PLATO.

We come now to Plato (427–347 B.C.). In the astronomy of Plato, as we find it in the Dialogues, there is so large an admixture of myth and poetry that it is impossible to be sure what his real views were on certain points of detail. In the Phædo we have certain statements about the earth to the effect that it is of very large dimensions, the apparent hollow (according to Plato) in which we live being a very small portion of the whole, and that it is in the middle of the heaven, in equilibrium, without any support, by virtue of the uniformity in the substance of the heaven. In the Republic we have a glimpse of a more complete astronomical system. The outermost revolution is that of the sphere of the fixed stars, which carries round with it the whole universe including the sun, moon and planets; the latter seven bodies, while they are so carried round by the general rotation, have slower revolutions of their own in addition, one inside the other, these revolutions being at different speeds but all in the opposite sense to the general rotation of the universe. The quickest rotation is that of the fixed stars and the universe, which takes place once in about twenty-four hours. The slower speeds of the sun, moon and planets are not absolute but relative to the sphere of the fixed stars regarded as stationary. The earth in the centre is unmoved; the successive revolutions about it and within the sphere of the fixed stars are (reckoning from the earth outwards) those of the moon, the sun, Venus, Mercury, Mars, Jupiter, Saturn; the speed of the moon is the quickest, that of the sun the next quickest, while Venus and Mercury travel with the sun and have the same speed, taking about a year to describe their orbits; after these in speed comes Mars, then Jupiter and, last and slowest of all, Saturn. There is nothing said in the Republic about the seven bodies revolving in a circle different from and inclined to the equator of the sphere of the fixed stars; that is, the obliquity of the ecliptic does not appear; hence the standpoint of the whole system is that of Pythagoras as distinct from that of the Pythagoreans.

Plato’s astronomical system is, however, most fully developed in the Timæus. While other details remain substantially the same, the zodiac circle in which the sun, moon and planets revolve is distinguished from the equator of the sphere of the fixed stars. The latter is called the circle of the Same, the former that of the Other, and we are told (quite correctly) that, since the revolution of the universe in the circle of the Same carries all the other revolutions with it, the effect on each of the seven bodies is to turn their actual motions in space into spirals. There is a difficulty in interpreting a phrase in Plato’s description which says that Venus and Mercury, though moving in a circle having equal speed with the sun, “have the contrary tendency to it”. Literally this would seem to mean that Venus and Mercury describe their circles the opposite way to the sun, but this is so contradicted by observation that Plato could hardly have maintained it; hence the words have been thought to convey a vague reference to the apparent irregularities in the motion of Venus and Mercury, their standings-still and retrogradations.

But the most disputed point in the system is the part assigned in it to the earth. An expression is used with regard to its relation to the axis of the heavenly sphere which might mean either (1) that it is wrapped or globed about that axis but without motion, or (2) that it revolves round the axis. If the word means revolving about the axis of the sphere, the revolution would be either (a) rotation about its own axis supposed to be identical with that of the sphere, or (b) revolution about the axis of the heavenly sphere in the same way that the sun, moon and planets revolve about an axis obliquely inclined to that axis. But (a) if the earth rotated about its own axis, this would make unnecessary the rotation of the sphere of the fixed stars once in twenty-four hours, which, however, is expressly included as part of the system. The hypothesis (b) would make the system similar to the Pythagorean except that the earth would revolve about the axis of the heavenly sphere instead of round the central fire. The supporters of this hypothesis cite two passages of Plutarch to the effect that Plato was said in his old age to have repented of having given the earth the middle place in the universe instead of placing it elsewhere and giving the middle and chiefest place to some worthier occupant. It is a sufficient answer to this argument that, if Plato really meant in the passage of the Timæus to say that the earth revolves about the axis of the heavenly sphere, he had nothing to repent of. We must therefore, for our part, conclude that in his written Dialogues Plato regarded the earth as at rest in the centre of the universe.

We have it on good authority that Plato set it as a problem to all earnest students “to find what are the uniform and ordered movements by the assumption of which the apparent movements of the planets can be accounted for”. The same authority adds that Eudoxus was the first to formulate a theory with this object; and Heraclides of Pontus followed with an entirely new hypothesis. Both were pupils of Plato and, in so far as the statement of his problem was a stimulus to these speculations, he rendered an important service to the science of astronomy.