312 
PLATO 
epicycle (which is from east to west) is in the contrary sense 
to the motion of Venus and Mercury on their epicycles 
respectively (which is from west to east) 1 ; and this would 
he a satisfactory explanation if Plato could be supposed to 
have been acquainted with the theory of epicycles. But the 
probabilities are entirely against the latter supposition. All, 
therefore, that can be said seems to be this. Heraclides of 
Pontus, Plato’s famous pupil, is known on clear evidence to 
have discovered that Venus and Mercury revolve round the 
sun like satellites. He may have come to the same conclusion 
about the superior planets, but this is not certain; and in any 
case he must have made the discovery with reference to 
Mercury and Venus first. Heraclides’s discovery meant that 
Venus and Mercury, while accompanying the sun in its annual 
motion, described what are really epicycles about it. Now 
discoveries of this sort are not made without some preliminary 
seeking, and it may have been some vague inkling of the 
truth that prompted the remark of Plato, whatever the precise 
meaning of the words. 
The differences between the angular speeds of the planets 
account for the overtakings of one planet by another, and 
the combination of their independent motions with that of the 
daily rotation causes one planet to appear to be overtaking 
another when it is really being overtaken by it and vice 
versa. 2 The sun, moon and planets are instruments for 
measuring time. 3 Even the earth is an instrument for making- 
night and day by virtue of its not rotating about its axis, 
while the rotation of the fixed stars carrying the sun with 
it is completed once in twenty-four hours; a month has passed 
when the moon after completing her own orbit overtakes the 
sun (the ‘ month ’ being therefore the synodic month), and 
a year when the sun has completed its own circle. According 
to Plato the time of revolution of the other planets (except 
Venus and Mercury, which have the same speed as the sun) 
had not been exactly calculated; nevertheless the Perfect 
Year is completed ‘ when the relative speeds of all the eight 
revolutions [the seven independent revolutions and the daily 
rotation] accomplish their course together and reach their 
1 Chalcidius on Timaeus, cc. 81, 109, 112.. 2 Timaeus, 39 a. 
3 lb. 41 E, 42 n.