About sixty million years after the Solar System formed, the newborn Earth still bore a molten surface. The most widely supported scientific model holds that a Mars-sized body — Theia — sharing Earth's orbit drifted inward and struck our planet. The collision was violent enough to disrupt Theia entirely, melt Earth's outer layers, and fling a vast cloud of debris into orbit. That material coalesced into the Moon over a relatively short interval — perhaps centuries, at most a few million years.

The evidence is indirect but consistent. Apollo lunar samples are strikingly similar to Earth's mantle in oxygen-isotope ratios, indicating shared material. The Moon's density matches Earth's silicate mantle and its core is tiny — the profile expected of a giant impact. Simulations show that an oblique strike naturally produces the Moon's mass, Earth's rapid spin, and the planet's axial tilt all at once.

The consequences reach beyond the sky. Tidal friction from the Moon has slowed Earth's rotation across billions of years (we owe the 24-hour day to that drag). By stabilizing axial tilt, the Moon kept the seasons steady — without that stability, the path to complex life would have been much harder. The rhythm of shallow tidal pools may have helped seed the first biochemical cycles. The Moon is not a passing neighbor in the sky; it is woven into Earth's formation.