Early Ideas

Beginnings of astronomy.
            People have studied the stars and other objects in space for a very long time. this study, known as astronomy, is though to be the oldest of the sciences.
            Ancient Chinese astronomers had worked out a yearly calendar by 1400 B.C. they improved their calendar so that by 1200 B.C., it included exactly 365¼ days. By 350 B.C., the astronomer Shih Shen had prepared a catalog listing more than 800 stars.
            The Babylonians also worked out a yearly calendar. Their calendar was based on the phases of the moon. The Babylonians also invented a sundial to keep track of the passing hours of the day.
            The Egyptians developed a different kind of calendar. They counted the passing of a year by watching for the brightest star in the winter sky, Sirius. When Sirius could be seen for the first time each year rising in the south just before the sun came up, the astronomers would foretell the overflow of the Nile River. This important event began the new year for the Egyptians.
Ups and downs in early astronomy.
            Early Greek astronomers put special emphasis on the use of mathematics in astronomy. They tried to figure the distance from the earth to the sun and to the moon. They also compared the size of the sun and the moon to the size of the earth.
            Most Greek astronomers believed that the earth was the center of the universe. They saw that the sun, the moon, and the stars seemed to move around the earth. To explain this movement, the astronomers said that the sky was really a huge, hollow sphere, or ball, surrounding the earth. The stars were located on the inside of this sphere. As this sphere turned, the stars turned with it around the earth. Greek astronomers later decided that there must be a series of hollow spheres, one inside the other. These additional spheres were needed to account for the differences noted in the movement of the sun and the stars as compared to that of the moon and of the planets.
            One branch of early astronomy was astrology. Although many people still believe in astrology, it is no longer studied as a part of astronomy. Astrologers believe that the planets and their motions influence the lives of people. A person's qualities and future are thought to be governed by the position of the planets in relation to the earth and to the stars at the time of the person's birth.
            Ancient astrologers studied the movement of the planets and kept careful charts of them. Their advice was constantly sought. Rulers of countries consulted astrologers before making every important decision.
Astronomy's revolution
            The period in which the Greeks had great influence in astronomy lasted from 600 B.C. to A.D. 400. After that, astronomy seemed to be at a standstill in Europe for thirteen centuries. People accepted without question the idea that the sky was a series of huge rotating spheres carrying the sun, the stars, the moon, and the planets around the earth. Finally in the sixteenth century A.D., a Polish astronomer named Nicolaus Copernicus offered a new explanation. He presented a model for the solar system.
            According to Copernicus the sun was the center of the solar system. "Solar" comes from a Latin word meaning sun. He said that the planets traveled around the sun in paths called orbits. The planets that were known at that time were Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Copernicus placed all of these planets in their proper order from the sun. He also showed that there are two motions of the earth. The earth rotates, or turns on a fixed axis, every day. The earth also revolves, or travels around the sun, once each year.
            Many people living at the time of Copernicus could not accept the idea that the earth was not the center of the universe. And they could not believe that the earth was moving, since they felt no movement. It was not until long after Copernicus's death that his ideas were accepted.
            Two other great scientists who contributed to the revolution in astronomy were Johannes Kepler and Isaac Newton. Kepler discovered that the orbits of the planets around the sun were ellipses, or ovals, not circles. He showed that at certain times each planet is closer to the sun than at other times.
            Kepler also came to the conclusion that a force from the sun pulled on each planet. This force was strongest when the planet was closest to the sun. Isaac Newton further studied this force exerted by the sun and other bodies. He explained that not only did the sun pull on other bodies in the solar system but these bodies also pulled on the sun and on all the other bodies as well. These forces are further explained in Newton's laws of gravitation.