ON THE HISTORY OF ASTRONOMT. 59S 



in the calculation of the sun's place, to which that of the stars was referred ; 

 but upon this supposition, he must also have been mistaken in three obser- 

 vations of the place of tiie equinoctial points. Ptolemy's principal work is 

 his mathematical system of astronomy, M-hich was afterwards called the great 

 syntax or body of astronomy, and is at present frequently quoted by the 

 Arabic name Almagest. He also wrote a treatise on optics, in which the 

 phenomena of atmospherical refraction are described, and which is extant 

 in manuscript in the National library at Paris. (Plate XXXVIII. Fig. 

 528.; 



Ptolemy was the last as well as the greatest of the Alexandrian astronomers* 

 and the science made no further progress till the time of the xArabians. The 

 first of these was Almamoun, was the son of the celebrated Aaron Reschid; 

 he reigned at Bagdad in 814, and having conquered the Greek emperor, 

 Michael the Third, he made it a condition of peace, that a copy of the works 

 of each of the best Greek authors should be delivered to him ; and among 

 them were the works of Ptolemy, of which he procured an Arabic transla- 

 tion. Almamoun also observed the obliquity of the ecliptic, and measured 

 the length of a degree in the plains of Mesopotamia. 



Among the astronomers protected by this prince and his successors, Al- 

 bategni was the most eminent. He ascertained with great accuracy, in 880, 

 the eccentricity of the solar motion, and discovered, the change of the place of 

 the sun's apogee, or of the earth's aphelion. 



Ibn Junis-made his observations at Cairo, about the year 1000; he was a 

 very assiduous astronomer, and determined the length of the year within 

 2 seconds of the truth. At this time the Arabians were in the liabit of em- 

 ploying, in their observations, the vibrations of a pendulum. 



The Persians soon after applied themselves to astronomy; and in the 

 eleventh century they invented the approximation of reckoning 8 bissextiles- 

 in 33 years, which was afterwards proposed by Dominic Cassini as an im- 

 provement of the Gregorian calendar. The most illustrious of this nation- 

 was Ulugh IJeigh, who observed in his capital Samarcand, about the year 

 1437| with very elaborate iostrumeuts. In the mean time Cocheouking. had 



