$ so] Aristotle 33 



present century by the discovery that such a motion of 

 the stars can be seen in a few cases, though owing to the 

 almost inconceivably great distance of the stars the motion 

 is imperceptible except by extremely refined methods of 

 observation (cf. chapter xin., 278, 279). The question 

 of the distances of the several celestial bodies is also 

 discussed, and Aristotle arrives at the conclusion that the 

 planets are farther off than the sun and moon, supporting 

 his view by his observation of an occultation of Mars by 

 the moon (i.e. a passage of the moon in front of Mars), and 

 by the fact that similar observations had been made in the 

 case of other planets by Egyptians and Babylonians. It 

 is, however, difficult to see why he placed the planets 

 beyond the sun, as he must have known that the intense 

 brilliancy of the sun renders planets invisible in its neigh- 

 bourhood, and that no occultations of planets by the sun 

 could really have been seen even if they had been reported 

 to have taken place. He quotes also, as an opinion of 

 " the mathematicians," that the stars must be at least nine 

 times as far off as the sun. 



There are also in Aristotle's writings a number of astro- 

 nomical speculations, founded on no solid evidence and of 

 little value ; thus among other questions he discusses the 

 nature of comets, of the Milky Way, and of the stars, why 

 the stars twinkle, and the causes which produce the various 

 celestial motions. 



In astronomy, as in other subjects, Aristotle appears 

 to have collected and systematised the best knowledge of 

 the time ; but his original contributions are not only not 

 comparable with his contributions to the mental and moral 

 sciences, but are inferior in value to his work in other 

 natural sciences, e.g. Natural History. Unfortunately the 

 Greek astronomy of his time, still in an undeveloped state, 

 was as it were crystallised in his writings, and his great 

 authority was invoked, centuries afterwards, by comparatively 

 unintelligent or ignorant disciples in support of doctrines 

 which were plausible enough in his time, but which subse- 

 quent research was shewing to be untenable. The advice 

 which he gives to his readers at the beginning of his ex- 

 position of the planetary motions, to compare his views 

 with those which they arrived at themselves or met with 



