Astronomy must also be taught up to a certain point, in order that the youth may imbibe correct belief respecting those great Divinities — Hêlios, Selênê, and the Planets — or may at any rate be protected from the danger of unconsciously advancing false affirmations about them, discreditable to their dignity. The general public consider it impious to study the Kosmos and the celestial bodies, with a view to detect the causes of what occurs:[256] while at the same time they assert that the movements of Hêlios and Selênê are irregular, and they call the planets Wanderers. Regular action is (in Plato’s view) the characteristic mark of what is good and perfect: irregularity is the foremost of all defects, and cannot without blasphemy be imputed to any of the celestial bodies. Moreover, many persons also assert untruly, that among the celestial bodies the one which is really the slowest mover, moves the fastest — and that the one which is really the fastest mover, moves the slowest. How foolish would it appear (continues Plato) if they made the like mistake about the Olympic runners, and if they selected the defeated competitor, instead of the victor, to be crowned and celebrated in panegyrical odes! How offensive is such falsehood, when applied to the great Gods in the heavens! Each of them has in reality one uniform circular movement, though they appear to have many and variable movements. Our youth must be taught enough of astronomy to guard against such heresies. The study of astronomy up to this point, far from being impious, is indispensable as a safeguard against impiety.[257] Plato intimates that these astronomical truths were of recent acquisition, even to himself.[258]
[256] Plato, Legg. vii. p. 821 A. We must observe that the Athenian (who here represents Plato himself) does not give this repugnance to astronomical study as his own feeling, but, on the contrary, as a prejudice from which he dissents. There is no ground, therefore, so far as this passage is concerned, for the charge of contradiction advanced by Velleius against Plato in Cicero De Nat. Deor. i. 12, 30.
[257] Plat. Legg. vii. pp. 821 B-822 C. καταψευδόμεθα νῦν, ὡς ἔπος εἰπεῖν, Ἕλληνες πάντες μεγάλων θεῶν, Ἡλίου τε ἅμα καὶ Σελήνης (821 B) … περὶ θεῶν τῶν κατ’ οὔρανον τούς γε ἡμετέρους πολίτας τε καὶ τοὺς νέους τὸ μέχρι τοσούτου μαθεῖν περὶ ἁπάντων τούτων, μέχρι τοῦ μὴ βλασφημεῖν περὶ αὐτά, εὐφημεῖν δὲ ἀεὶ θύοντας τε καὶ ἐν εὐχαῖς εὐχομένους εὐσεβως (821C-D). The five Planets were distinguished and named, and their periods to a certain extent understood, by Plato; but by many persons in his day the word Planet was understood more generally as comprehending all the celestial bodies, sun and moon among them — (except fixed stars) therefore comets also — τὰ μὴ ἐν τῇ αὐτῇ περιφορᾷ ὄντα, Xenoph. Memor. iv. 7, 5, where an opinion is ascribed to Sokrates quite opposed to that which Plato here expresses. See Schaubach, Geschichte der Astronomie, pp. 212-477.
[258] Plato, Legg. vii. pp. 819 D, 821 E.
This portion of the Leges is obscure, and would be hardly intelligible if it were not illustrated by a passage in the Timæus (p. 38). Even with such help it is difficult, and has been understood differently by different interpreters. Proklus (in Timæum, pp. 262-263) and Martin (Études sur le Timée, ii. note 36, p. 84) interpret it as alluding to the spiral line (ἕλικα) described by each planet (Sun and Moon are each counted as planets) round the Earth, arising from the combination of the force of the revolving sidereal sphere or Aplanês, carrying all the planets round along with it from East to West, with the counter-movement (contrary, but obliquely contrary) inherent in each planet. The spiral movement of each planet, resulting from combination of these two distinct forces, is a regular movement governed by law; though to an observer who does not understand the law, the movements appear irregular. Compare Derkyllides ap. Theon Smyrn. c. 41, f. 27, p. 330, ed. Martin.
The point here discussed forms one of the items of controversy between Gruppe and Boeckh, in the recent discussion about Plato’s astronomical views. Gruppe, Die Kosmischen Systeme der Griechen, pp. 157-168: Boeckh, Untersuchungen über das Kosmische System des Platon, pp. 45-57.
Gruppe has an ingenious argument to show that the novelty (παράδοξον) which Plato had in his mind, but was afraid to declare openly because of existing prejudices, was the heliocentric or Copernican system, which he believes to have been Plato’s discovery. Boeckh refutes Gruppe’s reasoning; and refutes it, in my judgment, completely. He sustains the interpretation given by Proklus and Martin.
Boeckh also illustrates (pp. 35-38-49-54), in a manner more satisfactory than Gruppe, the dicta of Plato about the comparative velocity of the Planets (Sun and Moon counted among them).
Plato declares the Moon to be the quickest mover among the planets, and Saturn to be the slowest. On the contrary Demokritus pronounced the Moon to be the slowest mover of all; slower than the Sun, because the Sun was farther from the Earth and nearer to the outermost or sidereal sphere. It was the rotation of this last-mentioned sphere (according to Demokritus) which carried round along with it the Sun, the Moon, and all the planets: the bodies near to it were more forcibly acted upon by its rotation, and carried round more rapidly, than the bodies distant from it — hence the Moon was the least rapid mover of all (Lucretius, v. 615-635. See Sir George Lewis’s Historical Survey of the Astronomy of the Ancients, ch. ii. pp. 139-140).
It appears to me probable that Plato, in the severe remarks which he makes on persons who falsely affirmed the quickest mover in the heavens to be the slowest, had in view these doctrines of Demokritus. Plato never once mentions Demokritus by name (see Mullach, Fragment. Demokrit. p. 25); but he is very sparing in mentioning by name any contemporaries. It illustrates the difference between the manner of Aristotle and Plato, that Aristotle frequently names Demokritus — seventy-eight times according to Mullach (p. 107) — even in the works which we possess.