Theia (also Thea, Thia, Euryphaessa, or Basileia) is a Titaness, daughter of Uranus and Gaia, consort of her brother Hyperion, and mother by him of Helios, Selene, and Eos — the sun, moon, and dawn. She scarcely acts in narrative myth beyond that maternity; Hyginus displaces her with Aethra as mother of the three lights. Pindar hails her as "Theia of many names," a goddess of brilliance through whom men honour gold. Distinct from the Oceanid Theia and from the modern astronomical name for a hypothetical Moon-forming planet.
Adapted from Wikipedia, "Theia" · CC BY-SA 4.0
I.Family
Early accounts make her the eldest daughter of Gaia and Uranus, sister of the Titans — Oceanus, Crius, Hyperion, Iapetus, Coeus, Themis, Rhea, Phoebe, Tethys, Mnemosyne, Cronus, and sometimes Dione — and of the Cyclopes and Hecatoncheires. By Hyperion she bore Helios, Selene, and Eos.
She is often identified with Aethra, whom Hyginus names as Hyperion's wife and mother of the same children. Robert Graves notes the Homeric Hymn's "cow-eyed Euryphaessa," a title that recalls moon-goddesses linked with bulls.
II.Myth and cult of light
The Homeric Hymn to Helios calls her "mild-eyed Euryphaessa, the far-shining one," mother of the three celestial lights; Catullus names them "Theia's illustrious progeny." Pindar's Fifth Isthmian praises Mother of the Sun, Theia of many names, for whose sake men honour gold above all, and through whose gift ships and chariot teams become marvels.
A scholium links Theia and Hyperion to Helios and Helios to gold, tying the goddess of sight and brilliance to solar wealth. Stesichorus has her dwell with her son in his palace; on the Pergamon Altar a goddess fighting beside Helios is conjectured to be his mother. Plutarch's fable of the Moon and her Mother (sometimes classed among Aesop's) has Selene ask for a fitted garment; her mother — unnamed, but read as Theia — cannot weave one, for the moon forever changes size and shape.
III.Diodorus's Basileia
Diodorus recounts an euhemerist tale in which Theia is Basileia ("queen"), eldest daughter of king Uranus, who raises her brothers and earns the title Great Mother. She marries Hyperion and bears beautiful Helios and Selene; envious brothers murder Hyperion and drown Helios in the Eridanus, after which Selene kills herself. Basileia, after a vision of her children's apotheosis as sun and moon, wanders the land in frenzy; the people raise altars and honour her with drums and cymbals.