How was the univere Born?

Long ago, human beings could only suppose that the universe was what it looked like to them. The Earth seemed to be no more than a round patch of flat ground. The sky seemed to be a dome that came down to meet the ground. The Sun and Moon traveled across the sky.
At night, there were many specks of light in the dome of the sky, and a few of them were brighter than the others and also moved. The ancient Egyptians viewed the sky as the domain of Nut, the sky goddess. According to ancient Greek myths, the god Helios drove a chariot that carried the Sun across the sky.
But later Greeks thought that the Earth was a large sphere at the center of the universe. They thought that the Moon, the Sun, and the planets all circled Earth, and that outside the orbits of these bodies were the sky and the stars.
In a time-exposure of the night sky, star trails seem to wheel across the sky. Small wonder that ancient peoples once thought Earth was the center of the universe. But in 1543, Nicolaus Copernicus, a Polish philosopher, doctor, and astronomer, showed that it made more sense to suppose that the Sun was at the center and that all the planets moved around it.
In the Copernican system, Earth was just one of the planets and went around the Sun, too. Beyond that were the stars, but they weren't attached to the sky. Later, Edmund Halley found out that the stars moved as well. In 1785, William Herschel showed that most of the stars seen from Earth formed a large collection shaped like a lens. We call this collection the Milky Way Galaxy.
If we could view our Milky Way from the outside, it would look like the great galaxy in Andromeda, seen here without the aid of a telescope. The Milky Way is our galaxy. It is 100,000 light-years across. Each light -year is almost 6 trillion miles (9.5 trillion kilometers) long. There are other galaxies as well. The closest large galaxy is the Andromeda Galaxy, which is over 2 million light-years away.
Many other galaxies are scattered through space. There might be a hundred billion in all. In 1842, Christian Doppler explained why anything noisy sounds more shrill when it comes toward you, and sounds deeper when it goes away from you. A similar kind of change, or shift, happens with light. Every star sends out light waves. The light appears bluer if the star is coming toward us, and redder if it is moving away.
When light from a star or galaxy is spread out into a rainbow of color, dark lines show up indicating where light has been absorbed by that star or galaxy. The dark lines in the light of more distant galaxies are shifted toward ever redder light. Astronomers have found that most other galaxies are moving away from us.
The farther away they are, the faster they move away. Quasars are galaxies that have the largest red shifts, so they must be very far away. Even the closest quasars are a billion light-years away. When we look at quasars, we are looking back into a time before our Sun was born. Radio telescopes created this image of a huge gas jet erupting from quasar 3C-273. The jet is a million light-years long.
The universe is a big place. We think of Earth as big, but here we see that Earth is just one of nine worlds orbiting the Sun. The known planets orbit the Sun in a region only about 7 billion miles (11 billion kilometers) in diameter. That's just a little over a thousandth of a light-year. Our Sun itself is just one of 200 billion stars in the Milky Way Galaxy. The nearest star is 4.2 light-years away.
That's thousands of times as far away as the farthest planet in our solar system. The farthest stars in our galaxy are 100,000 light-years away. The Milky Way is but one of many galaxies in our cluster. The Andromeda Galaxy is over 2 million light-years away, but it's our next-door neighbor.
The farthest known quasar is about 12 billion light-years away. In all the universe, there are about 100 billion galaxies, and each galaxy contains about 100 billion stars. Imagine how small our own Earth is in comparison. If we could look at the universe from a great distance and see it all at once, we might think it looked like soap bubbles. Galaxies would be like the soap film making up the bubbles, and the bubbles themselves would be empty and come in different sizes.
The universe is always expanding, growing larger. But suppose we look backward in time. As we go farther and farther back in time, the galaxies move closer and closer together. If we went back in time far enough, all the galaxies would crunch together into a small space. The whole thing must have exploded in a "big bang." The universe is still expanding as a result of that big bang. If we measure how fast the universe is expanding and how long it must have taken to reach its present size, we know that the big bang happened 15 to 20 billion years ago.
Imagine that galaxies are like the raisins in raisin bread dough. The raisins start off fairly close together. If you could stand on one of them as the dough expands, all the other raisins would appear to be moving away from you. As space expands, all the galaxies appear to be moving away from us.
This illustration shows the history of our expanding universe. The bright spot on the left represents the big bang itself. At the time of the big bang, all the matter and energy of the universe was squeezed into one tiny spot. It must have been very hot--trillions of degrees.
But as the universe expanded, it cooled off. There are still hot spots, like the stars, but overall, the universe has become much cooler. The light waves of the vast flash of the big bang stretched and grew longer as the universe cooled. Today, they are very long radio waves.
In 1965, those radio waves were detected. Scientists could hear the last faint whisper of the big bang of long ago. Light travels at a speed of 186,000 miles (300,000 kilometers) a second. If a star is 4.2 light-years from us, like the nearest star, Alpha Centauri (the bluish speck shown below Earth), its light takes 4.2 years to reach us.
Since the Andromeda Galaxy (the spiral shown lower left) is over 2 million light-years from us, its light takes over 2 million years to reach us. The light from the farthest known quasars (upper left) set out 12 to 15 billion years ago. This means that the farther out in space we look, the farther back in time we see.
Stars can explode with incredible violence. When a large star explodes, it becomes a supernova and spreads its material through space. In the big bang, only the simplest atoms, hydrogen and helium, were formed, but supernovas spread more complex atoms. Our Sun formed from a cloud with these more complex atoms. Almost all the atoms of Earth--and in ourselves--were formed in stars that exploded as supernovas long ago.
When a supernova explodes, what is left of it can collapse into a tiny object with gravity so strong that everything falls in, but nothing comes out. This object is called a black hole. There might be a black hole in the center of every galaxy.
The universe may expand forever, or someday its own gravity might slow or even stop it. It might then fall back together in a big crunch. And maybe a new universe will form in a new big bang. Maybe there was a big crunch, or even many big crunches, before the big bang that formed our universe.
We don't know. We're still trying to understand the big bang that created the present universe. That's a big enough puzzle for now.

Rockets, Probes, and Satellites

It could happen in the Americas, Europe, Asia, Australasia, or Africa. In any of these places, a rocket could be propelling a satellite through the Earth's atmosphere and into outer space. But where did rocketry begin? Actually, rockets were invented in the 1200s, long before there was a United States or a Soviet Union.
It was the Chinese who first packed gunpowder into a cardboard cylinder. On the right, you see a thirteenth-century rocket launcher used by the Mongolians; on the left, a seventeenth-century Chinese rocket-arrow launcher. In 1687, an English scientist, Isaac Newton, explained the science of how rockets move. His explanation is known as the law of action and reaction.
A modern rocket is used to illustrate Newton's law. In this rocket, liquid hydrogen and oxygen are sent to the combustion chamber,….where they mix and ignite. The hot gases created by the ignition rush out of the nozzle (action), causing the rocket to move in the opposite irection (reaction).
In the early 1800s, when Francis Scott Key wrote about "the rockets' red glare" in "The Star-Spangled Banner," he meant the rockets that were then sometimes used to carry explosives in war. Later on, by the twentieth century, some scientists had begun to realize that rockets were one way that objects could be pushed through space.
The first to do so in detail, beginning in 1903, was a Russian, K.E. Tsiolkovsky. An American, Robert H. Goddard, continued that work and in 1926 sent up the first rocket of a new kind. Instead of gunpowder, he used gasoline and liquid oxygen.
For the next fifteen years, Goddard kept designing and shooting off better and better rockets. Of course, working on rockets that would be big enough and powerful enough to use in space has never stopped people from using them to carry weapons on Earth. During World War II, the Germans developed rockets big enough and powerful enough to bomb London.
After the war, both the United States and the Soviet Union began to develop large rockets for exploring space. On October 4, 1957, as shown here, the Soviet Union's Sputnik 1 took its place as the Earth's first artificial satellite. On January 31, 1958, the United States launched Explorer 1, and what we call the Space Age had begun. Since 1958, many countries have sent satellites into orbit, including Canada, Japan, and Indonesia.
Such satellites circling the earth can do many kinds of work. Communications satellites receive radio waves from one place, make them stronger, and send them to a completely different place.
Special weather satellites, like this one, take photographs of Earth and send them down in the form of radio waves. When these radio waves are received on Earth, weather people can create the satellite pictures we see on the news each night. For the first time in history, we can see the clouds covering all of the Earth and watch how they move. This makes it much easier to predict the weather.
For example, we can see large, circular cloud formations that make up hurricanes. Before 1960, we couldn't always tell when a hurricane might hit. Now, people can board up their homes and leave before it comes. Countless lives have been saved in this way.
Satellites can also take pictures of Earth itself when clouds aren't in the way. This makes it possible to make very exact maps. This is a shot of Italy's boot. Such pictures can also help tell the condition of forests and croplands. They can pinpoint trouble areas and the spread of plant diseases.
The ocean, and schools of fish in it, can be studied, too. This map was sent back to Earth by a satellite that keeps track of the ozone layer. The central dark violet area over Antarctica shows a deep hole in the ozone that protects Earth from some of the Sun's ultraviolet rays. This warns us to be more careful about the chemicals we put into our atmosphere.
Since 1981, the United States has had space shuttles--rocket ships that can be used over and over again. This is a space shuttle launch at night. Shuttles can carry satellites into space and place these satellites in orbit. Once in orbit, members of the space shuttle crew can repair, rescue, or retrieve damaged satellites.
So satellites tell us many things about Earth. What else can they do? They can move outward and skim by other worlds, or actually land on them. They are then called probes. The nearest other world is Earth's Moon, which is only a quarter-million miles (or 400,000 kilometers) away. In 1959, a lunar probe skimmed by the Moon and sent back pictures.
This was the first time human beings had seen the far side of the Moon, for it is always turned away from Earth. With each lunar shot, the probes came closer and closer to the Moon. This photograph of the far side of the Moon was taken by the Apollo 8 astronauts in December 1968. Finally, on July 20, 1969, a probe piloted by humans landed on the Moon.
Neil Armstrong, here seen in his lunar module on the lunar surface, became the first human being who ever stepped onto another world. No human being has yet gone farther than the Moon, but unpiloted probes have gone much farther.
In 1973 and 1974, a probe called Mariner 10 skimmed by the planet Mercury several times. At one time, it came within 203 miles (327 kilometers) of the surface. It took photographs as it went. Mercury is the planet that is closest to the Sun, and until then, it could only be seen as a tiny circle. The probe showed us most of Mercury's surface in full detail. It looked very much like our Moon, with many craters on it.
Probes began to pass Venus in 1962. People have wondered for years what is beneath that planet's thick cloud cover. This gas balloon, dropped by a Soviet probe, transmitted data about the atmosphere, temperature, and wind movements on the surface of the planet. The thick atmosphere holds in heat so that Venus is very hot--hot enough to melt lead.
During the early 1900s, some astronomers thought they saw canals on the surface of Mars and speculated that intelligent beings might exist there. But beginning in 1965, probes passing near the planet sent back photographs like this one which showed no canals.



Instead, there were canyons, dead volcanoes, craters, and a very thin atmosphere. In 1976, two probes--Viking 1 and Viking 2--landed on Martian soil. This is a working model of the Viking lander, which features a soil sampler extending from the front and two camera "eyes" above it. These Viking probes tested the soil of Mars to see if simple life might exist there. Apparently none does.
This photograph of the Martian landscape was taken by the Viking 1 lander fifteen minutes before sunset. You can see part of the lander in the bottom of the picture. Because Mars is the planet most like Earth, everything we can do to understand it better might help us understand Earth better, too. So much for our neighbors. What lies beyond? Several probes, named Pioneer and Voyager, have gone past Mars to the farthest reaches of our solar system.
Beginning in 1973, they explored the giant planets that circle the Sun at great distances. They skimmed by Jupiter and its large natural satellites, or moons. On Io, Voyager 1 photographed a huge volcanic explosion. In this picture, the spectacular explosion appears just over Io's horizon.
Beyond Jupiter, these probes sent back close-up pictures of Saturn and its magnificent rings. The rings turned out to be full of complicated detail. Beyond Saturn, Voyager 2 has sent back photographs of even more distant Uranus. Aboard Voyager 2, which will continue beyond the solar system on out into our galaxy, is a recording of music and messages from Earth, just in case there are any extraterrestrials who care to listen.
In the future, a probe called Galileo will drop a package containing instruments on Jupiter, and for the first time we will be able to gather information about the inner regions of a giant planet's atmosphere. Closer to home, we are planning to build space stations between Earth and the Moon.
Such stations would give us a base for sending probes and people to other parts of the solar system. Satellites and probes give us eyes deep in space. Will we leave it at that? It's not likely. Many think we are getting closer and closer to the day when we may begin to colonize other worlds.

Science Fiction, Science fact.

For centuries, writers have imagined people traveling to the Moon. In 1638, Francis Godwin published The Man in the Moon, a story about a man whose trained geese go out of control and fly him to the Moon. By the 1800s, however, the Industrial Revolution was underway, and people saw how machines were changing the world. Writers imagined more and more just how our lives might change in years to come.
For example, a French science fiction writer, Jules Verne, wrote of a spaceship shot to the Moon by a cannon--from Florida. This picture from the movie A Trip to the Moon, based on Verne's story, shows the giant cannon firing. The result: the bulletlike spaceship is plunged right into the eye of our nearest neighbor. A Trip to the Moon, made in 1902, was the first science fiction movie.
In 1926, Amazing Stories, the first magazine devoted entirely to science fiction, appeared. From then on, more and more writers described space travel. They generally used rockets, the one method we have discovered that really works. They traveled not only to the Moon, the planets, and their satellites in their stories, but also to other stars.
We can now see where science fiction writers guessed wrong. Most described rocket ships zipping across the solar system as though they were airplanes going from New York to Los Angeles. Actually, it takes rockets months to travel to Mars, and years to reach the distant outer planets. If rockets kept their engines firing, they could go a little faster. But in real life, rockets simply can't carry enough fuel to do that for long.
In many science fiction stories, people on spaceships were pictured as leading normal lives, just as if they were on ocean liners. The writers knew that spaceships coasting through space would experience zero gravity, or weightlessness, but most just assumed that there would be some sort of artificial gravity.
In reality, so far at least, astronauts on real spaceships have had to live with weightlessness and cramped living conditions. Naturally, science fiction writers want their stories to be exciting. But making a story more exciting doesn't always make it more accurate.
Jules Verne described a trip to the Moon, but didn't have his heroes land. That wasn't exciting enough. In H.G. Wells's 1901 story, The First Men in the Moon, the heroes not only land on the Moon, but encounter an advanced civilization. This scene is from the 1964 movie version of Wells's story. But the Moon in fact has not been like the Moon in fiction. When human beings actually did land on the Moon in 1969, they found no civilization.
The Moon was a completely dead world. When the landing was made, however, hundreds of millions of people on Earth watched it on television. That was one achievement the early science fiction writers hadn't thought of-watching a Moon landing on television.
Early science fiction writers assumed that all the planets of our solar system were pretty much Earth-like. There would be grain fields on Mars, irrigated by water from the canals. Settlers on Venus would hunt dinosaurs. And as scientists learned that planetary atmospheres were not breathable, writers invented domed or underground cities with special atmospheres.
If, in the future, we do want to explore or settle other parts of the solar system, we may have to do something like this. In order to live in space, we will have to work in space. And to do that we will first have to build a space station close to Earth. It would be a place where people could permanently live, work, and put together new spaceships for exploration farther out in space. Soviet rocket scientist and space pioneer Konstantin Tsiolkovsky wrote about space stations in a 1920 science fiction story.
Since the 1950s, scientists have designed space stations as great spinning wheels. The spin would produce effects resembling gravity. Space stations haven't been built yet, but when they are, they will probably be smaller and simpler than those that have been imagined.
In order to make their stories more exciting, science fiction writers often imagine different forms of life on various planets. Usually, the writers include advanced, intelligent life forms that might often be hostile to humans.
In reality, we have no actual evidence--at least not yet--that any life exists beyond Earth. But still we dream of seeking and finding advanced life forms among the stars. The idea of star travel is appealing, but it's not very realistic. For instance, if there were a Galactic Empire or Federation, how would its members communicate?
And how would people travel from one star to the next? Ever since 1905, scientists have known that nothing we know of can possibly go faster than the speed of light, which travels at 186,000 miles (300,000 kilometers) a second.
Science fiction writers are forced to break the laws of nature and imagine something like "hyperspace," through which spaceships can go faster than light-like taking a shortcut through a long wall instead of having to walk around the end and back.
But scientists feel quite certain that there is no getting around the speed-of -light limit. That's why there would be little real chance of a Galactic Empire. Even if there are many intelligent civilizations in the galaxy, each will probably remain isolated from the others.
Those who someday explore the galaxy will find it a long, slow process in which different settlements could easily become isolated in the vastness of space. Science fiction writers of the past usually underestimated how fast things change. In 1900, most writers who visualized air flight thought of dirigibles or of small airplanes.
When they imagined rocket ships going to the Moon, they didn't think of all the ways in which we now use space. Back then, most didn't think of communications, weather, and navigational satellites. But they did think of spacesuits very much like those that real astronauts have used.
They also imagined things like television, microfilm, tape recorders, lasers, and even charge cards. On a lot of things, science fiction writers of long ago were right. Science fiction writers often envisioned war in space, but they tended to imagine air battles with spaceships maneuvered rapidly, shooting each other down with disintegrator rays. We still see that in science fiction movies.
This is a picture from Star Wars, a movie that tells the story of a galaxywide civil war. In real life, disintegrator rays have become laser beams manipulated by computers. But in spite of differences between science fact and fiction, real war in space could match or surpass fictional wars in one important way-the terrible power to actually destroy our planet.
So far, no war has ever been fought in space, but weapons designed to destroy satellites have been tested. Surely a war in space would be a tragic way for science fiction to predict science fact. Most people in the 1800s probably thought that the inventions we call television, air-conditioned skyscrapers, and spacecraft were far-off dreams, probably impossible fiction. But these and many other dreams of science fiction writers are everyday parts of modern life. Sometimes science fiction does become fact.

Mythology an the universe

The Sun gives us light and warmth. North and south of the equator, when the Sun is low in the sky, the days become shorter and cooler, and winter comes. Winter is a reminder that, without the Sun, there would be only darkness and freezing cold. So to the ancients, the Sun was a glorious and good god.
Different peoples had different images of the Sun god. The great eye of Ra represented the Sun god of the ancient Egyptians. Ra was considered the nation's protector. This warm and tranquil "Sun-being" was drawn in Europe during the Middle Ages.
This fierce dragon gliding beneath the fiery Sun is from eighteenth-century China. The Moon is much dimmer than the Sun, but its light at night is cool and helpful. In myths, the Moon is usually pictured as a gentle female. To the ancient Greeks, she was a beautiful maiden called Selene or Artemis. To the Egyptians, she was Isis. As it circles Earth, the Moon changes its appearance, going from thin crescent to full and back to crescent each month.
Ancient calendars were based on this monthly cycle, and twelve of these monthly cycles made up the cycle of the seasons. It therefore became very important to watch each month for the first sign of the new moon. In fact, both month and Monday come from the word moon.
From day to day and from night to night, the Sun and Moon change their positions against the stars in the sky. So do five bright, star like objects that we call planets, from the Greek word for "wanderers." This is a view of brilliant Venus and dim Mercury as they line up at sunset with a crescent moon. Venus is the brightest planet in the sky and is named for the goddess of beauty.
Mercury is the fastest-moving planet and is named after the quick-footed messenger of the gods. The ancient Babylonians watched the planets move across the sky and gave them the names of gods. The Greeks and Romans copied the Babylonians in this, and we use the Roman names to this day. Mars is named after the god of war; and Saturn, after the god of agriculture.
The second brightest planet in the sky, Jupiter, was named for the chief god. Jupiter is not as bright as Venus, but it shines all night, while Venus appears only in the evening or at dawn. In modern times, people have found new planets that are too far away for the ancients to have seen. These planets have been given names from mythology, too.
Beyond ringed Saturn is Uranus, named for the god of the sky, who was Saturn's father. Farther still is Neptune, a sea-green planet named for the god of the sea. Beyond Neptune is Pluto, named for the god of the underworld because it is so far from the light of the Sun.
Every so often, something unusual happens in the sky: the Sun or Moon is eclipsed and hidden from our view. The Sun is eclipsed because the Moon moves in front of it and hides its light.
During a lunar eclipse, the Moon's bright face is turned a dusky red as it slips into Earth's shadow. Ancient people didn't know these causes, so they invented causes of their own. Some thought the Sun and Moon were chased by wolves, dragons, or other monsters that caught up with them now and then. Here the Hindu dragon Rahu causes a solar eclipse as he tries to swallow the Sun.
Of course, the Sun and Moon have always come back from their eclipses. And they will continue to do so for billions of years, even though according to Norse myths, at world's end a giant wolf will finally swallow the Sun.
Comets appear in the sky now and then. They are hazy objects with long tails. With a little imagination, they might look like the heads of mourning women with long, streaming hair--and in fact, the word comet comes from the Greek word for "hair." Sometimes comets look like swords, so people had several reasons to think of them as unpleasant omens. It's no wonder, then, that most people thought comets were messages sent by the gods, warning of war, plague, and destruction.
People would pray or ring church bells in order to try to ward off the evil. But evil always came when there were comets in the sky. Of course, evil always came when comets were not in the sky, too--but people somehow didn't notice that. When you look at the stars, you may imagine that they form patterns. Some of these patterns are triangles, crosses, or squares. Some are shaped like a W. Some form wiggly lines. Two bright stars might be close together and appear to be related when viewed from Earth.
Ancient people imagined many shapes in the sky, including even people and animals. These shapes made it easier to locate the stars. A star might be in the "tail of the scorpion" or in the "head of the hunter." These patterns are called constellations, a word that comes from two Latin words which basically mean "stars together." The constellations were given names, many of them in Latin. The ancients also created stories about these imaginary figures in the sky.
The Sun, Moon, and planets each pass through the same constellations as they make a large circle in the sky. This circle was divided into twelve constellations, so that the Sun took one month to go through each. Most of the constellations were pictured as animals, so the band in which the planets move is known as the zodiac, which means "circle of animals."
In this thirteenth-century painting, celebrating the month of May, the Sun moves from the constellation Taurus (the Bull) into Gemini (the Twins), while Venus, the love goddess, watches over the people on Earth from her blue chariot.
Some constellations in the Northern Hemisphere never set. One of these, Ursa Major (the Great Bear), contains the Big Dipper. Sailors in old times noticed that Ursa Major was always in the northern sky. This meant that they could look for it and always tell which direction was north. Thanks to the Dipper, sailors could voyage out of sight of land and find their way home.
We know that both ancient and modern cultures have seen figures in the constellations. Sometimes these figures are similar. Babylonians as well as ancient Mongols saw the Milky Way as a seam sewn in the two halves of heaven. And several cultures from different times and places-the Sumerians, Vikings, and some American Indians--believed the Milky Way was a bridge between Earth and the sky for the dead. But most cultures differ in their reading of the stars. The Inca Indians, for instance, interpreted the dark clouds of the Milky Way, rather than the stars, and saw in them animals such as a bird, fox, llama, toad, and serpent.
To the Norsemen, it was a huge spike driven through the universe around which the heavens revolved. To the Mongols, it was the Golden Peg, a stake that kept the heavens from whirling apart. The Chinese likened it to an emperor, the chief star that ruled the others.
In India, it was the place where a holy young prince faithfully meditated. "It" is the Pole Star, that stable star in the north around which all others seem to revolve.
As shown here, the two stars at the end of the Big Dipper's bowl point toward the Pole Star. But, in reality, there has not been just one Pole Star. Because Earth's axis wobbles a bit, various stars have been the Pole Star: Alderamin, Deneb, Vega, Thuban, and our current Pole Star, Polaris. And, of course, during those years, there have been periods when there was no star exactly to the north.
People talk about objects in the sky in different ways. Astronomers talk about the skies in familiar ways. But astrologers talk about the skies in ways that are less familiar. The practice of astrology, dating from ancient times, is to work out methods for predicting the future by using the position of the planets in the zodiac.
Even today, many newspapers carry a horoscope for those who seek advice from the stars. Astronomers, who use the methods and tools of modern science, are skeptical about astrology. Yet many people believe it to be true, just as ancient peoples found their stories of the skies to be true. So history shows us that while we are still uncovering secrets about the universe, one thing remains certain: our endless desire to make sense out of the objects above and around us.

ANCIENT ASTRONOMY

Ancient people were fascinated by the sky and the patterns of the stars. They noticed that the Moon changed its shape from night to night and changed its position against the stars. They traced out constellations that looked like people and animals and made up stories about them.
The first astronomical observations were painted on the walls of caves 30,000 years ago. Ancient priests were among the first astronomers. They studied the sky to make sure that their calendars, based on the changes of the Moon, were accurate.
At least 5,000 years ago, ancient astronomers began using large stones to chart the movement of the Sun and the stars. The most famous ancient observatory of this kind is called Stonehenge, in England.
American Indians also built circles of stones lined up with the Sun and stars to figure out sunrise and the start of summer. Some stars and constellations, like the Big Dipper, always stay in the northern part of the sky. Ancient sailors used these stars to guide them. The Polynesians found their way to distant islands over the vast Pacific Ocean by watching the stars. The Mayans, who lived in southern Mexico, watched the movements of the Moon and the planet Venus carefully.
By about the year A.D. 800, they had worked out a calendar that was more accurate than the one being used in Europe at the time. They may have built special buildings like this one to study the sky. The lives of the ancient Egyptians depended on the Nile River. When the river flooded their fields, it made it possible for them to grow their crops.
Their priests carefully recorded when the floods came and found that they came about every 365 days. So the Egyptians were the first to use a calendar with a 365-day year. The ancient Babylonians viewed the universe as a disk of land with water surrounding everything.
They were the first people to study the movements of the planets and kept detailed records of their paths. Like most ancient peoples, the Babylonians believed that studying planetary movements could help them predict the future.
One biblical story tells how the people of a Babylonian city tried to build a stairway to the stars--the Tower of Babel. Early Greek astronomers probably picked up most of their knowledge from the ancient Babylonians.
Around 550 B.C., the Greek philosopher Pythagoras pointed out that the Evening Star and the Morning Star were really the same body.
Today, we know that this body is the planet Venus. At that time most people thought that the Earth was flat. One early Greek view described the world as a floating disk inside a great hollow ball.
But later some Greek astronomers thought that the Earth itself might have the shape of a ball. Others even thought that the light of the Moon was really reflected sunlight.
Ptolemy described the Earth as a huge ball at the center of the universe with the objects in the sky moving around it in great circles. Each planet moved in a separate circle. The Moon was lowest. Then came Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The stars were farthest out. To explain why the planets changed direction, Ptolemy, using the older calculations of Hipparchus, worked out a detailed scheme of the planetary motions.

Ptolemy did his work in about A.D. 150; Hipparchus, about 130 B.C. So it took about 280 years to come up with the scheme. It was very complicated, but it could be used to work out future positions of the planets.
In about 240 B.C., a Greek astronomer in Egypt, Eratosthenes, found that when the Sun was directly overhead in one city, it cast a shadow in another city 500 miles (800 kilometers) to the north. Eratosthenes figured out this meant Earth's surface curved.
He also figured out that Earth was a ball about 25,000 miles (40,000kilometers) around. Today, we know he was right. After Ptolemy, Greek science faded, but the Arabs, beginning in A.D. 632, set up a large empire, discovered Greek books on science and mathematics, translated them into Arabic, and studied them. In some cases, they improved on the Greeks.
In about 900, an Arab named al-Battani worked out new ways of figuring out planetary positions. "Star-finders," or astrolabes, like this one, were created by Arab astronomers to solve complicated problems in astronomy.
One side often contained a detailed star map. If it hadn't been for the Arabs, Greek science might have been totally lost. In July 1054, a star blazed out in the heavens. For three weeks it was so bright it could be seen in daylight. Europeans at the time took no interest, and the only reason we know that the star appeared was because Arab, American Indian, Japanese, and Chinese astronomers carefully noted it.
Eventually, Europeans began to translate Arabic versions of Greek books into Latin. To many European astronomers, the Greek scheme of the universe seemed too complicated. During the sixteenth century, the Polish astronomer Copernicus decided that a simpler scheme would be to place the Sun at the center of the universe and have all the planets circle it.
Earth would have to circle the Sun, too. This seemed against common sense, but Copernicus wrote that his idea would make it much easier to figure out planetary positions. For more than fifty years, astronomers argued whether Copernicus was right or not.
European astronomers were beginning to find out that the Greeks were indeed wrong now and then. In 1572, a Danish astronomer, Tycho Brahe, spotted and studied a bright new star, or supernova, in the sky. Eventually, the new star faded away. But the Greeks had thought that the sky never changed.
Tycho Brahe recorded the position of the supernova so precisely that modern astronomers have photographed its remains. From his observatory in Denmark, Tycho Brahe also discovered that comets were farther from the Earth than the Moon.
But the Greeks had thought comets were inside our atmosphere. All this made Europeans more ready to accept new ideas--like Copernicus's idea that the Earth circled the Sun. The turning point came when a telescope was invented in Holland. An Italian astronomer, Galileo, heard of this, built his own, and in 1609 pointed it at the heavens. He found that the Moon was a world with craters, mountains, and what looked like seas. He found that the planet Jupiter had four moons that moved about it, and that Venus changed shape, just as the Moon did. At once he discovered many stars too dim to be seen without his telescope. All of this didn't fit with the Greek view of the Earth-centered universe.
But it did fit the views of Copernicus, and at that moment, modern astronomy had begun. Today, in addition to optical telescopes, astronomers have instruments to pick up radio waves from objects too far away to see. They have even sent instruments into space.
We use these instruments to learn things that ancient astronomers never dreamed of. But in many ways, we want to learn about the universe for many of the same reasons the first astronomers did long ago.