Poseidon was a son of Cronus and Rhea. In most accounts he is swallowed by Cronus at birth but later saved, with his other brothers and sisters, by Zeus.
However in some versions of the story, he, like his brother Zeus, did not share the fate of his other brother and sisters who were eaten by Cronus.
He was saved by his mother Rhea, who concealed him among a flock of lambs and pretended to have given birth to a colt, which she gave to Cronus to devour.
According to John Tzetzes the kourotrophos, or nurse of Poseidon was Arne, who denied knowing where he was, when Cronus came searching; according to Diodorus Siculus Poseidon was raised by the Telchines on Rhodes, just as Zeus was raised by the Korybantes on Crete.
According to a single reference in the Iliad, when the world was divided by lot in three, Zeus received the sky, Hades the underworld and Poseidon the sea.
In the Odyssey (v.398), Poseidon has a home in Aegae.
Athena became the patron goddess of the city of Athens after a competition with Poseidon.
Yet Poseidon remained a numinous presence on the Acropolis in the form of his surrogate, Erechtheus. At the dissolution festival at the end of the year in the Athenian calendar, the Skira, the priests of Athena and the priest of Poseidon would process under canopies to Eleusis.
They agreed that each would give the Athenians one gift and the Athenians would choose whichever gift they preferred. Poseidon struck the ground with his trident and a spring sprang up; the water was salty and not very useful, whereas Athena offered them an olive tree.
The Athenians or their king, Cecrops, accepted the olive tree and along with it Athena as their patron, for the olive tree brought wood, oil and food. After the fight, infuriated at his loss, Poseidon sent a monstrous flood to the Attic Plain, to punish the Athenians for not choosing him.
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The depression made by Poseidon's trident and filled with salt water was surrounded by the northern hall of the Erechtheum, remaining open to the air. "In cult, Poseidon was identified with Erechtheus," Walter Burkert noted; "the myth turns this into a temporal-causal sequence: in his anger at losing, Poseidon led his son Eumolpus against Athens and killed Erectheus."
The contest of Athena and Poseidon was the subject of the reliefs on the western pediment of the Parthenon, the first sight that greeted the arriving visitor.
This myth is construed by Robert Graves and others as reflecting a clash between the inhabitants during Mycenaean times and newer immigrants. It is interesting to note that Athens at its height was a significant sea power, at one point defeating the Persian fleet at Salamis Island in a sea battle.
Poseidon and Apollo, having offended Zeus, were sent to serve King Laomedon of Troy.
He had them build huge walls around the city and promised to reward them well, a promise he then refused to fulfill.
In vengeance, before the Trojan War, Poseidon sent a sea monster to attack Troy. The monster was later killed by Heracles.
Poseidon was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon.
Linear B tablets show that Poseidon was venerated at Pylos and Thebes in pre-Olympian Bronze Age Greece, but he was integrated into the Olympian gods as the brother of Zeus and Hades.
Poseidon has many children. There is a Homeric hymn to Poseidon, who was the protector of many Hellenic cities, although he lost the contest for Athens to Athena.
Poseidon was a major civic god of several cities: in Athens, he was second only to Athena in importance, while in Corinth and many cities of Magna Graecia he was the chief god of the polis.
In his benign aspect, Poseidon was seen as creating new islands and offering calm seas. When offended or ignored, he supposedly struck the ground with his trident and caused chaotic springs, earthquakes, drownings and shipwrecks.
Sailors prayed to Poseidon for a safe voyage, sometimes drowning horses as a sacrifice; in this way, according to a fragmentary papyrus, Alexander the Great paused at the Syrian seashore before the climactic battle of Issus, and resorted to prayers, "invoking Poseidon the sea-god, for whom he ordered a four-horse chariot to be cast into the waves."
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According to Pausanias, Poseidon was one of the caretakers of the oracle at Delphi before Olympian Apollo took it over. Apollo and Poseidon worked closely in many realms: in colonization, for example, Delphic Apollo provided the authorization to go out and settle, while Poseidon watched over the colonists on their way, and provided the lustral water for the foundation-sacrifice.
Xenophon's Anabasis describes a group of Spartan soldiers in 400–399 BC singing to Poseidon a paean—a kind of hymn normally sung for Apollo.
Like Dionysus, who inflamed the maenads, Poseidon also caused certain forms of mental disturbance.
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A Hippocratic text of ca 400 BC, On the Sacred Disease says that he was blamed for certain types of epilepsy.
A feminine variant, po-se-de-ia, is also found, indicating a lost consort goddess, in effect a precursor of Amphitrite.
Tablets from Pylos record sacrificial goods destined for "the Two Queens and Poseidon" and to "the Two Queens and the King". The most obvious identification for the "Two Queens" is with Demeter and Persephone, or their precursors, goddesses who were not associated with Poseidon in later periods.
The illuminating exception is the archaic and localised myth of the stallion Poseidon and mare Demeter at Phigalia in isolated and conservative Arcadia, noted by Pausanias (2nd century AD) as having fallen into desuetude; the violated Demeter was Demeter Erinys.
In Mycenaean Knossos, Poseidon is already identified as "Earth-Shaker" (e-ne-si-da-o-ne), a powerful attribute (earthquakes had accompanied the collapse of the Minoan palace-culture).
In the heavily sea-dependent Mycenaean culture, no connection between Poseidon and the sea has yet surfaced.
Homer and Hesiod suggest that Poseidon became lord of the sea following the defeat of his father Kronos, when the world was divided by lot among his three sons; Zeus was given the sky, Hades the underworld, and Poseidon the sea, with the Earth and Mount Olympus belonging to all three.
Demeter and Poseidon's names are linked in one Pylos tablet, where they appear as po-se-da-wo-ne and da-ma-te, in the context of sacralized lot-casting.