DECLINE OF ALEXANDRIAN SCIENCE 131 



nearly sufficient for the requirements of practical life, but in the matter 

 of astronomical theory and speculation, in which their best thinkers 

 were very much more interested than in the detailed facts, they re- 

 ceived virtually a blank sheet on which they had to write (at first with 

 indifferent success) their speculative ideas. A considerable interval 

 of time was obviously necessary to bridge over the gulf separating 

 such data as the eclipse observations of the Chaldeans from such 

 ideas as the harmonical spheres of Pythagoras; and the necessary 

 theoretical structure could not be erected without the use of mathemati- 

 cal methods which had gradually to be invented. That the Greeks, 

 particularly in early times, paid little attention to making observations, 

 is true enough, but it may fairly be doubted whether the collection 

 of fresh material for observations would really have carried astronomy 

 much beyond the point reached by the Chaldean observers. When 

 once speculative ideas, made definite by the aid of geometry, had 

 been sufficiently developed to be capable of comparison with observa- 

 tion, rapid progress was made. The Greek astronomers of the scientific 

 period, such as Aristarchus, Eratosthenes, and above all Hipparchus, 

 appear moreover to have followed in their researches the method which 

 has always been fruitful in physical science namely, to frame pro- 

 visional hypotheses, to deduce their mathematical consequences, and 

 to compare these with the results of observation. There are few better 

 illustrations of genuine scientific caution than the way in which Hip- 

 parchus, having tested the planetary theories handed down to him 

 and having discovered their insufficiency, deliberately abstained from 

 building up a new theory on data which he knew to be insufficient, 

 and patiently collected fresh material, never to be used by himself, 

 that some future astronomer might thereby be able to arrive at an 

 improved theory. 



Of positive additions to our astronomical knowledge made by the 

 Greeks the most striking in some ways is the discovery of the ap- 

 proximately spherical form of the earth, a result which later work has 

 only slightly modified. But then- explanation of the chief motions 

 of the solar system and their resolution of them into a comparatively 

 small number of simpler motions was, in reality, a far more important 

 contribution, though the Greek epicyclic scheme has been so re- 

 modelled, that at first sight it is difficult to recognize the relation be- 

 tween it and our modern views. The subsequent history will, however, 

 show how completely each stage in the progress of astronomical science 

 has depended on those that preceded. 



