The Ancient Greeks
Astronomy owes a great debt to the ancient Greeks. From them came the idea of a science influenced to some small extent by earlier Babylonian discoveries, but characterized by a completely different outlook. Whereas the priests of Bablyonia and Egypt tended to keep their astronomical knowledge to themselves, the thinkers of ancient Greece were philosophers, or “lovers of wisdom”, who operated quite openly through observation, discussion and argument.
The new approach to understanding the nature of the physical world was almost wholly deductive. True, the Greeks made valuable observations, but the general trend was to fit observation to theory instead of basing theory on observation. In astronomy they were concerned with a knowledge of the nature and structure of no less than the entire physic universe. This study, known as cosmology, was based largely on geometry, for to the Greek mind all celestial motions were derived from the principle of uniform motion in a circle.
About 600 B.C. Thales of Miletus, the first of the great Greek astronomers, taught that the changes in the heavens were not due to the caprices of gods but to known physical processes. Anaxagoras, one of his flowers, understood the causes of eclipses and stated that just as the sun and moon were material and not spiritual bodies, so the stars were merely white-hot stones whirled round by the vault of heaven. But the most important contribution in this earlier Greek period came from Pythagoras. Considerations based on geometry mixed with mysticism led him to depart from the old idea of a flat earth. The earth, he declared, is round like a ball and is fixed at the centre of the universe without any visible signs of support. Philolaus, one his disciples, went even further by teaching that the earth, in common with the sun, moon, and planets, moves in a circle about an invisible central fire which he called “the hearth of the universe”.
The Universe according to Aristotle
The ideas of Philolaus were decidedly unpopular with other Greek thinkers. To them nothing seemed more obvious than that the Sun and stars should travel about the Earth. They welcomed the idea of a round earth, for this accorded with observation. For one thing, a comparatively short journey North or South brought about a change in appearance of the starry sky. Certain southern stars seen in Egypt were invisible in more northerly latitudes, while some of the northern stars that rose and set in Egypt stayed permanently above the horizon on the Greek mainland. These changes could not possibly take place if the earth were flat. Again, as Aristotle pointed out, when the moon was being eclipsed the earth`s shadow had a circular edge, which in fact indicated that the earth had a circular boundary.
Aristotle, born in 384 B.C., a pupil of Plato and the friend and tutor of Alexander the Great, had a profound effect on subsequent thought. According to him the universe was arranged in concentric layers like those of an onion. At the centre was the fixed spherical earth, above whose rocky core stretched the layers of the elements water, air, and fire. The last extended to a crystalline shell or sphere which completely enclosed the earth and in which the moon was embedded.
Within the sphere of the moon all was change and corruption, a scene fit for brutes and men and such transient things as rainbows, shooting stars and comets. Above the lunar sphere stretched regions celestial, divided by the successive crystalline shells of Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. Here all motion tended to be circular, uniform, and perpetual. Enclosing the whole was the vast sphere in the full sense of the word, which came under the direct influence of the Primum Mobile or Divine Mover. This outer Sphere rotated uniformly once in 24 hours and communicated its motion to all other spheres.
Diagram of Aristotle`s Universe. A fixed central earth is surrounded by the concentric shells of the heavenly bodies