he did this. He subdivided the circle into 360°, an idea introduced into Greek astronomy from Babylonia through the work of his contemporary, Hypsicles of Alexandria.

One of Hipparchus' greatest achievements was his discovery of the precession of the equinoxes. He discovered that the points at which the ecliptic crosses the celestial equator move slowly with respect to the stars. He was able to do this because Babylonian astronomical data became available in the Greek world and he examined systematically old observations and compared them with his new ones in order to discover changes that were too slow to be detected by astronomers using only data gathered during their own lifetimes. The arrival of large quantities of data covering observations made over many centuries helped transform Greek astronomy. It was now possible to take the geometrical models that had been developed and use them to produce procedures for accurate quantitative prediction. The work of Hipparchus, based as it was on a merging of Greek and Babylonian approaches, marks the transition between qualitative and quantitative mathematical astronomy.4

Hipparchus' contributions to astronomy were enormous. In the words of the French astronomer J.-B. J. Delambre:

When we consider all that Hipparchus invented or perfected, and reflect upon the number of his works and the mass of calculations which they imply, we must regard him as one of the most astonishing men of antiquity, and as the greatest of all in the sciences which are not purely speculative, and which require a combination of geometrical knowledge with a knowledge of phenomena, to be observed only by diligent attention and refined instruments.

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