CHAPTER II 

 ASTRONOMY 



FROM its beginnings Greek Astronomy, like 

 Geometry, sought to model itself after the 

 type of a rational science ; having to explain 

 physical facts, it tried to do so by physical causes, 

 that is to say causes of the same nature as these facts. 



To primitive peoples, celestial phenomena are divine, 

 that is, they depend entirely on the more or less 

 capricious will of divinities. Doubtless, as we have 

 seen, the Egyptians and Chaldeans already possessed 

 some amount of astronomical knowledge, but this 

 knowledge consisted, after all, in ascertaining the 

 periodicity of celestial phenomena, without giving any 

 explanation of these. 



From the first, Greek astronomy launched out in 

 another direction, as the works of the Ionian school 

 show. These works appear incredibly daring if we 

 compare them with the religious beliefs of the Chaldeans 

 and Egyptians. 



Thales, for example, lays down as a principle that 

 water is the unique element from which all things arise 

 by the action of purely physical causes, for water can 

 be solidified into ice, be changed into vapour, that is, 

 air, etc. Having once laid down this principle, Thales 

 deduces from it a cosmology which, in spite of its 

 childish simplicity, remains physically rational. 



However it was only with difficulty that Greek 

 astronomy succeeded in specifying its ideal and object. 

 It passed through a series of stages which may be 



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