After Plato and Aristotle, as before them, the doctrine of the Great Years, though by no means universal, was very popular in antiquity.[24] The Stoics adopted it in its more extreme form involving successive burnings and liquefactions of the universe.[25] It entered into Neoplatonism and was ultimately taken over by the Jews. It seems to have penetrated to India, where the Greek elaboration of the theory gave precision to ideas that were probably already in existence there. The Indian belief in the recurrent reincarnations of Brahma was brought into connection with Hellenic calculations of the duration of the Great Years.[26] From the Hindus and from the Greeks the conception was transferred to the Arabs and by them to the knowledge of the Latin West. DURATION OF THE GREAT YEARS Numerous endeavors were made in antiquity to calculate the length of a Great Year.[27] The figure that was adopted by the Arabs and passed on to the Christian world originated in Hipparchus’ discovery of the precession of the equinoxes, or apparent gradual revolution of the fixed stars around the pole of the ecliptic.[28] Ptolemy calculated that the period of this revolution was 36,000 years,[29] a figure which became known to the Hindus and Arabs and ultimately to medieval Christendom.[30] The actual figure is approximately 25,800 years. _SHAPE, MOVEMENTS, AND SIZE OF THE EARTH_ SPHERICITY OF THE EARTH Nearly all scholars of antiquity after the fifth century before Christ thought that the earth was a globe.[31] The earlier opinion of a disk-shaped earth resting upon the waters, which appears to have been held by Anaximander (although some students have thought that he, too, believed in a spherical earth [32]), was discarded by the Pythagoreans and Plato, and after their time no serious thinkers questioned the theory of sphericity. The Pythagoreans based their opinion on speculative and philosophical grounds rather than on physical and experimental proofs; they thought that since the sphere is the most perfect mathematical form, the earth must therefore be a sphere. The whole tendency of Aristotle’s thought, less speculative and less hypothetical than Plato’s,[33] led him to look for proofs of sphericity,[34] and these he enunciated with great emphasis. Cleomedes,[35] Pliny,[36] Ptolemy,[37] Martianus Capella,[38] and other ancient writers likewise adduced more or less convincing proofs, which were well known and often cited in the medieval period. IMMOBILITY OF THE EARTH