D'Arcy Thompson on Plato and Planets
PLATO'S THEORY OF THE PLANETS
There is a passage of great interest to the astronomer and of well-known difficulty to the classical scholar, in Plato's allegory of the Vision of Er, where the Spirits survey the heavens and the earth and all the harmonious motions of the universe. They see the heavenly bodies set in revolving whorls, which, whorl within whorl, combine to form the Spinning-whorl on the Spindle of Necessity; and the Goddess holds the spindle on her knee, and spins the thread which the Fates wind, unwind and cut.
The whorls are described as follows:
[Now the first and outermost whorl had the broadest circular rim, that of the sixth was second, and third was that of the fourth, and fourth was that of the eighth, fifth that of the seventh, sixth that of the fifth, seventh that of the third, eighth that of the second.]
The heavenly bodies, or the spheres or whorls in which they lie, are arranged one within another in the following order:
- The Fixed Stars.
- The Sun.
- The Moon.
- The Fixed Stars.
- The Moon.
- The Sun.
The spheres of Eudoxus have been described and explained by many astronomers and historians of astronomy, but best, and far best of all, by Schiaparelli. Let me epitomize as best I can, and as far as need be, the Italian astronomer's interpretation. To account for the apparent movements of the Sun and Moon Eudoxus postulates three concentric spheres in each case, and four spheres for each of the other planetary bodies. The two outermost spheres are alike in all, Sun and Moon included. The first or outermost is identical, or coaxial, with the sphere of the fixed stars; its poles are the poles of heaven, and all the heavenly bodies share in its uniform rotation, which is that of night and day. The second sphere, sharing in the revolution of the first but with its own proper motion superadded, revolves about the axis of the ecliptic; its poles are fixed within the former sphere at points corresponding with the poles of the ecliptic, and its rotation carries Sun or Moon or planet along its oblique pathway, through the circle of the zodiacal signs. The second spheres, then, are all alike in configuration and orientation, but differ in their speed of rotation: that of the Moon revolving in a month, that of the Sun in a year, and those of the planets according to their own proper orbital periods.
Without recapitulating further details for Sun and Moon, we come to a very great difficulty in the case of the planets, namely to explain, by any simple imaginary mechanism, what are known as their stations and retrogradations. For the planets pursue their apparent course around the zodiac not with an even movement but in a jerky, sinuous way, alternately accelerated and reversed.
Eudoxus, who is said to have been the first to analyse the apparent motion of a planet into a system of interlinked figures-of-eight, and who gave to this figure-of-eight curve the name 'hippopede,' accounted for this peculiar motion in a very masterly way, without departing from the general plan of a system of concentric spheres. In addition to the two spheres already indicated, he conceived each planet to be controlled by two other concentric spheres, and to be set upon the equator of the fourth or innermost. The third sphere had its poles in the equator of the second, that is to say in the plane of the ecliptic, and therefore revolved, approximately, in a north and south direction. The fourth sphere, placed within it and revolving with equal and opposite velocity, counteracted the motion of the third and prevented the planet being carried, as the third would of itself have carried it, to first one and then the other pole of the ecliptic. But the fourth sphere was not coaxial with the third, but was set somewhat obliquely to it (just as the first sphere was to the second), and thus introduced another component in the form of a simple harmonic motion, causing the planet to perform apparently a pendulum-like vibration in the plane of the ecliptic, while all the while it was being carried around that circle by the proper motion of the second sphere. The phase of this vibration is obviously conditioned by the angle which the axis of the fourth sphere bears to that of the third; and Eudoxus tells us that these angles differ for the different planets, though he does not tell us what they severally were. The calculation, a very simple one, has been performed by Schiaparelli, who shows the angles to be as follows:
|Venus||. . .||46° |
|Mars||. . .||34° |
|Mercury||. . .||23° |
|Jupiter||. . .||13°|
|Saturn||. . .||6°|
Now these numbers place the planets, other than the Sun and Moon, in the precise order of Plato's text. Moreover, though not identical with these motions of the planets properly so called, the excursions of the Sun and Moon to either side of the equator, across the intertropic belt, are in a way comparable to them: so far at least, that they might with equal truth be described as [ motions in breadth], or ascribed to a range of freedom on either side of a theoretic orbit; and the analogy is all the better seen when we think of the identical mechanism of axial obliquities to which in each case, according to Eudoxus, the phenomenon is due. We may be justified in including in our series the angle of the ecliptic for the Sun, that is to say about 23° 28', or rather, if we assume the epoch of Eudoxus (as Dr Copeland suggested to me long ago) about 23° 45': which angle, in the [ artificial sphere] of Eudoxus, is the angle which the axis of the Sun's second sphere makes with that of his first; and in including for the Moon an angle which, as Eudoxus tells us, is in her case somewhat greater than the Sun's, in fact about 5° more. Then, having intercalated these two planets with their appropriate angles, we find the whole seven to follow the precise order of Plato's description; and these seven planets might be arranged in five thousand different ways!
As for the Fixed Stars, they extend over the whole surface of their sphere, or in other words the breadth of their whorl is a zone of 180°. So we at least should say; or perhaps, as the ancients knew nothing of the antarctic stars, which were permanently invisible to them, it might be more reasonable to assume a zone or whorl some 30° less. As our final result, our whole eight whorls with their respective breadths fall as follows into the Platonic order:
|1.||The Fixed Stars||. . .||130° - 180°|
|2.||Venus||. . .||46°|
|3.||Mars||. . .||34°|
|4.||The Moon||. . .||28°|
|5.||The Sun||. . .||23° 45'|
|6.||Mercury||. . .||23°|
|7.||Jupiter||. . .||13°|
|8.||Saturn||. . .||6°|
A chief source of obscurity in the whole passage is the simple circumstance that Plato was talking in riddles and in allegory, after the fashion of antiquity and the East, and therefore did not choose to tell us many things which he must have known. But he was utilizing, and simplifying for the purposes of his allegory, all the astronomic knowledge of his time. Plato never supposed that a single sphere could account for all the complicated motions of each planet. He omitted from his description, I take it, all those spheres, the [spheres, the ones carrying no planets], which merely bore other and dependent spheres, but which themselves bore no heavenly body: even to the vision of the Spirits these spheres were invisible. He suppresses all those portions of the spheres into which the planetary orbits never range, and by so doing he reduces them to his [circular whorl], or 'whorls'.
He draws his picture for us of what the eye does not see but insight discerns - the [true embroidery], the [true figures] of the celestial fabric. [There are also heavens beyond the perceptible heavens]. It is this transcendental heaven which is brought down to our comprehension under the guise of homely allegory: [One must use the model of embroidery of the heavens for the sake of learning these things].
We cannot wholly elucidate what Plato left purposely obscure. We cannot reconstruct his system of spheres or whorls as we can those of Eudoxus or Callippus, for which material is expressly and carefully supplied: except in so far as we are justified in illustrating the Master's words by help of the pupil's. But so reading Plato's riddle in the light of Eudoxus' facts we cannot doubt that we have before us two settings of the same story, nor fail to see veiled in Plato's poetic language the finest technical knowledge of the age.
While writing on this subject let me add, in parenthesis, that the very ancient and very decorative 'Greek key-pattern' seems to me to be nothing more nor less than an archaic representation of a planet's apparent course, a series of simplified hippopedes. It is, indeed, intermediate between the typical hippopede of Saturn or of Jupiter and the open one of Mercury, and is just such a figure as would be devised and used to depict the retrogressional course of a planet, prior to its more perfect representation by means of the hippopede itself.
JOC/EFR April 2007
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