DECLINE OF ALEXANDRIAN SCIENCE 117 



the earth a negligible factor. Another reference to Aristarchus, 

 in Plutarch, mentions an opinion that he 



ought to be accused of impiety for moving the hearth of the world, 

 as the man in order to save the phenomena supposed that the heavens 

 stand still and the earth moves in an oblique circle at the same time as 

 it turns round its axis. 



How far this remarkable anticipation of the Copernican theory 

 was a conviction rather than a mere fortunate speculation cannot be 

 known, but at any rate it failed of that acceptance necessary to its 

 permanence. In the next century the rotation of the earth on 

 its axis was indeed taught by Seleucus, an Asiatic astronomer, but 

 it was 1700 years before these daring theories were again advanced. 

 Seleucus also observed the tides, saying "that the revolution of 

 the moon is opposed to the earth's rotation, but the air between the 

 two bodies being drawn forward falls upon the Atlantic Ocean, and 

 the sea is disturbed in proportion." 



PLANETARY IRREGULARITIES. - - The earlier theory of homocen- 

 tric spheres, while accounting more or less successfully for the ap- 

 parent motions of the heavenly bodies, had maintained each of 

 them at a constant distance from the earth, and thus quite failed 

 to explain the differences of brightness which were soon discovered, 

 as well as the variations in the apparent size of the moon. The 

 conception of motion in neither a straight line nor a circle was re- 

 pugnant to the Greek philosophers, and the difficulty was therefore 

 met, first by supposing the earth not to be exactly at the centre of 

 the circular orbits about it, second by introducing subsidiary 

 circles or epicycles. 



EXCENTRIC CIRCULAR ORBITS. The complete planetary system 

 according to the excentric circle theory was therefore as follows. In 

 the centre of the universe the earth, round which moved the moon 

 in 27 days, and the sun in a year, probably in concentric circles. 

 Mercury and Venus moved on circles, the centres of which were al- 

 ways on the straight line from the earth to the sun, so that the earth 

 was always outside these circles, for which reason the two planets 

 are always within a certain limited angular distance of the sun, from 



