Page:Popular Science Monthly Volume 64.djvu/329

Rh The Arabian school of astronomers added nothing to the theory of Ptolemy. They transmitted the text of the Almagest to the west accompanied by intelligent comment and almost without criticism except in the cases of Alpetragins and Geber. The Arab observations were very numerous, and resulted in fixing new and much more accurate values of the constant of precession, the length of the year, the obliquity of the ecliptic, the eccentricity of the sun's orbit and the motion of its apogee. Their arithmetic was the clumsy sexagesimal arithmetic of the Greeks, until in the eleventh century the Hindu decimal system began to make its way in Egypt, Spain and Europe. Geometry is not indebted to the Arabs for any marked advances. On the other hand, trigonometry was greatly improved.

As observers the astronomers of the Arab school had great merit. They grasped the need for continuous observations, whereas the Greeks in general had contented themselves with making observations at certain critical times only—at the solstices and equinoxes, for instance. The Arabs were the first to assign the exact time at which any phenomenon occurred—a fundamental datum. They measured the altitude of the sun at the beginning and ending of solar eclipses, for example, in order that the time might be known. The calculation of a spherical triangle enabled the instants of beginning and ending to be accurately assigned. The Greeks never employed this device and the times of phenomena recorded by them are seldom known with any accuracy. Indeed Ptolemy has no formula by which to calculate the time when the sun's altitude is given, and it is noteworthy that the Arab device was not known in Europe until 1457, when Purbach used it for the first time. Yet it was employed at Bagdad at the solar eclipse of A. D. 829, six hundred 3ears earlier. Even the times of phenomena recorded by Tycho Brahe in 1600 are seldom known so close as a quarter of an hour. Short intervals of time were measured by the Arabs by counting the beats of pendulums.

A few of the greatest Arabians are named in what follows. Albategnius was an Arab prince of Syria who flourished at the end of the ninth century of our era. His observations were made at Aracte (Eachah) in Mesopotamia and at Antioch, between the years 878 and 918. After studying the Syntaxis of Ptolemy he set himself to correct the errors of its catalogue of stars by observations of his own, made with apparatus fashioned after Ptolemy's descriptions. It appears that some of his instruments could be read to single minutes (1′) and were divided possibly to 2′ (or it may be to 6′). He detected the change of position of the sun's apogee, determined the obliquity of the ecliptic, the length of the year, the precession-constant (54″), observed and calculated solar and lunar eclipses and computed new tables of the planetary motions, although he did not seek to improve Ptolemy's planetary theory. He was original and inventive as an