162 SCIENCE IN GRECO-ROMAN ANTIQUITY 



roughly indicated as follows : in the first phase 

 astronomy is entirely confused with meteorology ; in 

 the second, the physical and geometrical hypotheses 

 which it needs are distinguished more or less clearly ; 

 in the third and last phase an attempt is made to give 

 a mathematical representation as exact as possible of 

 the movement of the heavenly bodies. 



1. METEOROLOGICAL IDEAS 



As long as the earth and the sky were regarded as 

 being situated on the confines of one another, celes- 

 tial phenomena were assimilated to meteorological 

 phenomena and an explanation of the former was 

 sought in the latter. The meteorological ideas them- 

 selves were very confused. Vapour was simply con- 

 densed air. Furthermore up to the eighth century 

 B.C. darkness was considered as a material thing, 

 composed of vapour. Heraclitus, for instance, affirms 

 that darkness is a concrete vapour, which, rising from 

 the sea and the bottom of the valleys, is able by its 

 aqueous nature to extinguish the sun. Plato likewise 

 makes the Pythagorean Timaeus say that fog and 

 darkness are condensed air [Timaeus, 58 D, 2). The air 

 possesses different properties according as it is hot or 

 cold : in the first case it is light and mobile ; in the 

 second it is heavy and stable. On the other hand, when 

 it is compressed in the form of vapour, it is partially 

 changed into invisible fire which suddenly bursts forth 

 as lightning, when, for lack of compression, the cloud 

 is rent. For a long time the Greeks, like the Chaldeans 

 and the Hebrews, regarded daylight as distinct from 

 sunlight. Shadow even had a concrete reality of its 

 own, it was not a function of light ; it was only 

 strengthened by its opposition to light. These ideas 

 persisted until the time of Empedocles, when the 

 reflection of light and the true nature of vapour, 

 shadow, and darkness were discovered. 



