes were not visible. I noticed the size of the Moon apparently changing from large to small and back again as I drove.

The traditional method of disrupting your brain’s perception of the Moon’s size is to stand with your back to the Moon, and then bend over, looking upside down at the Moon through your legs. You will then no longer see the “moon illusion” of a large lunar disk.

Q: Why does the Moon have more impact craters than Earth?

A: The Moon now has more impact craters than Earth for two reasons. First, Earth never had as many, and second, most of the craters Earth once had have been eroded away.

Unlike the Moon, Earth has a dense atmosphere, so most meteoroids coming at Earth burn up in Earth’s atmosphere. Only the larger ones survive.

Most craters on the Moon apparently formed between 3 and 4 billion years ago. The Moon has been geologically inactive for billions of years, however, so any craters that formed there are still present. The craters that formed on Earth by whatever meteorites did survive their passage through our atmosphere have long since been eroded by a variety of terrestrial processes.

Several craters remain on Earth, but they are large ones. The Meteor Crater in northern Arizona is the most famous and was the first to be recognized. Chicxulub in Mexico’s Yucatan Peninsula is hidden under ground and water but is now being revealed. Chicxulub is thought to be the crater caused by the meteorite that some scientists believe resulted in the mass extinction of the dinosaurs and many other species about 65 million years ago.

Q: Why did the Moon appear larger in the sky in the distant past?

A: Several thousand years ago, before human history, the Moon was close enough to Earth that it would have appeared noticeably larger than it does now. As a result of tidal forces and the conservation of angular momentum, the Moon is gradually receding from Earth by about 5 cm (2 in) per year. However, the effect is difficult to measure because the Moon changes in apparent size (as viewed from Earth) throughout the month as a result of its elliptical orbit.

The tidal bulge goes around faster in the Earth-Moon system than it would were the Moon’s gravity alone making it revolve, because the Earth is rotating. That tidal bulge pulls on the Moon, speeding up its revolution around Earth and making it move farther from Earth.

With the Moon farther out, its angular momentum is increased. Because angular momentum is conserved (according to Newton’s Third Law), the Earth’s angular momentum has to decrease to compensate.

Q: Why can we see only one side of the Moon from Earth?

A: The Moon’s rotation is locked to its orbit around Earth by a bulge of material under the Moon’s surface. This material is denser than the rest of the Moon’s surface material, and Earth’s gravity pulls on it especially strongly. The side of the Moon containing the bulge always faces Earth. We call it the near side and we call the other side, which we never see from Earth, the far side. People often mistakenly use the term “dark side” when referring to the far side of the Moon, forgetting that the far side is fully illuminated by the Sun whenever we see a new moon from Earth.

If we were to look from high above the Earth-Moon system, we would see the Moon rotate once per month with respect to the stars. This is its sidereal rotation period and lasts 27 1/3 days. The Moon revolves around Earth in the same amount of time.

The Moon’s elliptical orbit, combined with other factors, gives the Moon a slightly irregular velocity as it revolves around Earth, so we sometimes see a bit around the Moon’s edge to one side or the other. These “librations” allow us to see about five-eighths of the Moon’s surface from Earth.

Q: How were the sages of India able to accurately calculate eclipses and even the presence of nine planets without the help of the sophisticated equipment we have today? I have heard other nonindustrialized societies such as the Dogon of Mali were also excellent astronomers.

A: There are periodicities in the recurrence of eclipses that allowed scientists as far back as 2,000 years ago to predict them, or at least to predict the seasons when they could occur. The best known of these periodicities is the “saros,” named by Halley around 1700.

Certainly the ancient Indian sages did not know about nine planets. Indeed, even today we are debating whether Pluto is a planet, so any nontelescopic information about the number of planets past Saturn was not based on reality.

The Jantar Mantar in Jaipur, as well as several other observatories constructed in the 18th century by the Maharaja Jai Singh, have giant instruments that measure the positions of objects in the sky and give time on giant sundials. But the emissaries the Maharaja sent to the Western world were misled by the people they consulted, and they did not bring back news of the telescope, even though it had been invented and Jai Singh would surely have had one built, due to his great interest in modern science.

There was a flurry of interest about the astronomical knowledge of the Dogon of Mali some years ago, but the report they had strangely acquired modern astronomical knowledge turned out to be false. The interest originally dealt with a report that they knew Sirius had a companion, but it turns out contact with travelers in the late 19th century could easily have brought knowledge of that widely reported telescopic discovery.

Q: How are astronomers who view massive, distant objects able to discriminate between redshift components that are due to recessional velocities versus those due to gravitational redshift? Errors in such discrimination may make massive objects seem more distant than they really are, or may make distant objects (strongly redshifted) appear more massive than they really are.

A: Redshift is a change, or shift, in the light radiated by an object, such as a star or galaxy, that indicates the object’s motion. Scientists have used redshifts to measure the velocities (speed and direction) of distant galaxies.

The question you ask was important at the time of the discovery of quasars in 1963. Redshifts of over 13 percent were found in the spectra of objects, and it seemed incredible they could be receding at such a pace. If Hubble’s law—which links redshift and distance—were accepted, the quasars would be the farthest things in the universe. They would have had to be radiating at prodigious rates, given their tremendous distances. No obvious way was known that such huge amounts of radiation could be produced in the volume of quasar emission, which was known to be small because of the speed over which it varied.

Gravitational redshift thus seemed to be a viable alternative. But over the years, the distances of quasars have been verified to be at their Hubble-law (“cosmological”) distances, as direct methods of finding distances have reached farther out into the universe. The linear progression of distances with redshift has been verified for galaxies, and quasars are sometimes found in association with distant galaxies.

The “fuzz” around quasars was eventually resolved conclusively, most recently with the Hubble Space Telescope. On the gravitational redshift model, the amount of redshift should diminish with distance from the quasar; but observationally, it does not. So the gravitational redshift model for quasar redshifts is no longer viable.

Indeed, with the theory of energy production from matter raining down into black holes from their accretion disks, the problem of producing so much energy in a small volume is no longer thought to be a limit.

Hubble’s law is now very well established, most recently from the Key Project on the Cosmic Distance Scale carried out by Wendy Freedman of the Carnegie Observatories and her colleagues over the last decade. (The final results were released in May 2001.) The scientists determined the distances to galaxies using Cepheid variable stars, which Hubble can see farther out than ground-based telescopes can. Secondary distance methods then carried the measurements farther out.

Not only has gravitational redshift been ruled out for producing redshifts of galaxies and quasars but the inverse Compton effect has also been ruled out. Joseph Silk and I, both then graduate students at Harvard, found that such a method of producing redshifts would produce blurring that has not been found.

Q: Why is Earth’s rotation slowing down?

A: In an isolated system, the amount of rotation or revolution, known as angular momentum, doesn’t change. The angular momentum of a rotating object depends on its speed of rotation, its mass, and the distance of the mass from the axis. Thus when an ice skater draws his or her arms in, thereby bringing the distribution of mass closer to the center, the skater spins faster. The amount of angular momentum is conserved.

In the Earth-Moon system, the tidal bulge goes around faster than the Moon’s gravity would make it revolve, because the Earth is rotating. That tidal bulge pulls on the Moon, speeding up its revolution around Earth and making it go farther out. This is due to Newton’s third law of motion, which states that an object experiences a force because it is interacting with some other object. Thus, even the weaker object exerts some force over the stronger one.

With the Moon farther out, its angular momentum is increased. Because the Moon’s angular momentum is conserved, Earth’s angular momentum has to decrease to compensate. Thus Earth’s rotation slows. To slow the rotation, the water at the points of high tide are pulled back by the Moon’s gravity and run into the continents that are rotating forward.

The speed of Earth’s rotation is best measured by studying historical records of eclipses. A total solar eclipse is so dramatic that even written descriptions from thousands of years ago can be interpreted to show whether a total eclipse occurred at the writer’s location. A slowing Earth shifts the positions affected by a total solar eclipse on Earth’s surface. As a hypothetical example, calculations based on Earth’s current rate of rotation might predict that people in ancient Rome would have experienced a total eclipse, when in fact historical records show that the Sun was completely eclipsed over Alexandria, Egypt, instead.

The day is getting longer by about two milliseconds per century, although shifting structures on and in the Earth make this rate vary slightly. Hundreds of millions of years ago, Earth was rotating about four times faster than it is now, making the day about six hours long.

Q: Are SETI (Search for Extraterrestrial Intelligence) projects likely to yield important discoveries?

A: SETI has already yielded an important discovery: There aren’t strong, easy-to-detect signals from other civilizations bombarding the Earth. We wouldn’t know that if we hadn’t searched.

Project Ozma began the search for extraterrestrial civilizations in 1960, using a radio telescope at the National Radio Astronomy Observatory. Now a significant number of astronomers are looking hard for signals from such civilizations, mainly using radio telescopes. The SETI Institute in California is a major leader in the search, especially since the U.S. Congress shut off public funding for such investigations. The SETI Institute has raised private money to continue their efforts.

The SETI Institute collects radio signals from afar and studies them with computers to search for intelligent signals. Another group of scientists, at the University of California at Berkeley, invented the idea of SETI@home, where data are sent out to home computers of people all over the world. These home computers have lots of time when they aren’t computing, either when people aren’t using them at all or even between keystrokes of word processing. Seti@home puts that downtime to use, analyzing the radio signals for signs of intelligence. These two projects identify many signals of potentially intelligent origin that must subsequently be analyzed.

To me, the chance of finding something through these radio searches seems small. However, since the discovery of a distant civilization would be so important, it is still worth pursuing.

Q: Why haven’t astronomers detected planets similar to Earth’s size circling other stars?

A: One of the more exciting things going on in astronomy is the discovery of planets circling other stars. Astronomers detect the planets by observing the effect of their gravitational pull on the stars they circle. Imagine a pair of dancers swirling around each other in a waltz. Even if one dancer were invisible, you would notice the other dancer spinning around some point in front of herself in a way that would be impossible if she were not balanced by the weight of her invisible partner. Similarly, if we look at a star and see it moving alternately toward us and away from us, we know that some other object must be moving in the opposite directions to compensate.

Over the last few years, several groups of astronomers have discovered more than 50 new planets outside the solar system. But the planets have to be massive for them to pull on the stars we are looking at enough for us to be able to measure the effect. At first only planets more massive than Jupiter, which is 318 times the mass of Earth, were detectable. Astronomers have refined their methods and have discovered some planets even less massive than Saturn, which is 95 times the mass of Earth. As methods are further refined, we can expect to detect even smaller planets.

One future space mission that will help in the search is NASA’s Space Interferometry Mission, but you will have to wait until at least 2006 for that. Even farther in the future is the Terrestrial Planet Finder, but when that goes up in two or three or four decades, it should be able to detect the effect even of minuscule planets like Earth.

Q: Will the Sun die someday?

A: Stars evolve. They start as concentrations of gas that collapse under the force of their own gravity. As they collapse, they gain energy from the collapse and heat up. When the temperature and density become sufficiently high, nuclear reactions begin and the star itself is born. The nuclear reactions generate energy that creates an outward pressure, and for most of a star’s lifetime the outward pressure balances the inward force of gravity. This state of equilibrium is known as the main sequence. The more massive a star, the more rapidly it uses up its nuclear fuel and the shorter its main sequence lifetime. For the Sun, and for other stars of the same mass, this main sequence lifetime is about 10 billion years. The Sun is about halfway through this lifetime.

In about 5 billion years, the Sun will have used up so much of the hydrogen in its core, transmuting it into helium, that its insides will heat up and its outsides will swell. The Sun will become a red giant star, so large that its outer layers will cover the orbits of the inner planets. Eventually, the outer layers will drift off, making a type of gas cloud, called planetary nebulae, around the star. These outer shells will dissipate in 50,000 years or so.

The inside of the Sun will then collapse, until the electrons in it cannot be pressed closer together. The Sun will then be a white dwarf. A white dwarf is a dead star. It will cool gradually for tens of billions of years.

As the Sun ends its lifetime and its outer layers drift off, it will lose some of its mass. The planets will then orbit farther out than they do now. When the Sun becomes a red giant, the planets will be seared, any water will boil off, and all life that remains may well be extinguished (though who knows what we humans can dream up in 5 billion years). The dead planets may well keep orbiting the dead Sun. Indeed, we know of two or possibly three planets orbiting a single neutron star, a star even more compressed than a white dwarf. Since most of the Sun’s mass will remain, the planets can stay in orbit.

Q: Is it true the Sun’s magnetic cycles may be linked to the ice age cycle the Earth seems trapped in?

A: Nobody knows for sure how the Sun’s magnetic cycles are linked to the Earth’s weather or climate. People have found various supposed links over the years, but most of them have proven wrong once more time has passed or more data has come in.

The most suspicious long-term relation was figured out by John Eddy a few decades ago, reviving something noticed by E. Walter Maunder a hundred years ago. Eddy and Maunder noticed there was a period of decades in the 17th century when there did not seem to be any sunspots on the Sun, and that this period coincided with the “Little Ice Age,” when Europe was cooler than average.

Paintings from that period by Pieter Bruegel and Hendrick Avercamp show people ice skating on Dutch canals, which seldom freeze now.

The dominant cause of Earth’s major ice ages is thought to be related to the Milankovitch cycle, which is not related to the Sun’s magnetic cycles. The Milankovitch cycle is the 41,000-year period of the changing tilt of the Earth’s axis with respect to the plane of its orbit around the Sun, and with the 22,000-year precession cycle of the Earth’s axis; as well as variations in how far out of round the Earth’s orbit is. Predictions based on these cycles forecast at least 100,000 years without an ice age in our future.

Q: Why does the Sun appear yellow?

A: When you heat an iron poker in a fire, it first glows red and then may glow blue-white as it gets hotter. Similarly, the coolest stars are red in color, and intermediate-temperature stars give off more of their energy in the yellow part of the spectrum. The hottest stars give off most of their radiation in the blue or even in the ultraviolet parts of the spectrum. The Sun is a star of intermediate temperature so most of its radiation is given off in or near the yellow.

The kind of radiation any gas gives off depends on its temperature. In the 19th century physicist Wilhelm Wien figured out that the temperature of the gas multiplied by the wavelength at which the continuous radiation from a gas is at its highest yields a constant. So the hotter the gas, the higher the temperature, and the shorter the wavelength. Red light has a long wavelength, yellow has an intermediate wavelength, and violet has the shortest wavelength of all visible light.

Even though the Sun’s radiation peaks in the yellow, all the light from the Sun together is called white light. Our eyes compensate a lot for the mix of colors we see. Some of the pictures of the Sun you see make the Sun look yellow because of the filter photographers use to cut off most of the sunlight. They use filters because the Sun is about a million times brighter than the full moon—too bright to look at directly without a special filter.

Q: Why are the other solar systems that astronomers are detecting arranged so differently from our own?

A: This is such a good question that nobody knows the answer. Until recently, astronomers had only studied our solar system, which has small, rocky planets near the Sun and giant, gaseous ones farther away. We thought that the solar wind and the higher temperatures close to the Sun stripped the inner planets of their mantles of hydrogen and helium, while the outer planets were so cold and had so much gravity that they retained these light elements. But in the last few years, astronomers have found dozens of “hot Jupiters,” giant planets around other sunlike stars but with orbits even smaller than that of the planet Mercury.

Current theories hold that these giant planets were indeed formed at distances comparable to that of our own Jupiter but have migrated toward the Sun over long periods of time. Indeed, there had already been theories like that about our own solar system, theories that suggest Jupiter and Saturn had migrated out a bit while Uranus and Neptune had migrated out much farther from the distances at which they were formed. The reason for the migration might be gravitational interaction.

Your question indicates the problems that can arise when we make generalizations based on only one example, as we had been doing with our solar system. Let us hope we aren’t making the same mistake in generalizing about cosmology from observations of the one universe we know.

Q: How did the invention of the reflecting telescope advance astronomical observation?

A: Isaac Newton invented the reflecting telescope because of a mistake he made! He wrongly assumed that the original type of telescope used by Galileo during the early 17th century, the refracting telescope, could only focus light of different colors at different places, blurring the images. So instead of the refracting telescope, which uses a main lens to bring light to a focus, he invented a reflecting telescope, which uses a mirror. Mirrors reflect light of all colors at the same angle. Scientists later discovered ways of using lenses made of different types of glass to bring several different colors to the same focus even in a refracting telescope.

Newton’s new idea was important in allowing scientists to make bigger telescopes than ever before. In 1845 in Ireland the earl of Rosse made a telescope that had a 79-in (200-cm) mirror. His telescope was so sensitive that he discovered, for example, that many nebulae in the sky had spiral shapes. We now know that these are galaxies, giant collections of billions of stars along with gas and dust that are equivalent to our own Milky Way Galaxy. This telescope was the biggest in the world until 1917, when a huge reflecting telescope with a 100-in (250-cm) mirror was opened in an observatory on Mt. Wilson in California. A telescope twice as big across was opened in an observatory on Palomar Mountain in California in 1950. Huge new reflecting telescopes include the twin Keck telescopes on Mauna Kea in Hawaii. Each has a 400-in (1,000-cm) mirror, composed of several smaller mirrors. The largest refracting telescope is the 40-in (100-cm) refractor of the Yerkes Observatory in Wisconsin. So the largest reflectors are ten times bigger across and 100 times bigger in area than the largest refractor. The reflecting telescopes are therefore much more sensitive.

Q: Why are large telescopes often built on mountaintops?

A: At first, telescopes were set up wherever the astronomers were, starting with Galileo in Venice. In the 19th century Charles Piazzi Smyth, the Astronomer Royal for Scotland, made a trip to the Canary Islands and wrote a report on how good the observation conditions were. In the late 19th century, observatories such as the Lick Observatory on Mount Hamilton were set up in California. From these observatories in the high altitudes of mountains, astronomers found clearer, steadier air than at lower levels, as well as more nights of good weather for observing. Steadier air means that stars twinkle less, making them better to observe.

In the 20th century, with the increased outdoor lighting in commercial and residential areas, it became important for astronomers to choose observation sites far from civilization. That’s another reason why astronomers went to mountaintops.

From 1845 to 1917, the largest telescope in the world belonged to the Earl of Rosse, at Birr Castle in central Ireland. This telescope, the Leviathan of Parsontown, had a mirror that was 6 ft (1.83 m) across; it is now on display at the Science Museum in London. But because of the telescope’s location, few nights a year were suitable for observing. When George Ellery Hale opened his observatory north of Los Angeles on Mount Wilson, he found stable air by first conducting solar observation with a 60-in (152.4-cm) reflector in 1904 and then with a 100-in (254-cm) reflector in 1917. Even now, with the sky too bright for observing dark skies because of the growth of Los Angeles, Mount Wilson is still a useful observatory for studying bright objects because of the steady air. The 100-in (254-cm) telescope on Mount Wilson was superseded in 1950 by the 200-in (508-cm) telescope on Palomar Mountain in southern California, also credited to George Ellery Hale.

Starting in the 1970s, a wonderful mountaintop site was developed at Mauna Kea in Hawaii. Its 4,000-m (14,000-ft) height meant there was little water vapor above, making the site good for infrared observations. The twin 400-in (10-m) Keck telescopes, put up in the mid-1990s, are the largest telescopes on Mauna Kea; other telescopes on Mauna Kea include the 320-in (8-m) telescopes of the Japanese National Astronomical Observatory and the Gemini project.

Chile and the Canary Islands also host major contemporary observatory sites. The European Southern Observatory’s Very Large Telescope in Chile consists of four 26-ft (8-m) telescopes and several smaller telescopes. Texas has a large telescope that can point at limited parts of the sky, and a twin model is being constructed in South Africa.

These telescopes are for optical and infrared observations. Conditions for observing different parts of the spectrum vary. Most radio telescopes do not need high-altitude sites and can be built in areas where there is no radio interference.

Telescopes to study X rays in space must be placed on rockets or satellites. X rays do not come through the Earth’s atmosphere, so scientists who want to study X rays given off by objects in space must place telescopes on rockets or satellites. The Chandra X-ray Observatory is the largest X-ray telescope in space.

Q: How does a total eclipse affect the solar winds and radiation hitting the Earth and could this affect the magnetic field of the Earth in any ways that could be adverse?

A: A total eclipse of the Sun is a relatively minor event at the solar-system scale and differs only slightly from a normal monthly new moon. The solar wind goes by the Moon fairly easily and can continue on to the Earth even when a solar eclipse is occurring.

The change in radiation hitting the Earth has more of an effect, at least locally. The region in totality on Earth cools by about 11 Celsius degrees (20 Fahrenheit degrees)—as measured at the June 21, 2001, eclipse in Africa—and this cooling (over a period of 90 minutes or so during the partial phases, with the most extreme cooling occurring abruptly at the onset of totality) can cause a local change in weather, especially in cloud formation.

Fortunately, the eclipse cooling did not affect the completely clear day that blessed most of southern Africa for the June 21 total eclipse.

A total eclipse, like a new moon, has no effect on the Earth’s magnetic field.

Q: If the universe were to contract, would time reverse?

A: We are all familiar with the forward-pointing arrow of time: We can stir milk into coffee but can’t unstir the light-colored coffee to separate it into black coffee and milk. If you watch a movie of a natural process, such as running water, backward you can always eventually tell that it is moving backward rather than forward. What makes time seem always to go forward?

Scientists don’t know what time really is. They think that the second law of thermodynamics, which specifies that the universe tends to become more and more disordered, is fundamental. But that law cannot be proved to always be valid.

Until recently, cosmologists argued over whether the universe will always expand or whether it will eventually contract, due to the gravity of the mass in the universe pulling all the matter back together. Current theories, though, indicate that the universe will expand forever. There are even strong indications that the expansion of the universe is accelerating rather than slowing down. If these observations are verified, then we will know that the universe will never contract.

Theoreticians disagree as to whether the arrow of time would be reversed in a contracting universe. At present, it seems as though we will never have the observational opportunity to find out.

Q: What made the universe expand? What force could overcome all that gravity to set things apart so drastically that the universe is still expanding and there’s no chance of it contracting?

A: Once things start moving, they keep moving unless some force acts on them to stop the motion. So once the big bang started things moving outward, the question became: How much gravity is there to pull things back? As it turns out, the universe doesn’t have too much gravity, because there isn’t that much matter in it! Whatever matter exists is so spread out that it hardly counts. You might think that Earth is massive, but by the time its mass is spread over the empty space in the solar system, and all the other planets and stars and galaxies are also spread out, the cosmic density is only about 10-30 g/cu cm, compared with 1 g/cu cm for water.

One way of finding out the density of our universe is to measure how much deuterium there is. Since all the deuterium was formed in the first three minutes after the big bang, while the universe was expanding rapidly, measuring the amount of deuterium tells us how much gravity was pulling back on the expansion at that crucial stage. It turns out that only about 3 percent of the amount of matter necessary to pull the universe back in on itself is present.

Scientists’ latest measurements indicate that another 30 percent or so of the mass in the universe is in the form of “dark matter,” which we detect only by its gravity. That still leaves another 67 percent—the mass needed to pull the universe back on itself.

Discoveries since 1997 have indicated that the remaining 67 percent is in the form of a “dark energy” that makes the universe expand faster, not slower. Scientists are increasingly accepting the idea that the universe’s expansion is accelerating, not slowing down.

I should also introduce the idea of “inflation,” which is a rapid expansion of the universe in its first tiny fraction of a second after the big bang. As the universe inflated to perhaps 1050 times its original size in the first 10-32 second, the amount of “vacuum energy” in each volume of space remained constant. Thus the inflation produced most of the mass and energy in the universe. After about 10-32 second, the inflationary era ended, and the universe resumed its expansion at a more reasonable rate.

We now measure its expansion rate with Hubble’s law, which says that the constant relationship between distance from Earth and the speed of expansion means the universe is expanding uniformly. The Hubble Space Telescope was designed in large part to measure Hubble’s law, and scientists using it gave their final value in the spring of 2001. For each 3.26 million light-years of distance to us, galaxies and clusters of galaxies recede by an additional 72 km/second.

Q: When does something enter orbit? When is something in orbit?

A: Isaac Newton, in his System of the World (published in 1726), drew a series of paths for objects projected forward from a high mountain. He showed that the objects fell to Earth in parabolic arcs, as long as their paths went no more than halfway around the Earth. But once a projectile passed halfway around, it continued forward toward its starting point rather than falling to Earth on its other hemisphere. The object is said to be in orbit.

Something is in orbit around another body when it is held in a fixed path by gravity. Gravity keeps pulling the orbiting object toward the center of the main object. Let us say that the orbiting object is a spacecraft and the main object is the Earth. If the curvature of the Earth is such that the Earth curves away at the rate the spacecraft falls, then the spacecraft always remains at the same distance from the center of the Earth. It is thus in free fall and stays in its orbit. To be in orbit, it must have exactly the proper speed to maintain its distance from Earth. That orbital velocity is about 28,967 km/h (18,000 mph) for an object orbiting within a few hundred miles of Earth’s surface.

Johannes Kepler found three laws governing orbits, which he published in 1609 and 1618. The first law shows the orbit is an ellipse with the central object at one of the ellipse’s foci. The second law describes how the speed of the object varies in its orbit, depending on its distance from the central object. The third law links the period of the orbit (the time it takes to orbit the object once) with its size.

From Kepler’s third law, one can see how an object orbits with a longer period the farther it is from Earth. At about six Earth radii, an object orbits in 24 hours instead of the 90 minutes of an object within a few hundred miles of Earth’s surface. It thus appears to hover overhead since a point on the equator below it is rotating at the same speed that the spacecraft is orbiting. The geosynchronous satellites that relay television signals are in such high orbits.

Q: Do wormholes—shortcuts through time and space—exist?

A: Isaac Newton in 1687 advanced the law of gravity, describing the strength of the force that pulls any two masses together. Albert Einstein in 1916, in his general theory of relativity, explained gravity as a warping of space. An analogy would be to put a heavy weight in the middle of your bed, causing the mattress to sag. If you then rolled a ball across the bed, its path would curve when it entered the sagging part of the mattress. You might think that the weight was pulling the ball to it by gravity. This example shows a two-dimensional surface curving into an extra dimension. Similarly, scientists describe the universe as a four-dimensional space-time, and Einstein’s theory shows how a massive object causes it to curve.

If the weight were extremely heavy, the dent in the mattress would be very deep. A black hole is a curvature of space so extreme that nothing, not even light, can escape. Some scientists have proposed that the opposite also exists: a white hole, out of which matter comes. The question then is whether the black hole and the white hole can be connected, so that matter that goes into the black hole comes out of the white hole. We call this link a wormhole. Once you enter the black hole you are pulled inward and are eventually crushed. Wormholes can, in principle, connect two regions of space without the extreme warp of a black hole.

In the 1980s Carl Sagan, in writing his novel Contact (later made into a major motion picture of the same name), consulted the Caltech theoretician Kip Thorne to see if a wormhole could transport his heroine from one place in the universe to another. Physicists had previously calculated that the throat connecting the two ends of a wormhole would pinch off too quickly for anything to pass through. But following Sagan’s question, Thorne and a student figured out that a theoretical kind of matter, which they call “exotic matter,” might be able to keep the throat open. This type of matter cannot exist in our current understanding of the constituents of the universe, but new laws of physics may be discovered that permit it. When matter is very compressed, for example, we would need a quantum theory to explain gravity, and no such theory yet exists.

So no one knows yet whether wormholes can exist. They can’t in our current understanding of the universe, but we still have a lot to learn. In 2000 Thorne stated, “There is growing evidence, but nothing at all firm, that the laws of quantum field theory in curved space-time may prevent the existence of the kind of stress-energy required to hold open a macroscopic wormhole,” in which case, they can’t exist.

Study of Other Emissions

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Sometimes astronomers study emissions from space that are not electromagnetic radiation. Some of the particles of interest to astronomers are neutrinos, cosmic rays, and gravitational waves. Neutrinos are tiny particles with no electric charge and very little or no mass. The Sun and supernovas emit neutrinos. Most neutrino telescopes consist of huge underground tanks of liquid. These tanks capture a few of the many neutrinos that strike them, while the vast majority of neutrinos pass right through the tanks.

Cosmic rays are electrically charged particles that come to Earth from outer space at almost the speed of light. They are made up of negatively charged particles called electrons and positively charged nuclei of atoms. Astronomers do not know where most cosmic rays come from, but they use cosmic-ray detectors to study the particles. Cosmic-ray detectors are usually grids of wires that produce an electrical signal when a cosmic ray passes close to them.

Gravitational waves are a predicted consequence of the general theory of relativity developed by German-born American physicist Albert Einstein. Since the 1960s astronomers have been building detectors for gravitational waves. Older gravitational-wave detectors were huge instruments that surrounded a carefully measured and positioned massive object suspended from the top of the instrument. Lasers trained on the object were designed to measure the object’s movement, which theoretically would occur when a gravitational wave hit the object. At the end of the 20th century, these instruments had picked up no gravitational waves. Gravitational waves should be very weak, and the instruments were probably not yet sensitive enough to register them. In the 1970s and 1980s American physicists Joseph Taylor and Russell Hulse observed indirect evidence of gravitational waves by studying systems of double pulsars. A new generation of gravitational-wave detectors, developed in the 1990s, uses interferometers to measure distortions of space that would be caused by passing gravitational waves.

Some objects emit radiation more strongly in one wavelength than in another, but a set of data across the entire spectrum of electromagnetic radiation is much more useful than observations in any one wavelength. For example, the supernova remnant known as the Crab Nebula has been observed in every part of the spectrum, and astronomers have used all the discoveries together to make a complete picture of how the Crab Nebula is evolving.

Radio Astronomy

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Radio waves have the longest wavelengths. Radio astronomers use giant dish antennas to collect and focus signals in the radio part of the spectrum (see Radio Astronomy). These celestial radio signals, often from hot bodies in space or from objects with strong magnetic fields, come through Earth's atmosphere to the ground. Radio waves penetrate dust clouds, allowing astronomers to see into the center of our galaxy and into the cocoons of dust that surround forming stars.

Infrared Astronomy

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Infrared astronomers study parts of the infrared spectrum, which consists of electromagnetic waves with wavelengths ranging from just longer than visible light to 1,000 times longer than visible light. Earth’s atmosphere absorbs infrared radiation, so astronomers must collect infrared radiation from places where the atmosphere is very thin, or from above the atmosphere. Observatories for these wavelengths are located on certain high mountaintops or in space (see Infrared Astronomy). Most infrared wavelengths can be observed only from space. Every warm object emits some infrared radiation. Infrared astronomy is useful because objects that are not hot enough to emit visible or ultraviolet radiation may still emit infrared radiation. Infrared radiation also passes through interstellar and intergalactic gas and dust more easily than radiation with shorter wavelengths.

Further, the brightest part of the spectrum from the farthest galaxies in the universe is shifted into the infrared. The Next Generation Space Telescope, which NASA plans to launch in 2006, will operate especially in the infrared.

Ultraviolet Astronomy

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Ultraviolet light has wavelengths longer than X rays, but shorter than visible light. Ultraviolet telescopes are similar to visible-light telescopes in the way they gather light, but the atmosphere blocks most ultraviolet radiation. Most ultraviolet observations, therefore, must also take place in space. Most of the instruments on the Hubble Space Telescope (HST) are sensitive to ultraviolet radiation (see Ultraviolet Astronomy). Humans cannot see ultraviolet radiation, but astronomers can create visual images from ultraviolet light by assigning particular colors or shades to different intensities of radiation.

Gamma-Ray and X-Ray Astronomy

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Gamma rays have the shortest wavelengths. Special telescopes in orbit around Earth, such as the National Aeronautics and Space Administration’s (NASA’s) Compton Gamma-Ray Observatory, gather gamma rays before Earth’s atmosphere absorbs them. X rays, the next shortest wavelengths, also must be observed from space. NASA’s Chandra X-Ray Observatory (CXO) is a school-bus-sized spacecraft scheduled to begin studying X rays from orbit in 1999. It is designed to make high-resolution images.

Optical Astronomy

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Until the 20th century, all observational astronomers studied the visible light that astronomical objects emit. Such astronomers are called optical astronomers, because they observe the same part of the electromagnetic spectrum that the human eye sees. Optical astronomers use telescopes and imaging equipment to study light from objects. Professional astronomers today hardly ever actually look through telescopes. Instead, a telescope sends an object’s light to a photographic plate or to an electronic light-sensitive computer chip called a charge-coupled device, or CCD. CCDs are about 50 times more sensitive than film, so today's astronomers can record in a minute an image that would have taken about an hour to record on film.

Telescopes may use either lenses or mirrors to gather visible light, permitting direct observation or photographic recording of distant objects. Those that use lenses are called refracting telescopes, since they use the property of refraction, or bending, of light (see Optics: Reflection and Refraction). The largest refracting telescope is the 40-in (1-m) telescope at the Yerkes Observatory in Williams Bay, Wisconsin, founded in the late 19th century. Lenses bend different colors of light by different amounts, so different colors focus slightly differently. Images produced by large lenses can be tinged with color, often limiting the observations to those made through filters. Filters limit the image to one color of light, so the lens bends all of the light in the image the same amount and makes the image more accurate than an image that includes all colors of light. Also, because light must pass through lenses, lenses can only be supported at the very edges. Large, heavy lenses are so thick that all the large telescopes in current use are made with other techniques.

The Hubble Space Telescope (HST), a reflecting telescope that orbits Earth, has returned the clearest images of any optical telescope. The main mirror of the HST is only 94 in (2.4 m) across, far smaller than that of the largest ground-based reflecting telescopes. Turbulence in the atmosphere makes observing objects as clearly as the HST can see impossible for ground-based telescopes. HST images of visible light are about five times finer than any produced by ground-based telescopes. Giant telescopes on Earth, however, collect much more light than the HST can. Examples of such giant telescopes include the twin 32-ft (10-m) Keck telescopes in Hawaii and the four 26-ft (8-m) telescopes in the Very Large Telescope array in the Atacama Desert in northern Chile (the nearest city is Antofagasta, Chile). Often astronomers use space- and ground-based telescopes in conjunction. See also Space Telescope.

Astronomers usually share telescopes. Many institutions with large telescopes accept applications from any astronomer who wishes to use the instruments, though others have limited sets of eligible applicants. The institution then divides the available time among successful applicants and assigns each astronomer an observing period. Astronomers can collect data from telescopes remotely. Data from Earth-based telescopes can be sent electronically over computer networks. Data from space-based telescopes reach Earth through radio waves collected by antennas on the ground.

Astronomy, History of

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Astronomy, History of, history of the science that deals with all the celestial bodies in the universe. Astronomy includes the study of planets and their satellites, comets and meteors, stars and interstellar matter, star systems known as galaxies, and clusters of galaxies. The field of astronomy
has developed from simple observations about the movement of the Sun and Moon into sophisticated theories about the nature of the universe. See also Astronomy.

How astronomers work

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Professional astronomers usually have access to powerful telescopes, detectors, and computers. Most work in astronomy includes three parts, or phases. Astronomers first observe astronomical objects by guiding telescopes and instruments to collect the appropriate information. Astronomers then analyze the images and data. After the analysis, they compare their results with existing theories to determine whether their observations match with what theories predict, or whether the theories can be improved. Some astronomers work solely on observation and analysis, and some work solely on developing new theories.

Seven Wonders of Modern Astronomy

By George Musser

Choosing only seven wonders out of the myriad accomplishments of modern astronomy is an impossible task—just the sort I like. The mere attempt encourages a tour of the golden age of astronomy in which we are now living, a time of big questions and proportionately big efforts to answer them.

For many people, astronomy sounds like a quaint science—they imagine a recluse perched on a mountain, quietly pondering the inky skies. To a large extent it is indeed a battle of the solitary mind with the almighty heavens. But sky-watching was also the first Big Science. Nineteenth-century astronomers wielded huge budgets, commanded armies of peons and reigned over megafacilities at a time when physicists' labs were simple affairs, just some magnets and oil droplets. And the tradition extends even further back: consider the great observatories of Jaipur and Delhi, the sky temples of the Maya, Stonehenge.

Nowadays the term "Big Science" is generally reserved for particle accelerators and genome projects. Yet astronomy still qualifies, even if you leave aside planetary exploration, a subject perhaps better thought of as an offshoot of geology. A major observatory is like a factory, filled with pallets of equipment, DANGER signs, gangways, metal ladders, bustling workers and the buzzing of great machinery—all to catch a sliver of light from the dawn of time.

The wonders are many; any big adventure is really a succession of small victories. Another list might focus on the cosmic marvels themselves, but those already tend to get the attention. Some lists concentrate on the technological breakthroughs, whose size is often in inverse proportion to their importance: for instance, charge-coupled-device (CCD) microchips, the exquisitely sensitive detectors that have supplanted photographic film in observatories big and small over the past decade. Or a list might preview the mindblowers soon to come: the plans to detect new forms of radiation, say, or to see the continents and oceans of a distant planet. But here I present my own idiosyncratic selection of seven noteworthy telescopes now in operation or just gearing up.

THE SHARPEST

What would a list of astronomical wonders be without Hubble [Space Telescope]? The space telescope, after all, has broken all kinds of records, including probably the most newspaper headlines produced by any single astronomical project. Although its 2.4-meter (94-inch) mirror is a runt by today's standards, Hubble and its ilk are still the most complex robotic spacecraft ever built. One reason is the tracking mechanism. Above the madding clouds and turbulent distortion of Earth's atmosphere, the optics can attain its theoretical limit of resolution, but only so long as the spacecraft remains rock-steady despite the orbital motion and various buffeting forces. Hubble effects this stability using an interlinked system of mini-telescopes and flywheels.

Nine years ago, however, Hubble would have been placed on the list of projects that never made it—a victim of bureaucratic mismanagement, space program politics and technical snafus. Most infamously, the space telescope became a $1.6-billion example of the difference between accuracy and precision: because of a faulty measuring device, its mirror had been sculpted with utmost care to the wrong shape. But since astronauts fixed it in a dramatic series of space walks six years ago, even seasoned researchers have seen the universe in a new light. The gleam of comet crashes, the dainty arcs of gravitational lenses, the stellar corpses that look uncannily like eyeballs or sperm—Hubble is the Ansel Adams of our age.

THE BIGGEST

The nicest thing about the Very Large Telescope (VLT) is the charm of its lyrical names. Its four constituent telescopes were recently rechristened Antu, Kueyen, Melipal and Yepun—which mean the sun, moon, Southern Cross and Sirius in the indigenous Mapuche language of Chile. It is something of an improvement on Unit 1, Unit 2, Unit 3 and Unit 4.

Each of those 8.2-meter instruments is itself a very large telescope. Ten years ago such devices were impossible, but since then engineers have developed various ways to fabricate and support their huge, unwieldy mirrors. The European Southern Observatory—the consortium that built the VLT in northern Chile for $500 million—decided on single pieces of glass just 18 centimeters (seven inches) thick. Too thin to maintain their shape on their own, they are each propped up by 150 pistons, which are readjusted whenever the telescope shifts to a new position.

What justifies the "V" in VLT, however, is the way the individual scopes will work in unison to achieve the resolving power of a whopping 200-meter device. Beginning in 2002, their light will be funneled into a central lab and merged in a technique known as interferometry. Although the technique has long been used in radio astronomy, its arrival in optical astronomy awaited two recent developments. First, laser rangers can now gauge distances to one part in a billion, the precision needed to align and merge the shorter wavelengths of visible light. Second, new adaptive optics—in the VLT's case, a small extra mirror fine-tuned 100 times a second—can correct for atmospheric distortion so that the interferometer won't merely take a sharper picture of a blurred star. Similar interferometers should even be able to detect minute disturbances in the fabric of space itself, such as might occur during the birth of a black hole.

THE FARTHEST FLUNG

In high school physics, my favorite lab exercise—to be honest, the only thing I remember at all—was the wave tank. For one of the experiments, we had to send a wave of water toward a barrier with two gaps. Two pieces of the wave squeezed through the gaps and then blended into a distinctive pattern. Little did I know at the time that such patterns would make possible a radio telescope bigger than planet Earth.

A telescope, too, is a gap in a barrier. It only lets through part of a wave of light; the rest gets chopped off at the edge. An observer notices this chopping as a slight smearing of the image. The larger the scope is relative to the wavelength, the less the smearing. Because radio astronomers deal with wavelengths measured in centimeters or meters, rather than in millionths or billionths of a meter, they suffer from such smearing more than their optical colleagues do.

So in the late 1940s they decided to punch another hole in the barrier. That is, they built two dishes and blended their outputs—two pieces of the same wave from a cosmic source. From the resulting pattern, they could calculate what the unsmeared light must look like. It was as though they had constructed two segments of a single telescope equal in size to the separation between the dishes.

Researchers have now taken this technique of interferometry to an extreme. Six years ago the National Radio Astronomy Observatory opened the $85-million Very Long Baseline Array: 10 radio dishes scattered from Hawaii to the U.S. Virgin Islands. Collectively they act as a single telescope more than 8,000 kilometers across. Astronomers record the signals—along with the exact time as measured by an atomic clock—and later merge them computationally. When they also mix in signals from a new Japanese radio satellite, the effective size swells to over 20,000 kilometers. For short radio wavelengths, the system produces sharper images than even the Hubble does. In fact, it is so sensitive that continental drift shows up in some of its observations.

THE MOST EXTENSIVE

Here's a subversive thought: Instead of observing the cosmos piecemeal, pointing your telescope at this galaxy today and that one tomorrow, what if you just took one big picture of the whole sky? Crudely speaking, that is the goal of astronomical sky surveys, such as the Palomar Observatory sky survey in the mid-1950s. Such surveys have not replaced observations of individual celestial bodies; rather they offer a macro view of the heavens, revealing the broad patterns.

Recently astronomers embarked on the most ambitious effort yet: the Sloan Digital Sky Survey. Over the next five years, this $77-million American-Japanese collaboration will scan a quarter of the sky (avoiding the crowded Milky Way) out to a distance of 1.5 billion light-years from Earth. The researchers expect to tabulate 100 million stars, one million galaxies and 100,000 quasars. They say that if the completed data set were to be printed and bound in books by someone who had little concern for the world's trees, it would nearly fill the Library of Congress.

The survey utilizes a 2.5-meter telescope on Apache Point in southern New Mexico, specially designed to capture as much of the sky as possible at a time. The light alternately feeds one of two instruments. The first is said to be the most complex camera ever built: 54 CCDs that take images in green and red light as well as in ultraviolet and near-infrared. The second is a pair of spectrographs, fed by a forest of optical fibers so that they can analyze the light of more than 600 objects in one go.

Sloan is expected to answer a key question in cosmology: How far do you need to zoom out before the matter in the universe, which on smaller scales is blatantly clumped into planets, stars and galaxies, begins to arrange itself uniformly? By determining where this transition occurs, Sloan could help resolve the age-old debate over the fate of the universe: Will it end in fire or ice, or something else?

THE SWIFTEST

Once upon a time pagers were only for doctors. And astronomers. As eternal and unchanging as the night sky might sometimes seem, it is actually filled with flickers and flashes, explosions and eruptions that flare up and fade out in a matter of seconds or hours. To catch these flighty phenomena, scientists have to be standing by at all hours, ready to reposition satellites and swivel telescopes at a moment's notice. In fact, this is one of the areas of astronomy where amateur astronomers, by virtue of their wide fields of view and sheer numbers, have made crucial discoveries.

The latest entrant in the fast lane is ROTSE, the Robotic Optical Transient Search Experiment, in Los Alamos, N.M. Its first incarnation, ROTSE-1, looks like something from the camera bags of the paparazzi: a set of four 200-millimeter telephoto lenses cobbled together on a high-speed mount. The recently installed ROTSE-2 is a pair of half-meter telescopes. Whereas the standard telescope drive relies on precision gears, like a clock, ROTSE-2 uses position encoders and a feedback control loop, like a robot.

As does a sky survey, ROTSE sacrifices sensitivity and incisiveness for speed and sweep. It can capture 1 percent of the sky in a single exposure; during normal operation, it photographs the entire sky twice a night. Whenever an event of interest occurs, ROTSE suspends surveying, swings around and snaps away. In January the instrument proved its mettle. Satellites saw a gamma-ray burst—an intense but ephemeral blast of high-energy radiation—and sent out rough position information via the Internet. Within 10 seconds ROTSE had pinpointed the burst. Never before had astronomers caught such an event in visible light while it was still flaring in gammas.

THE DEADLIEST

It is enveloped in poisonous vapors that can cause progressive kidney and brain damage. It can look only straight up; slewing would create an instant toxic waste dump. In short, a mercury mirror is not for everyone. But how else could you build a six-meter telescope for $500,000?

Any swirling liquid naturally assumes a parabolic form, whereas glass requires expensive grinding and hefty supports even to approximate that shape. Over the past two decades astronomers have built several bargain-basement telescopes using mercury, the shiniest element known.

The largest will soon be the Large Zenith Telescope (LZT) near Vancouver. A collaboration among Canadian and French astronomers, the LZT contains 28 liters (30 quarts) of mercury in a large pan that spins at the rate of one rotation every 8.5 seconds. The only real hassle has been the bearing. A mechanical bearing would have been too jerky, and no air bearings of the required size were available commercially, so the team had to design its own. Still, the observatory has cost a hundredth as much as one with a glass mirror.

The restriction on pointing straight up might seem a bit of a disadvantage. But it works just fine for studying representative samples of stars, galaxies and even space junk in Earth orbit. By synchronizing the CCD output rate to Earth's rotation, the telescope can electronically track objects as they move through its field of view. Even the mercury isn't as much trouble as you might think. It oxidizes on contact with air, partially trapping the noxious vapors. To be sure, no one will go near the mirror during normal operation, and the building is sealed to contain any spill.

THE WEIRDEST

Most telescopes look up. This one looks down. Most capture some sort of light. This one seeks an invisible subatomic particle. Most telescopes are in remote locations, but this one goes to extremes: it is buried under more than a mile of ice at the South Pole.

The Antarctic Muon and Neutrino Detector Array (AMANDA) is the world's largest detector of the mysterious neutrino—and the first that can claim to be an astronomical instrument rather than a physics experiment. It trades sensitivity for the sheer size needed to catch a meaningful number of high-energy neutrinos from distant objects, which include many of the violent felons on astronomers' most wanted list: the swirling gas around black holes, the innards of stellar explosions, the decomposition of the unidentified matter that dominates our cosmos.

So far the observatory, a $7-million collaboration among U.S., Belgian, Swedish and German universities, consists of 424 glass orbs, each the size of a basketball. They watch for the eerie blue glow indirectly emitted when neutrinos collide with atomic nuclei in the ice or underlying rock. The orbs point downward so that Earth will screen out extraneous particles. To deploy them, workers first used pressurized hot water to melt a column of ice half a meter across and 2,400 meters deep. Then they lowered in the orbs, strung on a cable like beads on a necklace, and let them freeze in place. Ultimately, scientists want 5,000 orbs on 80 cables throughout a cubic kilometer of ice.

It turns out that ice is a friendly place for neutrino detectors. At depth it is crystal-clear, so the orbs can spot flashes of light hundreds of meters away. AMANDA exemplifies a new breed of telescope that has redefined what it means to "see."

George Musser is an editor at Scientific American.

Source: Reprinted with permission. Copyright © 1999 by Scientific American, Inc. All rights reserved.

Astronomy is such a broad topic that astronomers specialize in one or more parts of the field. For example, the study of the solar system is a different area of specialization than the study of stars. Astronomers who study our galaxy, the Milky Way, often use techniques different from those used by astronomers who study distant galaxies. Many planetary astronomers, such as scientists who study Mars, may have geology backgrounds and not consider themselves astronomers at all. Solar astronomers use different telescopes than nighttime astronomers use, because the Sun is so bright. Theoretical astronomers may never use telescopes at all. Instead, these astronomers use existing data or sometimes only previous theoretical results to develop and test theories. An increasing field of astronomy is computational astronomy, in which astronomers use computers to simulate astronomical events. Examples of events for which simulations are useful include the formation of the earliest galaxies of the universe or the explosion of a star to make a supernova.

Astronomers learn about astronomical objects by observing the energy they emit. These objects emit energy in the form of electromagnetic radiation. This radiation travels throughout the universe in the form of waves and can range from gamma rays, which have extremely short wavelengths, to visible light, to radio waves, which are very long. The entire range of these different wavelengths makes up the electromagnetic spectrum.

Astronomers gather different wavelengths of electromagnetic radiation depending on the objects that are being studied. The techniques of astronomy are often very different for studying different wavelengths. Conventional telescopes work only for visible light and the parts of the spectrum near visible light, such as the shortest infrared wavelengths and the longest ultraviolet wavelengths. Earth’s atmosphere complicates studies by absorbing many wavelengths of the electromagnetic spectrum. Gamma-ray astronomy, X-ray astronomy, infrared astronomy, ultraviolet astronomy, radio astronomy, visible-light astronomy, cosmic-ray astronomy, gravitational-wave astronomy, and neutrino astronomy all use different instruments and techniques.

Amateur Astronomy

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Astronomers use tools such as telescopes, cameras, spectrographs, and computers to analyze the light that astronomical objects emit. Amateur astronomers observe the sky as a hobby, while professional astronomers are paid for their research and usually work for large institutions such as colleges, universities, observatories, and government research institutes. Amateur astronomers make valuable observations, but are often limited by lack of access to the powerful and expensive equipment of professional astronomers.

A wide range of astronomical objects is accessible to amateur astronomers. Many solar system objects—such as planets, moons, and comets—are bright enough to be visible through binoculars and small telescopes. Small telescopes are also sufficient to reveal some of the beautiful detail in nebulas—clouds of gas and dust in our galaxy. Many amateur astronomers observe and photograph these objects. The increasing availability of sophisticated electronic instruments and computers over the past few decades has made powerful equipment more affordable and allowed amateur astronomers to expand their observations to much fainter objects. Amateur astronomers sometimes share their observations by posting their photographs on the World Wide Web, a network of information based on connections between computers.

Amateurs often undertake projects that require numerous observations over days, weeks, months, or even years. By searching the sky over a long period of time, amateur astronomers may observe things in the sky that represent sudden change, such as new comets or novas (stars that brighten suddenly). This type of consistent observation is also useful for studying objects that change slowly over time, such as variable stars and double stars. Amateur astronomers observe meteor showers, sunspots, and groupings of planets and the Moon in the sky. They also participate in expeditions to places in which special astronomical events—such as solar eclipses and meteor showers—are most visible. Several organizations, such as the Astronomical League and the American Association of Variable Star Observers, provide meetings and publications through which amateur astronomers can communicate and share their observations.

Astronomy

2009-04-20T22:54:00.000-07:00

Astronomy, study of the universe and the celestial bodies, gas, and dust within it. Astronomy includes observations and theories about the solar system, the stars, the galaxies, and the general structure of space. Astronomy also includes cosmology, the study of the universe and its past and future. People who study astronomy are called astronomers, and they use a wide variety of methods to perform their research. These methods usually involve ideas of physics, so most astronomers are also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas of astronomy also use techniques of chemistry, geology, and biology.

Astronomy is the oldest science, dating back thousands of years to when primitive people noticed objects in the sky overhead and watched the way the objects moved. In ancient Egypt, the first appearance of certain stars each year marked the onset of the seasonal flood, an important event for agriculture. In 17th-century England, astronomy provided methods of keeping track of time that were especially useful for accurate navigation. Astronomy has a long tradition of practical results, such as our current understanding of the stars, day and night, the seasons, and the phases of the Moon. Much of today's research in astronomy does not address immediate practical problems. Instead, it involves basic research to satisfy our curiosity about the universe and the objects in it. One day such knowledge may well be of practical use to humans. See also History of Astronomy.

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2009-04-14T22:25:00.000-07:00

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