50 THE UNIVERSE A VAST SYSTEM OF PARTS knowledge for nearly two thousand years. One of his greatest contributions was the creation of the inductive method of reasoning and the working out of the principles of logic. Aristotle improved nearly every field of learning he touched except those of physics and astronomy. He considered the earth to be a sphere in the center of the universe and taught that the planets were supernatural beings entirely unlike the the earth. Aristotle added a fifth element/ ether, which was supposed to move in circles. He rejected the ideas of previous philosophers that matter could be divided into ultimate particles called atoms. Of all the Greeks, Aristarchus (died 230 h.c.) held the most modern ideas of astronomy. He taught that the spherical earth and the planets revolved about the sun. He concluded that the sun was about twenty times more distant from the earth than the moon. The actual ratio is about 1 to 400, but his errors were due to inadequate instruments; the principles of his experiments were sound. He accounted for the relative immobility of the stars by rightly assuming that they were very much farther away than the sun. Unfortunately the experiments of Aristarchus were rejected for the teaching of Aristotle. Eratosthenes (died about 195 B.C.) was the first man to measure the size of the earth on the assumption that it was a sphere. He noted that at noon on June 21 at Syene, Egypt, the sun was directly overhead and that exactly a year later at Alexandria the sun was a little more than 7 degrees south of the line passing directly overhead. He concluded that this was due to the fact that the earth was round. Inasmuch as the distance between these two cities (about 480 miles by his measurements) represented 7/360 of the entire circumference of the earth, he figured the circumference to be about 24,000 miles, which is within about 900 miles of the present accepted value for the circumference at the equator. Hipparchus (died 125 B.C.) was the last of the great Greek astronomers. He catalogued about 1080 stars, classified according to their brightness, and mapped the course of the sun among the stars. His measurements of the size of the moon and its distance from the earth were within about 10 per cent of present-day values. He discovered the precession of the equinoxes (see page 98) and determined its amount. The next great man was Claudius Ptolemy, a Greek born in Egypt, who, between the years 127 and 151 a.d., made observations in Alexandria, the center of Hellenic culture for centuries. On the basis of 1 When the ancients used the word "element," it meant "elementary quality" rather than "elementary substance," which is the modern meaning of the word, first clearly defined by Lavoisier.
THE NATURE OF THE UNIVERSE 57 these observations he worked out a system of astronomy that dominated the minds of men for the next thirteen centuries. His great contribution was the compilation of his Almagest, or encyclopedia of astronomy, which preserved the work of Hipparchus. Ptolemy believed that the earth is the center of the universe. He also erroneously believed that the earth is stationary, but he was correct in his assumption that the earth is a sphere, balanced without supports in space. He concluded that the fixed stars were fastened to the inside of a vast dome that revolved about the earth once every twenty-four hours. The planets supposedly moved in independent orbits between the earth and this star-studded dome. His system was so satisfactory that it was widely accepted and never seriously challenged until the time of Copernicus , thirteen centuries later. The thirteen hundred years following Ptolemy were a period of retrogression, in which people returned to the old astrology, and, forgetting the relatively advanced teachings of Ptolemy, generally considered the earth to be fiat. Copernicus Was the First of the Renaissance Astronomers. Nikolaus Copernicus (1473-1543) was a skilled painter and student of medicine, mathematics, and astronomy. He taught from the still dominant Almagest and was greatly influenced by the teachings of Aristarchus. Fortunately he did not accept traditional theories without question but made observations of the stars for himself. He found so many errors in the Ptolemaic tables that he came to distrust all of Ptolemy's teachings. In 1507, he wrote his famous book De Revolutionibus Orbium Caelestium ("Concerning the Revolutions of the Heavenly Bodies"). His main thesis was that the sun is the center of the solar system and that the earth is but one of the planets revolving around the sun. He also advanced the modern concepts that the moon revolves around the earth, accompanying it in its revolution around the sun, and that the earth turns on its axis from west to east, thus accounting for day arid night and the apparent motion of the stars. Other Renaissance Astronomers. , Copernicus' follower, Bruno (1548-1600), went even farther and! stated that the universe is infinite and that the stars are scattered throughout space. He was regarded as a heretic, however, and was burned at the stake in 1600. Tycho Brake (1546-1601), whose interest in astronomy was aroused by an eclipse of the sun coming at the exact time predicted in 1560,