The glory of the Alexandrian school departed with the life of Ptolemy. The school itself continued about five centuries after the spirit of discovery had fled, and was with difficulty kept alive by industrious commenta tors, who obscured the writings of Hipparchus and Ptolemy, without adding to their discoveries. The hu man mind was replunged in its former lethargy, and science was doomed to languish during a long period of inglorious repose. Even in Rome,—the metropolis of the world, the seat of learning and civilization, the cen tre on which the eyes of all Europe were converged with idolatrous regard,—even in Rome, the science of astronomy was completely neglected, amid the turbu lence of contending factions, and'the pursuit of military fame. Eloquence, poetry, and the fine arts, were the subjects of Roman study ; while the heavens were con templated with a barbarous indifference, which has never been exhibited by a civilized people. After the division of this mighty empire, and its subsequent overthrow by the hordes of the north, the taste and genius of the Romans began to decline, and literature and the fine arts accompanied science in her fall.
The periods of languor which mark the history of knowledge, are frequently less fatal to its progress than is generally believed. While the collected materials of former ages are preserved from the outrages of time, science may be stationary, but never can be retrograde. No sooner does the mind recover from its lethargy, than new facts are added to the stock that is perpetually ac cumulating, and the river of knowledge runs deeper and stronger by the accession of every tributary stream. Mighty convulsions, however, sometimes take place, which not only absorb these stipendiary rivulets, but which reach the source of the current, and cut off the very springs which gave it birth. Such was the revolu tion which agitated the nations at the beginning of the 7th century, when Mahomet carried victory and deso lation into the powerful kingdoms of the East. Instigated by an ungovernable zeal to propagate their new faith, the successors of the prophet committed the most fatal ravages in Persia, Syria, and Egypt. The Alexandrian library, the treasury of all the wisdom of antiquity, was wantonly burned caliph Omar ; and though sor row and repentance followed the barbarous deed, the precious fruits of ancient genius and learning were irre coverably lost.
When the Arabian caliphs had reduced under their dominion the western kingdoms of Europe and Africa, their ambition of conquest was amply gratified, and their enthusiasm was directed to more worthy objects. The desire of repose, and the love of science, induced them to return to their country, and to rekindle those in tellectual lights which but a century before they had been so zealous to extinguish.
The revival of astronomy in Arabia commenced in the reign of Almansor, who not only encouraged but culti vated the sciences. His son Haran-al-Rashicl extended to them the same liberal protection ; and in an embassy which he sent to Charlemagne in 799, he presented that monarch with an ingenious clepsydra, or water clock, of the most elegant workmanship. It was not till the reign of Almamon, however, the youngest son of Al-Rashid, that astronomy received any important accessions from the Arabs. This illustrious prince invited to his capital the most distinguished philosophers of the age ; and Christians, Mahometans, and Jews, were all assembled in the academy which he formed at Bagdad. While a war of persecution was raging among the Christians of the West, universal toleration reigned throughout the dominions of Almamon ; and the rays of science were gilding the walls of Bagdad, when they had left, in ut ter darkness, the highest mountains of Europe. Even the force of arms was employed by the Saracen con queror in the cause of science. One of the conditions of peace which he imposed on the Greek emperor Mi chael III. was, the surrender of all the Greek manu scripts possessed by the emperor of Constantinople. The Almagest of Ptolemy, which happened to be among the manuscripts, was, by the command of Almamon, translated into the Arabic language, and an original treatise on astronomy, which still exists, was composed, under the title of ?stronomia elaborata a compluribus D. D. jussu Regis iUaimon. By means of great instruments constructed for the purpose, Almamon determined the obliquity of the ecliptic, and employed the ablest geome ters to measure a degree of the meridian on an extensive plain in Mesopotamia. See, ALMAMON and ALMANSOR.
During the reign of Almamon, the science of astro nomy was cultivated by Allraganus, Thebit-Ibn-Chora, and Albategnius. Aifraganus wrote an elementary work on astronomy, and a treatise on sun dials, and on the astrolabe ; and from the facility with which he perform ed the most complicated calculations, he received the name of the Calculator. Thebit-Ibn-Chora determined the length of the year to be 365 days 6h. 9' 12". Trust ing to erroneous observations, he imagined that the mo tion of the equinoctial points was alternately direct and retrogade ; and from this motion of the equinoxes, he concluded that the obliquity of the ecliptic was varia ble, and sustained an alternate increase and diminution.