§2.8#2ἔτι δʼ ἐπεὶ σφαιροειδῆ τὰ ἄστρα, καθάπερ οἵ τʼ ἄλλοι φασὶ καὶ ἡμῖν ὁμολογούμενον εἰπεῖν, ἐξ ἐκείνου γε τοῦ σώματος γεννῶσιν, τοῦ δὲ σφαιροειδοῦς δύο κινήσεις εἰσὶ καθʼ αὑτό, κύλισις καὶ δίνησις, εἴπερ οὖν κινοῖτο τὰ ἄστρα διʼ αὑτῶν, τὴν ἑτέραν ἂν κινοῖτο τούτων· ἀλλʼ οὐδετέραν φαίνεται.
Furthermore, since they generate the stars as spherical—just as others say and as is consistent for us to say—from that body, and since there are two proper motions of a sphere, namely rolling and spinning, if then the stars were to move by themselves, they would move with one of these; but they are seen to move with neither.
δινούμενα μὲν γὰρ ἔμενεν ἄν ἐν ταὐτῷ καὶ οὐ μετέβαλλε τὸν τόπον, ὅπερ φαίνεταί τε καὶ πάντες φασίν.
For if they spun, they would remain in the same place and would not change their position, which they are seen to do and everyone says they do.
ἔτι δὲ πάντα μὲν εὔλογον τὴν αὐτὴν κίνησιν κινεῖσθαι, μόνος δὲ δοκεῖ τῶν ἄστρων ὁ ἥλιος τοῦτο δρᾶν, ἀνατέλλων ἢ καὶ οὗτος οὐ διʼ αὑτὸν ἀλλὰ διὰ τὴν ἀπόστασιν τῆς ἡμετέρας ὄψεως· ἡ γὰρ ὄψις ἀποτεινομένη μακρὰν ἑλίσσεται διὰ τὴν ἀσθένειαν.
Furthermore, it is reasonable that all stars should move with the same motion, but of the stars only the sun seems to do this, when rising or [setting]—and indeed even this one does so not of itself, but because of the distance of our sight; for our sight, when stretched out over a long distance, wavers because of its weakness.
ὅπερ αἴτιον ἴσως καὶ τοῦ στίλβειν φαίνεσθαι τούς ἀστέρας τοὺς ἐνδεδεμένους, τούς δὲ πλάνητας μὴ στίλβειν· οἱ μὲν γὰρ πλάνητες ἐγγύς εἰσιν, ὥστʼ ἐγκρατὴς οὖσα πρὸς αὐτοὺς ἀφικνεῖται ἡ ὄψις· πρὸς δὲ τοὺς μένοντας κραδαίνεται διὰ τὸ μῆκος, ἀποτεινομένη πόρρω λίαν.
Which is perhaps also the cause of the bound stars appearing to twinkle, while the planets do not twinkle; for the planets are near, so that our sight reaches them while still strong; but towards the fixed ones it trembles because of the distance, being stretched too far away.
ὁ δὲ τρόμος αὐτῆς ποιεῖ τοῦ ἄστρου δοκεῖν εἶναι τὴν κίνησιν· οὐθὲν γὰρ διαφέρει κινεῖν τὴν ὄψιν ἢ τὸ ὁρώμενον.
And its trembling makes the motion seem to belong to the star; for it makes no difference whether the sight or the thing seen is moved.
ἀλλὰ μὴν ὅτι οὐδὲ κυλίεται τὰ ἄστρα, φανερόν· τὸ μὲν γὰρ κυλιόμενον στρέφεσθαι ἀνάγκη, τῆς δὲ σελήνης ἀεὶ δῆλόν ἐστι τὸ καλούμενον πρόσωπον.
But furthermore, that the stars do not roll either is clear; for that which rolls must turn, but the so-called face of the moon is always visible.
ὥστʼ ἐπεὶ κινούμενα μὲν διʼ αὑτῶν τὰς οἰκείας κινεῖσθαι κινήσεις εὔλογον, ταύτας δʼ οὐ φαίνεται κινούμενα, δῆλον ὅτι οὐκ ἂν κινοῖτο διʼ οὑτῶν.
So since, if they moved by themselves, it would be reasonable for them to move with their own proper motions, but they are not seen to move with these, it is clear that they would not move by themselves.
πρὸς δὲ τούτοις ἄλογον τὸ μηθὲν ὄργανον αὐτοῖς ἀποδοῦναι τὴν φύσιν πρὸς τὴν κίνησιν· οὐθὲν γὰρ ὡς ἔτυχε ποιεῖ ἡ φύσις, οὐδὲ τῶν μὲν ζώων φροντίσαι, τῶν δʼ οὕτω τιμίων ὑπεριδεῖν, ἀλλʼ ἔοικεν ὥσπερ ἐπίτηδες ἀφελεῖν πάντα διʼ ὧν ἐνεδέχετο προϊέναι καθʼ οὑτά, καὶ ὅτι πλεῖστον ἀποστῆσαι τῶν ἐχόντων ὄργανα πρὸς κίνησιν.
In addition to these things, it is unreasonable that nature should have given them no organ for motion; for nature does nothing at random, nor does she care for animals while overlooking things so honorable, but she seems as if on purpose to have taken away all means by which they could progress by themselves, and to have distanced them as far as possible from things that have organs for motion.
διὸ καὶ εὐλόγως ἂν δόξειεν ὅ τε ὅλος οὐρανὸς σφαιροειδὴς εἶναι καὶ ἕκαστον τῶν ἄστρων.
Therefore, it would also seem reasonable both that the whole heaven is spherical and each of the stars.
πρὸς μὲν γὰρ τὴν ἐν τῷ αὐτῷ κίνησιν ἡ σφαῖρα τῶν σχημάτων χρησιμώτατον (οὕτω γὰρ ἂν καὶ τάχιστα κινοῖτο καὶ μάλιστα κατέχοι τὸν αὐτὸν τόπον), πρὸς δὲ τὴν εἰς τὸ πρόσθεν ἀχρηστότατον· ἥκιστα γὰρ ὅμοιον τοῖς διʼ αὐτῶν κινητικοῖς· οὐδὲν γὰρ ἀπηρτημένον ἔχει οὐδὲ προέχον, ὥςπερ τὸ εὐθύγραμμον, ἀλλὰ πλεῖστον ἀφέστηκε τῷ σχήματι τῶν πορευτικῶν σωμάτων.
For, on the one hand, for motion in the same place, the sphere is the most useful of shapes (for in this way it would move most swiftly and best keep the same place), but, on the other hand, for forward motion, it is the most useless; for it is least like things that are self-motive; for it has nothing detached or projecting, like a rectilinear body, but is furthest removed in shape from progressive bodies.
ἐπεὶ οὖν δεῖ τὸν μὲν οὐρανὸν κινεῖσθαι τὴν ἐν ταὐτῷ κίνησιν, τὰ ἄλλα δʼ ἄστρα μὴ προιέναι διʼ αὑτῶν, εὐλόγως ἄν ἑκάτερον εἴη σφαιροειδές· οὕτω γὰρ μάλιστα τὸ μὲν κινήσεται τὸ δʼ ἠρεμήσει.
Since, then, the heaven must move with motion in the same place, and the other stars must not progress by themselves, both would reasonably be spherical; for in this way, most of all, the one will move and the other will remain at rest.