in one lecture-room at the University of Padua in 1592, Galileo was teaching the Euclid's Elements in another. It is easier to comprehend how students flocked to listen when a few years later Galileo began his lectures upon astronomy, although by the conditions of his professorship he was only permitted to expound the astronomy of Sacro Bosco.
Aristotle taught that the earth was spherical and gave reasons, good and bad, for his belief. The distance of the sun was fixed by a most ingenious method invented by Aristarchus of Samos (270 B. C.) who concluded that the sun was about 19 times more distant than the moon (it is, in fact, 390 times more distant). Hipparchus determined the moon's distance for himself[1] and took the sun to be 19 times more distant. He did not leave the earth in the central point of the sun's orbit, but shifted that center towards the sixth degree of Gemini by one twenty-fourth of the radius so as to account for observed inequalities in the annual motion. Ptolemy adopted this result without question, and it was accepted by astronomers for twelve centuries. It was not until the time of Kepler that it was proved that the sun must be at least fifty times as far away as the moon. This was one of the consequences of Tycho's accurate observations.
The Chaldeans and Egyptians held the earth to be a flat disc canopied by the sky—the firmament—and this was the view of the Hebrews. A distinctly Christian theory of the figure of the earth and heavens, drawn from scripture, was formulated by the Egyptian monk and traveler Cosmas Indicopleustes. According to this theory, the earth was a flat parallelogram surrounded by the four seas. "We say, therefore, with Isaiah, that the heaven embracing the universe is a vault; with Job, that it is joined to the earth; and with Moses, that the length of the earth is greater than its breadth." This explanation of appearances was very generally accepted as orthodox, and was held by the common people long after the learned had been convinced of the earth's sphericity by the arguments of Ptolemy and Aristotle. Isidore of Seville in the seventh century, and the Venerable Bede in the eighth, declared for the opinion of Aristotle; Dante in the thirteenth century supported it, and Columbus proved it in the fifteenth. In the sixteenth, Magellan's voyage of circumnavigation settled the vexed question once and for all.
There is in the library of the University of Cambridge, so Dr. Whewell reports, a French poem of the time of Edward the Second (1307–27) illustrated with drawings that show men standing upright on all parts of a spherical earth. By way of illustrating the tendency
- ↑ He fixed the greatest distance of the moon at 78, the least at 67, semi-diameters of the earth. The mean distance is, in fact, 60. The distance of the sun, according to Hipparchus, was 1,300 semi-diameters. It is really about 23,000.
