176 THE GREEK ASTRONOMY. 



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I shall here close my account of the astronomical progress of the 

 Greek School. My purpose is only to illustrate the principles on which 

 the progress of science depends, and therefore I have not at all pre- 

 tended to touch upon every part of the subject. Some portion of the 

 ancient theories, as, for instance, the mode of accounting for the motions 

 of the moon and planets in latitude, are sufficiently analogous to -what 

 has been explained, not to require any more especial notice. Other 

 parts of Greek astronomical knowledge, as, for instance, their acquaint- 

 ance with refraction, did not assume any clear or definite form, and 

 can only be considered as the prelude to modern discoveries on the 

 same subject. And before we can with propriety pass on to these, 

 there is a long and remarkable, though unproductive interval, of which 

 some account must be given. 



Sect. 8. Arabian Astronomy. 



THE interval to which I have just alluded may be considered as ex 

 tending from Ptolemy to Copernicus ; we have no advance in Greek 

 astronomy after the former; no signs of a revival of the power of dis- 

 covery till the latter. During this interval of 1350 years, 39 the princi- 

 pal cultivators of astronomy were the Arabians, who adopted this 

 science from the Greeks whom they conquered, and from whom the 

 conquerors of western Europe again received back their treasure, when 

 the love of science and the capacity for it had been awakened in their 

 minds. In the intervening time, the precious deposit had undergone 

 little chano-e. The Arab astronomer had been the scrupulous but 



O 



unprofitable servant, who kept his talent without apparent danger 

 of loss, but also without prospect of increase. There is little in Ara- 



cycle was to the Periodic Time of the Epicyclical Centre on the Deferent, as the 

 synodlcal Revolution of the Planet to the tropical Revolution of the Earth above 

 the Sun. For the three superior Planets, Mars, Jupiter, and Saturn, the Radius of 

 the Deferent was equal to the Radius of the P.lanet's orbit, and the Radius of the 

 Epicycle was equal to the Radius of the Earth's orbit ; the Periodic Time on the 

 Planet in its Epicycle was to the Periodic Time of the Epicyclical Centre on the 

 Deferent, as the synodlcal Revolution of the Planet to the tropical Revolution of the 

 same Planet. 



Ptolemy might obviously have made the geometrical motions of all the Planets 

 correspond with the observations by one of these two modes of construction ; but 

 he appears to have adopted this double form of the theory, in order that in the 

 inferior, as well as in the superior Planets, he might give the smaller of the two 

 Radii to the Epicycle : that is, in order that he might make the smaller circle move 

 round the larger, not vice versa. Littrow's Notes. 



" Ptolemy died about A. D. 150. Copernicus was living A. D. 1500. 



