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Plato is aware of the periodicity of celestial phenomena and of the relative velocities of the planets, which are the elements of contemporary Babylonian astronomy.
Plato, Timaeus 39a-e:
Now, when all the stars which were necessary to the creation of time had attained a motion suitable to them, - and had become living creatures having bodies fastened by vital chains, and learnt their appointed task, moving in the motion of the diverse, which is diagonal, and passes through and is governed by the motion of the same (= the sphere of the fixed stars), they revolved, some in a larger and some in a lesser orbit - those which had the lesser orbit revolving faster, and those which had the larger more slowly. Now by reason of the motion of the same, those which revolved fastest appeared to be overtaken by those which moved slower although they really overtook them; for the motion of the same made them all turn in a spiral, and, because some went one way and some another, that which receded most slowly from the sphere of the same, which was the swiftest, appeared to follow it most nearly (= Saturn). That there might be some visible measure of their relative swiftness and slowness as they proceeded in their eight courses, God lighted a fire, which we now call the sun, in the second from the earth of these orbits, that it might give light to the whole of heaven, and that the animals, as many as nature intended, might participate in number, learning arithmetic from the revolution of the same and the similar.
Thus then, and for this reason the night and the day were created, being the period of the one most intelligent revolution. And the month is accomplished when the moon has completed her orbit and overtaken the sun, and the year when the sun has completed his own orbit. Mankind, with hardly an exception, have not remarked the periods of the other stars, and they have no name for them, and do not measure them against one another by the help of number, and hence they can scarcely be said to know that their (= planets) wanderings, being infinite in number and admirable for their variety, make up time. And yet there is no difficulty in seeing that the perfect number of time fulfils the Perfect Year when all the eight revolutions, having their relative degrees of swiftness, are accomplished together and attain their completion at the same time, measured by the rotation of the same and equally moving. After this manner, and for these reasons, came into being such of the stars as in their heavenly progress received reversals of motion, to the end that the created heaven might imitate the eternal nature, and be as like as possible to the perfect and intelligible animal.
Source (list of abbreviations) (source links will open in a new browser window)
Plato, Timaeus 39a-e
Links (external links will open in a new browser window)
Cf. Plato on the celestial phenomena (1)
Cf. Plato on the celestial phenomena (2)
Cf. Plato on the celestial phenomena (4)
Amar Annus
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