Greek myth is full of gods pursuing mortals and the mortals suffering for it. The Endymion story is the one that goes the other way, and it is treated with a tenderness that has almost no parallel.
Endymion was a beautiful young man — a shepherd on Mount Latmos in Caria, or a king of Elis, depending on the source. Selene saw him asleep and fell in love. In the standard version she asked Zeus to grant him whatever he wished, and he chose eternal sleep, remaining ageless and undying. She comes down to him nightly in his cave. Apollodorus reports that she bore him fifty daughters, which is usually taken as a lunar calendar count — fifty months between Olympic festivals.
Nobody is punished. Nothing is lost, except that Endymion never wakes. Pausanias records that the people of Elis showed Endymion's tomb at Olympia while the Carians of Heracleia claimed he was still sleeping in a cave on Latmos, which is a good example of two communities holding incompatible versions of the same story and both showing visitors the evidence.
What the sources say about her
A brief Homeric Hymn is devoted to her, and it is unusually pictorial: her light appears from heaven, her rays illuminate the immortals, she bathes in Ocean, puts on her far-gleaming garments, yokes her strong-necked shining colts, and drives them at evening in the middle of the month when her great orb is fullest. It ends by connecting her to the poets who sing of demigods.
She is the daughter of the Titans Hyperion and Theia, and the sister of Helios and Eos — sun, moon and dawn as siblings, which is a tidier arrangement than most of the Greek pantheon manages. Her chariot is drawn by two horses, sometimes by oxen whose horns suggest the crescent; Helios has four.
Her other lovers are fewer and briefer: Pan, who according to Virgil seduced her with the gift of a fleece of white wool, and Zeus, by whom she is sometimes given the daughter Pandia, and the Nemean lion is said in one strange tradition to have fallen from the moon.
Not the same goddess as Artemis
This needs stating clearly because it is misstated constantly. Homer and Hesiod keep Selene entirely separate from Artemis, who has nothing to do with the moon in archaic Greek religion. The identification is a Hellenistic and Roman development, running in parallel with the merger of Helios and Apollo, and it is completed by later mythographers who assembled the tidy triple scheme — Luna in the sky, Diana on earth, Hecate below — which Renaissance handbooks then taught to Europe as though it were ancient doctrine.
The same applies to Hecate, whose lunar associations belong to the imperial-era magical material rather than to Hesiod. Three distinct goddesses were flattened into one by later syncretism, and unpicking them is one of the most useful things a reader of Greek myth can learn to do.
Cult of Selene herself was thin. There are references to sacrifices and to a precinct or two, and Pausanias mentions images, but she had no great sanctuary. Greeks generally regarded the direct worship of sun and moon as something foreigners did — Aristophanes says so explicitly, contrasting Greek practice with Persian.
Drawing down the moon
Where she does become important is in magic. Thessalian witches were proverbially able to "draw down the moon", a claim so widespread that Aristophanes jokes about hiring a Thessalian witch to steal the moon and stop the calendar so that debts fall due later. Plato mentions the belief; Plutarch discusses it; Roman poets from Horace to Lucan use it constantly.
Theocritus' second Idyll is the great literary example: a woman working a binding spell by night, addressing the moon directly — "look kindly on my love, lady Selene" — and telling the whole story of how she was abandoned while she turns her magic wheel. It is the earliest sustained depiction of a private love spell in European literature, and it is spoken to this goddess.
By the Roman imperial period, in the magical papyri, Selene has been fused with Hecate, Artemis, Persephone and various Egyptian and Semitic names into a composite lunar power invoked in long strings of epithets. That composite is the direct ancestor of the moon goddess of modern esoteric practice.
Luna, lunacy and the long shadow
The Roman Luna had a temple on the Aventine and another on the Palatine, and appears with the crescent on her head and a two-horse chariot. What she gave the modern world is a vocabulary of belief: lunatic and lunacy, from the ancient and medieval conviction that the moon's phases affected mental illness — a belief so entrenched that it survived into English statute law, with the Lunacy Act, and into modern folklore about full moons and emergency rooms, where repeated studies have failed to find an effect.
Selene the element name is a related case: selenium, isolated by Berzelius in 1817, was named after the moon because it accompanies tellurium, which had been named after the earth.
The modern moon goddess
She has become far more prominent in the last century than she ever was in Greek cult, because modern paganism, fantasy and popular astrology all need a moon goddess and she is the one with the right name. Sailor Moon's Usagi is Princess Serenity; countless novels and games use Selene for a lunar or vampiric figure; the name and its Roman form are common given names, and Luna in particular has risen dramatically in English-speaking countries since the 2000s.
The Endymion story has had a separate and rich career: Keats' four-thousand-line Endymion of 1818, which opens with "A thing of beauty is a joy for ever" and was savaged by reviewers; Longfellow's and Drayton's versions; and a long line of paintings, most famously Girodet's Sleep of Endymion, in which the moonlight itself is the goddess and she does not appear as a figure at all.
Two horses, fifty daughters, and one etymology
Why does she have a chariot with only two horses?
Artistic convention distinguishing her from her brother's four — the moon is a smaller and quieter power, and the two-horse biga is the visual shorthand. In some depictions she rides a horse, a bull or a mule sidesaddle instead, and the bull's horns doubling as the crescent is a common and effective device on coins and gems.
Is the "fifty daughters" number meaningful?
Almost certainly. Fifty lunar months is the interval of the Olympic cycle in the eight-year oktaeteris used to reconcile lunar and solar calendars, and Endymion is associated with Elis, where the Olympic festival was held. A moon goddess with fifty daughters by a local king reads as a calendar encoded as a genealogy, which is not unusual in Greek material.
What does her name mean?
It is transparently from selas, brightness or a flame, so "the bright one" — one of the few Greek divine names with an uncontested Greek etymology. She is also called Mene, from the word for month, which preserves the older function: the moon as the measure of time before it is a light in the sky.
Was she involved in the Gigantomachy?
She appears on the Pergamon Altar frieze riding into the battle, alongside Helios and Eos, and the three celestial siblings arriving together is one of the most effective compositional passages in that enormous work. It is her most prominent appearance in monumental Greek art and one of the few places where she is an actor rather than a background light.