Albategnius was the greatest of the Arabian astrono mers. While governor in Syria, he made numerous observations at Antioch, and also at Aracte, a town in Mesopotamia. By a diligent comparison of ancient ob servations with his own, he found the annual precession of the equinoxes to be 51".2. His determination of the obliquity of the ecliptic, when corrected by refraction and parallax, is 23° 35' 40", which makes the secular va riation 51".5. The theory of the motions of the sun is particularly indebted to Albategnius. He reduced the eccentricity of the sun's orbit to17325, the radius being 1000000, which gives a diminution of 5110 in 870 years ; while the diminution, according to the theory of gravity, is 3967 ; and he discovered the proper motion of the sun's apogee, which preceding astronomers had regard ed as immoveable. Reasoning from analogy, Albateg nius conjectured that there were similar inequalities in the orbits of the other planets; but this conjecture re mained to be confirmed by modern astronomers. In ad dition to these labours, he constructed new astronomical tables for the meridian of Aracte, which were superior in correctness to those of Ptolemy, and were highly esteemed by his successors in science. The works of this learned astronomer were collected in one volume, and published under the title of De Scientia et Motibus Stellarune The history of Arabian astronomy terminates with the life of Albategnius ; but the spirit of discovery sur vived, and the sjences of the Arabs were introduced into all the countries that were subjected to their pow er. Ibn Junis, an Egyptian astronomer, made several interesting observations, that arc recorded in a manu script work of that astronomer, which is preserved in the library at Leyden, and which has been very lately published with a translation by professor Caussin. This work contains 28 observations of solar and lunar eclipses, from 829 to 1004 ; 7 observations of the equi noxes, from 830 to 851 ; an observation of the summer solstice in 832 ; and three eclipses observed at Cairo in 977, 978, and 979, which establish the acceleration of the moon's motion.
About the middle of the eleventh century, the Per sians emancipated themselves from the Arabian yoke, and, along with their independence, recovered their taste for astronomy. Their ancient calendar having given place to that of the Arabs, they resumed it in 1079, under the direction of Omar Cheyam, who adapt ed it to the length of the solar year, by adding seven times in succession a day to every fourth year, and then a day to the fifth year. • Holagu Ilekou-Khan, one of the descendants of Zin gis-Khan, conquered Persia in 1264, and established at Maragha, the capital of Media, a magnificent observa tory under the care of Nassir-Eddin. Encouraged by his patronage, crowds of astronomers flocked to Marag ha, and were united into an academy which long flou rished in Persia. Nassir-Eddin composed, among other works on astronomy, a Treatise on the Theory of the celestial motions, a Treatise on the Astrolabe, and As tronomical Tables.
Ulugh-Beigh, a prince of Tartary, and grandson of Tamerlane the Great, made a distinguished figure as an astronomical observer. He established at Samarcand,
the capital of his dominions, a society for the advance ment of astronomy, and erected magnificent instruments for its use. By means of a gnomon 180 feet high, lie found the obliquity of the ecliptic 23° 30' 20". He de termined the precession of the equinoxes to be 1° in 70 years. He formed a new catalogue of the fixed stars, and constructed astronomical tables, which were so ac curate, that they differed but little from those which were afterwards calculated by Tycho Brahe.
The observations of Ulugh-Beigh were the last ac cessions which astronomy received from the East. The iron hand of religious and political despotism has been stretched with fatal effect over the most delightful re gions of Asia, and under its deadly pressure, the love of science has been extinguished, and the human mind unfitted for any higher exertions than the reveries of magic and judicial astrology. It was enough, perhaps, for the inhabitants of the East to have cherished the arts and sciences in their infancy ; to have furnished the ele ments of their progress ; and to have preserved them amid the awful convulsions of human affairs. The un tried genius of the West was reserved to carry them on to maturity, and to perfect that fabric of human wisdom which astronomy was destined to form.
The study of astronomy was kept alive in Spain by the Arabs, who had established themselves in that coun try. Arzachel observed the obliquity of the ecliptic, and corrected the theory of the sun by a series of accu rate observations. Alhazen wrote upon the cause of the twilight, and found the height of the earth's atmos phere to be 52 miles. He explained also, in his treatise on optics, the refraction of the atmosphere, and the me thod of determining it.
Alphonso X. king of Castile, had the honour of being the first prince who contributed to the progress of astronomy in Europe. He founded at Toledo a college for the advancement of the science, and assembled, at a great expense, all the learned Arabians in Spain. By the united exertions of these philosophers, the Alphon sine Tables were published in 1252, which, though founded on the same principles as those of Ptolemy, were much more correct, in consequence of the superior accuracy of the mean motions.
The impulse which was now given to the human mind was rapidly propagated through every part of Europe, and the lists of fame were crowded with numerous competitors. Roger Bacon pointed out the necessity of correcting the calendar. Nicolas Cusa, a cardinal of the Romish church, discovered some errors in the Al phonsine Tables, and maintained the motion of the earth round the sun. Purbaehius undertook a translation of the Almcgest of Ptolemy : he composed a treatise on the theory of the planets ; and gave a more convenient form to the equations used by Ptolemy and Alphonso. He determined the obliquity of the ecliptic, and, along with several instruments for dialling, lie constructed ce lestial globes, on which the precessim* the stars was marked from the time of Ptolemy to 1450.