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.