456 LECTURE XLVIII. 



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

 the phenomena of atmospherical refraction are described, and which is 

 extant is manuscript in the National library at Paris.* (Plate XXXVIII. 

 Fig. 528.) 



Ptolemy was the last as well as the greatest of the Alexandrian astrono- 

 mers, and the science made no further progress till the time of the Arabians. 

 The first of these was Almamoun, 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 

 translation. 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, 

 Albategni 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 habit of 

 employing, 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 bissex- 

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

 improvement of the Gregorian calendar. The most illustrious of this 

 nation was Ulugh Beigh, who observed in his capital Samarcand, about 

 the year 1437, with very elaborate instruments. In the mean time 

 Cocheouking had made in China some very accurate observations, which 

 are valuable for the precision with which they ascertain the obliquity of 

 the ecliptic : their date is about 1278. 



It was not long after the time of Ulugh Beigh, that Copernicus laid the 

 foundation of the more accurate theories which modern improvements have 

 introduced into astronomy. Dissatisfied with the complicated hypotheses 

 of the Ptolemaean system, he examined the works of the ancients, in quest 

 of more probable opinions. He found from Cicero that'Nicetas and other 

 Pythagoreans had maintained, that the sun is placed in the centre of the 

 system, and that the earth moves round him in common with the other 

 planets. He applied this idea to the numerous observations which the 

 diligence of astronomers had accumulated, and he had the satisfaction to 

 find them all in perfect conformity with this theory. He quickly discarded 

 the Ptolemaean epicycles, imagined in order to explain the alternations of 

 the direct and retrograde motions of the planets; in these remarkable 

 phenomena, ^.Copernicus saw nothing but the consequences necessarily 

 produced by the combination of the motions of the earth and planets round 



* Composition Mathematique, Gr. et Fr. 2 vols. 4to, Paris, 1813. See also 

 Table Chronologique des Regnes, &c. trad, de 1'Allemagne de M. Ideler par Halma, 

 4to, Paris, 1819. Hypotheses et Epoques des Planetes de Ptol. &c. ibid. 4to, 1820.' 

 Tables Manuelles Astron. Gr. et Fr. 4to, 1823. 



t See Halley's Dissertation, in Ph. Tr. xvii. 913. 



