Three different positions of the equinox are given in three different Babylonian tablets, namely, 10°, 8° 15′, and 8° 0′ 30″ of Aries. (Kugler, _Mondrechnung_, p. 103; Ginzel, _Klio_, i. p. 205.) Given knowledge of this kind, and the practice of formulating recurrences in cycles, it is scarcely conceivable that the Babylonians should not have invented a cycle for precession. It is equally intelligible that they should only have reached a rough approximation; for the precessional period is really about 27,600 years and not 36,000. It is to be observed that Plato’s “perfect year” is also 36,000 solar years (Adam’s _Republic_, vol. ii. p. 302), and that it is probably connected with the precession of the equinoxes. (Cf. _Tim._ 39 d, a passage which is most easily interpreted if referred to precession.) This suggestion as to the origin of the “Great Year” was thrown out by Mr. Adam (_op. cit._ p. 305), and is now confirmed by Hilprecht, _The Babylonian Expedition of the University of Pennsylvania_ (Philadelphia, 1906). We shall see that Thales probably knew the cycle which the Babylonians used to predict eclipses (§ 3); but it would be a mistake to suppose that the pioneers of Greek science had any detailed knowledge of the Babylonian astronomy. It was not till the time of Plato that even the names of the planets were known,[40] and the recorded observations were only made available in Alexandrian times. But, even if they had known these, their originality would remain. The Babylonians studied and recorded celestial phenomena for what we call astrological purposes, not from any scientific interest. There is no evidence at all that their accumulated observations ever suggested to them the least dissatisfaction with the primitive view of the world, or that they attempted to account for what they saw in any but the crudest way. The Greeks, on the other hand, with far fewer data to go upon, made at least three discoveries of capital importance in the course of two or three generations. In the first place, they discovered that the earth is a sphere and does not rest on anything. In the second place, they discovered the true theory of lunar and solar eclipses; and, in close connexion with this, they came to see, in the third place, that the earth is not the centre of our system, but revolves round it like the other planets. Not very much later, certain Greeks even took, at least tentatively, the final step of identifying the centre round which the earth and the planets revolve with the sun. These discoveries will be discussed in their proper place; they are only mentioned here to show the gulf between Greek astronomy and everything that had preceded it. The Babylonians had as many thousand years as the Greeks had centuries to make these discoveries, and it does not appear that they ever thought of one of them. The originality of the Greeks cannot be successfully