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In the ninth century begins the fertile period of Arabian science, protected and encouraged by the Khalifs of Baghdad, especially by Al Mamun, son of Harun-al-Raschid, who spent much time in forming a collection of Greek works on science. Ptolemy's Syntaxis was translated in 817 by Isaac ben Honain, and was carefully studied by the astronomers of Baghdad and Damascus. His observations were carefully repeated, but without any marked advance of science till the time of Mohammed ben Geber ALBATANI; so called from Batan, in Mesopotamia, the place of his birth; in Western style he is known as Albategnius. He was a Syrian prince; able and willing to spend wealth on costly observatories established at Aracte and also at Antioch. The result of his labours is contained in a treatise on the science of the stars and their motions.
In trigonometry, Albategnius introduced an important innovation--the use of the semi-chord of the double arc for the chords employed by Hipparchus and Ptolemy. This semi-chord was called in Arabic gib, i.e. pleat or fold, translated into Latin as sinus; such, at least, is the most probable explanation of the word. The introduction of the sine simplified the labour of calculation. Of the other trigonometrical lines, the tangent appears to have been known to him, but not used; the cosine and secant were of later invention.
In astronomy, Albategnius, repeating with greater accuracy and better instruments the observations recorded by Ptolemy, determined the annual ammount of precession as 54", instead of 36"; a very much nearer approximation to the true amount. He discovered also what in Ptolemy's time was unknown--the annual motion of the solar apogee, which he fixed at 25". His determinations of the eccentricities of the solar orbit, or the obliquity of the ecliptic, and of the length of the year, were superior in accuracy to those of the Greeks. The year as fixed by him was two minutes and a half too short, an error much smaller than that of Hipparchus, and which, when examined, appears to be due to an incorrect, or even fictitious, observation of Ptolemy.
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| This biography is
reprinted from The New Calendar of Great Men. Ed. Frederic
Harrison. London: Macmillan and Co., 1920. |
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