PLANE´TAEPLANE´TAE s. STELLAE ERRANTES (πλανῆται s. πλανώμενοι ἀστέρες as opposed to τὰ ἀπλανῆ τῶν ἄστρων). The popular astronomy of the early Greeks was chiefly confined, as is pointed out elsewhere [ASTRONOMIA], to a knowledge of the morning and evening risings and settings of the brightest stars and most remarkable constellations, since upon these observations the formation and regulation of the primitive calendars in a great measure depended. No single star was more likely to attract attention under such circumstances than the planet Venus, and accordingly The Morning Star (Ἑωσφόρος) is placed first among the stellar progeny of Erigeneia in the Theogony (381)-- τοὺς δὲ μέτ᾽ (sc. ἀνέμους) “ ἀστέρα τίκτεν Ἑωσφόρον Ἠριγένεια
ἄστρα τε λαμπετόωντα τά τ᾽ οὐρανὸς ἐστεφάνωται,
” while both the Morning Star (Ἑωσφόρος) and the Evening Star (Ἕσπερος) are named in the Homeric poems (Il. 22.318, 23.226: cf. Od. 13.93), where they are evidently regarded as distinct from one another, and there is no hint that they are unlike the other stars in their nature. According to Apollodorus, in the second book of his work Περὶ θεῶν, Pythagoras (about B.C. 612) was the first who surmised that Φωσφόρος and Ἕσπερος were one and the same, but by Favorinus the honour of this discovery is ascribed to Parmenides. The latter certainly looked upon this body, which he called both Ἑῷος and Ἕσπερος, as altogether different in its nature from the fixed stars, for he placed it in his highest region, or aether; below it, but also in the aether, was the sun, and below the sun, in the fiery region (ἐν τῷ πυρώδει) which he calls οὐρανός were the fixed stars. Achilles Tatius assigns the discovery to Ibycus (circ. B.C. 540). The term πλανῆταιτ seems, if we can trust Plutarch and Stobaeus (Eel. Phys. 1.24), to have been recognised as early as the epoch of Anaximander, according to whom the sun stood highest in the universe; next below was the moon, and then the fixed stars and the planets (ὑπὸ δὲ αὐτοὺς τὰ ἀπλανῆ τῶν ἄστρων καὶ τοὺς πλανήτας). Empedocles supposed the fixed stars to be imbedded in the crystalline sphere which, according to his system, enveloped all things, but the planets to be detached from it, thus implying the necessity felt for some theory which should account for their erratic course. Democritus wrote a treatise τῶν πλανητῶν, among which he reckoned the sun, the moon, and Φωσφόρος, but, as yet, their number had not been determined. This is expressly affirmed by Seneca (Quaest. Nat. 7.3), “Democritus. subtilissimus antiquorum omnium suspicari ait se plures stellas esse quae currant; sed nec numerum illarum posuit, nec nomina, nondum comprehensis quinque siderum cursibus. Eudoxus ab Aegypto hos motus in Graeciam transtulit.” But, although Eudoxus may have been the first to communicate scientific details with respect to the orbits and movements of the planets, Philolaus,a Pythagorean, who flourished more than a century earlier, was certainly acquainted with the whole five, for he maintained that there was a central fire around which the ten heavenly bodies (δέκα σώματα θεῖα) revolved. Of these, the most remote from the centre was οὐρανός that is, the sphere containing the fixed stars, next in order were the planets, then the sun, then the moon, then the earth, and, below the earth, the Antichthon (ἀντίχθων, see Arist. de Caelo, 2.13), thus completing the number ten if we reckon the planets as five. In the Timaeus of Plato (p. 38), the planets are mentioned specifically as five in number (ἥλιος καὶ σελήνη καὶ πέντε ἄλλα ἄστρα ἐπίκλην ἔχοντα πλανητά), and, in the same passage, we for the first time meet with the name Hermes as connected with one of these (Ἑωσφόρον δὲ καὶ τὸν ἱερὸν Ἑρμοῦ λεγόμενον). It is not, however, until we come down to the Epinomis (p. 987), the work of some disciple of Plato, that the whole five are enumerated, each with a distinguishing appellation derived from a god: τὸν τοῦ Κρόνου, τὸν τοῦ Διός, τὸν τοῦ Ἄρεος, τὸν τῆς Ἀφροδίτης, τὸν τοῦ Ἑρμοῦ. In the tract Περὶ κόσμου, found among the writings of Aristotle, although probably not from his pen, we are furnished with a second set of names (p. 392a, 23)--Φαίνων for the star of Kronus; Φαέθων, for that of Zeus; Πυρόεις, for that of Ares; Φωσφόρος, for that of Aphrodite; Στίλβων, for that of Hermes: and these seem to have been the ordinary designations employed by men [p. 2.433]of science. It is here stated also, that Πυρόεις was by some termed the star of Herakles, and that Στίλβων was by some termed the star of Apollo. Pliny gives additional variations, for in his list they are catalogued as SIDUS SATURNI, JOVIS, MARTIS s. HERCULIS, VENERIS s. JUNONIS s. ISIDIS s. MATRIS DEUM (Lucifer, Vesper), MERCURI s. APOLLINIS; and these may be still farther increased from Achilles Tatius, the grammarians and the lexicographers. The Pythagoreans, regarding the earth as the centre of the universe, assumed the place of the five planets to be between that of the fixed stars on the one hand, and the sun and moon on the other, a doctrine followed by Plato (cf. Martin, Timée de Platon, ii. p. 64), Eudoxus and Aristotle (cf. Proclus, in Tim. p. 257 F). Archimedes, however, employing a fuller knowledge of mathematics (Macrob. in Somn. Scip. 1.19.2), assigned the following order:--1. Saturn; 2. Jupiter; 3. Mars; 4. The Sun; 5. Venus; 6. Mercury; 7. The Moon; and this order was generally adopted, e. g. by Cicero (de Div. 2.43, 91), Manilius (1.803, 6), Pliny (Plin. Nat. 2.6), &c. Macrobius says that the Pythagoreans learnt their doctrine from the Egyptians, whereas the latter was the view of the Chaldaeans (cf. Lewis, p. 246 f.). Saturnus was believed to perform a complete revolution in thirty solar years, Jupiter in twelve, calculations approaching very nearly to the truth. The period of Mars was fixed at two years, a determination less accurate than the two former, but not very wide of the truth. As to Venus and Mercury, not even an approximation was made, for they were both believed to perform their revolution in exactly or very nearly the same time as the sun (cf. Cicero, Somn. Scip. 4). Pliny, who affects great precision in this matter, fixes 348 days for Venus and 339 days for Mercury, the true period being nearly 225 days for the former, and about 88 days for the latter. Saturnus being thus removed to a great distance from the source of heat was naturally viewed as possessing a cold and icy character (gelidae ac rigentis naturae; frigida stella Saturni); Mars, on the other hand, as of a hot and fiery nature; while Jupiter, which lay between them, enjoyed a temperature made up by the combination of the extremes. The astrologers caught up these notions, and, uniting them with the legends of mythology, adapted them to their own purpose, uniformly representing the influence of Saturnus as malign, and that of Jupiter as propitious. “ Haec tamen ignorat, quid sidus triste minetur Saturni.
”--(Juv. 6.569.) “ Saturnumque gravem nostro Jove frangimus una.
” (Pers. 5.48.) “ Te Jovis impio
Tutela Saturno refulgens
”--(Hor. Carm. 2.16.22.) It must be understood that, in the above remarks, we have confined ourselves entirely to the popular notions which prevailed among the ancients, without attempting to trace the progress of scientific observation, a subject which belongs to a formal history of astronomy, but does not fall within our limits. (Plut. de Placitis Philos. 2.14-16; Stob. Ecl. Phys. 1.23.1; 25.1; D. L. 8.14, 9.23; Arat. Phaen. 454; Gemini, Elementa Astron. 100.1; Achill. Tat. Isag. ad Arat. Phaen. xvii.; Lydus, de Mens. v., &c.; Cic. de Nat. Deor. 2.2. 0, 51-54, with Mayor's notes; Plin. Nat. 2.6, 8; Tac. Hist. 5.4; Macrob. Somn. Scip. 4; cf. Lewis, Astronomy of the Ancients, pp. 62, 144, 152, 245, &c.) [W.R] [A.S.W]