Theia

Theia, also called Euryphaessa, the wide-shining, is a first-generation Titaness, daughter of Uranos and Gaia, and the goddess of sight and of the bright light of the clear sky. With her brother and husband Hyperion she is the mother of the three luminaries, Helios the Sun, Selene the Moon and Eos the Dawn, so that through her the Titan generation gives birth to the lights that cross the heavens.

The Greeks connected her especially with the shining of things, and the poet Pindar preserves a striking sidelight on her: he says it is because of Theia that people prize gold and value the sight of it above all else, treating her as the goddess who gives radiant things their worth in human eyes. Like the other archaic Titans she has little independent cult; her role is cosmogonic, a link in the chain by which Hesiod's Theogony accounts for where the sun, moon and dawn came from. Theia receives no attested cult of her own, and her chief afterlife is genealogical and poetic rather than religious: later mythographers such as Pseudo-Apollodorus repeat her role as mother of the three great lights with little elaboration, while Pindar's isolated remark connecting her to the value people place on gold and on the sight of radiant things remains one of the few moments any ancient author pauses to reflect on what her name and role might actually mean, rather than simply listing her among the Titans.

Theia's name has been given a second life in modern planetary science, attached not to a myth but to a hypothesis about the literal origin of the Moon. The giant-impact hypothesis, the leading scientific explanation for how Earth's Moon formed, proposes that a Mars-sized protoplanet collided with the early Earth roughly 4.5 billion years ago, and the debris thrown into orbit by that collision coalesced to form the Moon; planetary scientists who developed and refined this model in the late twentieth century named the hypothetical impactor Theia, a deliberate and fitting borrowing, since the mythological Theia was the mother of Selene, the Moon, making the name of the Moon's mythic parent an apt label for the object whose destruction is thought to have physically produced the Moon itself.

Theia the protoplanet has never been directly observed and cannot be, since the hypothesis holds it was destroyed and absorbed in the very collision that created the Moon, its former existence inferred indirectly from the chemical composition of lunar rock samples returned by the Apollo missions, whose isotopic similarities to Earth rock have themselves generated ongoing debate among planetary scientists about exactly how much of the Moon's material came from Theia rather than from Earth itself. The naming shows a small but genuine pattern in modern scientific practice, in which astronomers and planetary scientists reach for the same store of Greek mythological names ancient poets used for the sky, giving Theia a documented career in twenty first century cosmology considerably more prominent than any cult she held in antiquity.