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To Albatani, however, we owe two really valuable heirlooms. The one is the use of the sine, or half-chord of the double arc, instead of the chord of the arc itself, which had been employed by the Greek astronomers; the other, of even more practical benefit, was the introduction of the present decimal arithmetic, instead of the troublesome sexagesimal arithmetic of the Greeks.
Averroes, in quality of commentator on Aristotle, became his rival in the minds of the mediaeval schoolmen; Avicenna, in quality of commentator on Hippocrates and Galen, was for centuries the text-book of all European physicians; while Albatani and Aboul Wefa, as astronomers, commented on Ptolemy, not however without making a few important additions to his knowledge; for Aboul Wefa discovered a third inequality of the moon's motion, in addition to the two mentioned by Ptolemy, which he did, according to Professor Whewell, in a truly philosophic manner an apparently solitary instance, and one which, in its own day, had no effect; for the fact was forgotten, and rediscovered centuries after by Tycho Brahe.
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