POSEIDON (Ποσειδων). — God of the waters whose prerogative seems symbolized by the common root of his name (πόσις, ποτίζω, πόταμος; V. H. L. Ahrens, Über den Namen des Poseidon, in Philologus, 23 [1866], p. 1 sq.).
Originally his domain embraced the entire earth, which suffered his power manifested in seismic shocks (ἐννοσίγειος, ἐννοσίγων γαμήχος). One among many testimonies confirming this aspect of his as the Shaker is the sanctuary which the Rhodians erected to him under the title of ἀσφάλιος on a new islet formed near Thera, one of the Sporades, following a volcanic eruption (Strabo, I, 3, 16; ἀσφάλιος = terram stabilem, according to Macrobius, Sat., I, 17, 22). Elsewhere, as at Troezen, he was regarded as a god of vegetation (φυτάλμος; Plutarch, Thes., 6). Returning to the primitive conception of a terrestrial god, P. was chiefly worshipped in Thessaly, Boeotia (Etym. magn., 547, 16) and the Peloponnese (Diodorus Sic., XV, 49). Traces of fetishism are found in Arcadia, at Thelpusa (Pausanias, VIII, 25, 4, 10) and at Phigalia (ibid., VIII, 5, 5, 42, 4) where, in connection with the cult of Demeter Melaina, the god appears in the form of a horse (P. Hippios at Lycosura, Mantinea, etc.; cf. Iliad, XXIII, 584; Homeric Hymns, 22, V. 5).
It is probable that the Thessalian and Boeotian colonists, driven by new invasions to the shores of Asia Minor, modified the archaic prerogative of P. as protector of springs and rivers into that of lord of the sea. Indeed, with Homer (Iliad, XV, 187 sq.) P. is one of the three sons of Cronus and possesses in the sea a clearly defined sphere of dominion; in later tradition he is credited with the entire retinue of marine deities (husband of Amphitrite, etc.). His cult was widespread in the various localities of Greece and Ionia where temples and shrines were erected to him. Famous was that on the Isthmus of Corinth. Long considered as an indigenous god together with Athena, he was worshipped on the Acropolis of Athens as P. Erechtheus in the temple of that name (on an earlier sanctuary ἀρχαῖος νέος; V. W. Judeich, Topographie von Athen, Munich 1931, p. 259 seq.). At Rome, under the name of Neptunus, derived from the Greek «νεῦς», P. bore the same attributes of sea-god, and in his honor the Neptunalia were celebrated on July 23, becoming more important after the victory at Actium. As Neptunus Equester he was in a certain way assimilated to the Latin god Consus. An altar and a temple of Neptune stood near the Circus Flaminius (Livy, XXVIII, 11, 4; Pliny, Nat. hist., XXXVI, 26; CIL, VI, 8423). Classical art generally depicts P. as a vigorous man in his full maturity with the attributes of the trident and the dolphin.