The astronomy of Plato is based on the two principles of the same
and the other, which God combined in the creation of the world.
The soul, which is compounded of the same, the other, and the
essence, is diffused from the centre to the circumference of the
heavens. We speak of a soul of the universe; but more truly
regarded, the universe of the Timaeus is a soul, governed by
mind, and holding in solution a residuum of matter or evil, which
the author of the world is unable to expel, and of which Plato
cannot tell us the origin. The creation, in Plato’s sense, is
really the creation of order; and the first step in giving order
is the division of the heavens into an inner and outer circle of
the other and the same, of the divisible and the indivisible,
answering to the two spheres, of the planets and of the world
beyond them, all together moving around the earth, which is their
centre. To us there is a difficulty in apprehending how that
which is at rest can also be in motion, or that which is
indivisible exist in space. But the whole description is so ideal
and imaginative, that we can hardly venture to attribute to many
of Plato’s words in the Timaeus any more meaning than to his
mythical account of the heavens in the Republic and in the
Phaedrus. (Compare his denial of the ‘blasphemous opinion’ that
there are planets or wandering stars; all alike move in
circles—Laws.) The stars are the habitations of the souls of men,
from which they come and to which they return. In attributing to
the fixed stars only the most perfect motion—that which is on the
same spot or circulating around the same—he might perhaps have
said that to ‘the spectator of all time and all existence,’ to
borrow once more his own grand expression, or viewed, in the
language of Spinoza, ‘sub specie aeternitatis,’ they were still
at rest, but appeared to move in order to teach men the periods
of time. Although absolutely in motion, they are relatively at
rest; or we may conceive of them as resting, while the space in
which they are contained, or the whole anima mundi, revolves.
The universe revolves around a centre once in twenty-four hours,
but the orbits of the fixed stars take a different direction from
those of the planets. The outer and the inner sphere cross one
another and meet again at a point opposite to that of their first
contact; the first moving in a circle from left to right along
the side of a parallelogram which is supposed to be inscribed in
it, the second also moving in a circle along the diagonal of the
same parallelogram from right to left; or, in other words, the
first describing the path of the equator, the second, the path of
the ecliptic. The motion of the second is controlled by the
first, and hence the oblique line in which the planets are
supposed to move becomes a spiral. The motion of the same is said
to be undivided, whereas the inner motion is split into seven
unequal orbits—the intervals between them being in the ratio of
two and three, three of either:—the Sun, moving in the opposite
direction to Mercury and Venus, but with equal swiftness; the
remaining four, Moon, Saturn, Mars, Jupiter, with unequal
swiftness to the former three and to one another. Thus arises the
following progression:—Moon 1, Sun 2, Venus 3, Mercury 4, Mars 8,
Jupiter 9, Saturn 27. This series of numbers is the compound of
the two Pythagorean ratios, having the same intervals, though not
in the same order, as the mixture which was originally divided in
forming the soul of the world.
Plato was struck by the phenomenon of Mercury, Venus, and the Sun
appearing to overtake and be overtaken by one another. The true
reason of this, namely, that they lie within the circle of the
earth’s orbit, was unknown to him, and the reason which he
gives—that the two former move in an opposite direction to the
latter—is far from explaining the appearance of them in the
heavens. All the planets, including the sun, are carried round in
the daily motion of the circle of the fixed stars, and they have
a second or oblique motion which gives the explanation of the
different lengths of the sun’s course in different parts of the
earth. The fixed stars have also two movements—a forward movement
in their orbit which is common to the whole circle; and a
movement on the same spot around an axis, which Plato calls the
movement of thought about the same. In this latter respect they
are more perfect than the wandering stars, as Plato himself terms
them in the Timaeus, although in the Laws he condemns the
appellation as blasphemous.
The revolution of the world around earth, which is accomplished
in a single day and night, is described as being the most perfect
or intelligent. Yet Plato also speaks of an ‘annus magnus’ or
cyclical year, in which periods wonderful for their complexity
are found to coincide in a perfect number, i.e. a number which
equals the sum of its factors, as 6 = 1 + 2 + 3. This, although
not literally contradictory, is in spirit irreconcilable with the
perfect revolution of twenty-four hours. The same remark may be
applied to the complexity of the appearances and occultations of
the stars, which, if the outer heaven is supposed to be moving
around the centre once in twenty-four hours, must be confined to
the effects produced by the seven planets. Plato seems to confuse
the actual observation of the heavens with his desire to find in
them mathematical perfection. The same spirit is carried yet
further by him in the passage already quoted from the Laws, in
which he affirms their wanderings to be an appearance only, which
a little knowledge of mathematics would enable men to correct.
We have now to consider the much discussed question of the
rotation or immobility of the earth. Plato’s doctrine on this
subject is contained in the following words:—‘The earth, which is
our nurse, compacted (OR revolving) around the pole which is
extended through the universe, he made to be the guardian and
artificer of night and day, first and eldest of gods that are in
the interior of heaven’. There is an unfortunate doubt in this
passage (1) about the meaning of the word (Greek), which is
translated either ‘compacted’ or ‘revolving,’ and is equally
capable of both explanations. A doubt (2) may also be raised as
to whether the words ‘artificer of day and night’ are consistent
with the mere passive causation of them, produced by the
immobility of the earth in the midst of the circling universe. We
must admit, further, (3) that Aristotle attributed to Plato the
doctrine of the rotation of the earth on its axis. On the other
hand it has been urged that if the earth goes round with the
outer heaven and sun in twenty-four hours, there is no way of
accounting for the alternation of day and night; since the equal
motion of the earth and sun would have the effect of absolute
immobility. To which it may be replied that Plato never says that
the earth goes round with the outer heaven and sun; although the
whole question depends on the relation of earth and sun, their
movements are nowhere precisely described. But if we suppose,
with Mr. Grote, that the diurnal rotation of the earth on its
axis and the revolution of the sun and outer heaven precisely
coincide, it would be difficult to imagine that Plato was unaware
of the consequence. For though he was ignorant of many things
which are familiar to us, and often confused in his ideas where
we have become clear, we have no right to attribute to him a
childish want of reasoning about very simple facts, or an
inability to understand the necessary and obvious deductions from
geometrical figures or movements. Of the causes of day and night
the pre-Socratic philosophers, and especially the Pythagoreans,
gave various accounts, and therefore the question can hardly be
imagined to have escaped him. On the other hand it may be urged
that the further step, however simple and obvious, is just what
Plato often seems to be ignorant of, and that as there is no
limit to his insight, there is also no limit to the blindness
which sometimes obscures his intelligence (compare the
construction of solids out of surfaces in his account of the
creation of the world, or the attraction of similars to
similars). Further, Mr. Grote supposes, not that (Greek) means
‘revolving,’ or that this is the sense in which Aristotle
understood the word, but that the rotation of the earth is
necessarily implied in its adherence to the cosmical axis. But
(a) if, as Mr Grote assumes, Plato did not see that the rotation
of the earth on its axis and of the sun and outer heavens around
the earth in equal times was inconsistent with the alternation of
day and night, neither need we suppose that he would have seen
the immobility of the earth to be inconsistent with the rotation
of the axis. And (b) what proof is there that the axis of the
world revolves at all? (c) The comparison of the two passages
quoted by Mr Grote (see his pamphlet on ‘The Rotation of the
Earth’) from Aristotle De Coelo, Book II (Greek) clearly shows,
although this is a matter of minor importance, that Aristotle, as
Proclus and Simplicius supposed, understood (Greek) in the
Timaeus to mean ‘revolving.’ For the second passage, in which
motion on an axis is expressly mentioned, refers to the first,
but this would be unmeaning unless (Greek) in the first passage
meant rotation on an axis. (4) The immobility of the earth is
more in accordance with Plato’s other writings than the opposite
hypothesis. For in the Phaedo the earth is described as the
centre of the world, and is not said to be in motion. In the
Republic the pilgrims appear to be looking out from the earth
upon the motions of the heavenly bodies; in the Phaedrus, Hestia,
who remains immovable in the house of Zeus while the other gods
go in procession, is called the first and eldest of the gods, and
is probably the symbol of the earth. The silence of Plato in
these and in some other passages (Laws) in which he might be
expected to speak of the rotation of the earth, is more
favourable to the doctrine of its immobility than to the
opposite. If he had meant to say that the earth revolves on its
axis, he would have said so in distinct words, and have explained
the relation of its movements to those of the other heavenly
bodies. (5) The meaning of the words ‘artificer of day and night’
is literally true according to Plato’s view. For the alternation
of day and night is not produced by the motion of the heavens
alone, or by the immobility of the earth alone, but by both
together; and that which has the inherent force or energy to
remain at rest when all other bodies are moving, may be truly
said to act, equally with them. (6) We should not lay too much
stress on Aristotle or the writer De Caelo having adopted the
other interpretation of the words, although Alexander of
Aphrodisias thinks that he could not have been ignorant either of
the doctrine of Plato or of the sense which he intended to give
to the word (Greek). For the citations of Plato in Aristotle are
frequently misinterpreted by him; and he seems hardly ever to
have had in his mind the connection in which they occur. In this
instance the allusion is very slight, and there is no reason to
suppose that the diurnal revolution of the heavens was present to
his mind. Hence we need not attribute to him the error from which
we are defending Plato.
After weighing one against the other all these complicated
probabilities, the final conclusion at which we arrive is that
there is nearly as much to be said on the one side of the
question as on the other, and that we are not perfectly certain,
whether, as Bockh and the majority of commentators, ancient as
well as modern, are inclined to believe, Plato thought that the
earth was at rest in the centre of the universe, or, as Aristotle
and Mr. Grote suppose, that it revolved on its axis. Whether we
assume the earth to be stationary in the centre of the universe,
or to revolve with the heavens, no explanation is given of the
variation in the length of days and nights at different times of
the year. The relations of the earth and heavens are so
indistinct in the Timaeus and so figurative in the Phaedo,
Phaedrus and Republic, that we must give up the hope of
ascertaining how they were imagined by Plato, if he had any fixed
or scientific conception of them at all.