An Etymological Dictionary of Astronomy and Astrophysics
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A hypothetical → protoplanet that collided with → earth some 4.5 billion years ago. Debris from the collision, a mixture of material from both bodies, spun out into Earth orbit and → coalesced into the → Moon. This scenario explains many aspects of → lunar geology including the size of the Moon's → core and the → density and → isotopic  → composition of Moon rocks.

Named for Theia, the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.