26 A Short History of Astronomy [Cn. n. 



repugnant to uninstructed common sense, although presented 

 in such a crude form, without any of the evidence required 

 to win general assent, was, however, undoubtedly a valuable 

 contribution to astronomical thought. It is well worth 

 notice that Coppernicus in the great book which is the 

 foundation of modern astronomy (chapter iv., 75) especi- 

 ally quotes Philolaus and other Pythagoreans as authorities 

 for his doctrine of the motion of the earth. 



Three other Pythagoreans, belonging to the end of 

 the 6th century and to the 5th century B.C., Hicetas of 

 Syracuse, Heraditus, and Ecphantus, are explicitly mentioned 

 by later writers as having believed in the rotation of the 

 earth. 



An obscure passage in one of Plato's dialogues (the 

 Timaeus) has been interpreted by many ancient and modern 

 commentators as implying a belief in the rotation of the 

 earth, and Plutarch also tells us, partly on the authority 

 of Theophrastus, that Plato in old age adopted the belief 

 that the centre of the universe was not occupied by the 

 earth but by some better body.* 



Almost the only scientific Greek astronomer who believed 

 in the motion of the earth was Aristarchus of Samos, who 

 lived in the first half of the 3rd century B.C., and is best 

 known by his measurements of the distances of the sun 

 and moon ( 32). He held that the sun and fixed stars 

 were motionless, the sun being in the centre of the sphere 

 on which the latter lay, and that the enrth not only rotated 

 on its axis, but also described an orbit round the sun. 

 Seleucus of Seleucia, who belonged to the middle of the 

 2nd century B.C., also held a similar opinion. Unfor- 

 tunately we know nothing of the grounds of this belief in 

 either case, and their views appear to have found little 

 favour among their contemporaries or successors. 



It may also be mentioned in this connection that Aristotle 

 ( 27) clearly realised that the apparent daily motion of the 

 stars could be explained by a motion either of the stars or 

 of the earth, but that he rejected the latter explanation. 



25. Plato (about 428-347 B.C.) devoted no dialogue 

 especially to astronomy, but made a good many references 



* Theophrastus was born about half a century, Plutarch nearly 

 five centuries, later than Plato. 



