After gazing up at the moon for all those years, however, we're still not exactly sure how it came to be. Here's a brief rundown of the most prominent theories scientists have come up with to explain the moon's origin, including a few relatively wild ones with little observational evidence to back them up. [The Moon: 10 Surprising Lunar Facts]
FIRST STOP: A Captured Moon?
The capture idea isn't really an origin theory, of course; it just concerns how the moon came to orbit Earth. And it has some major problems, the most serious of which is the geochemical similarity of the Earth and moon. The two bodies have nearly identical oxygen isotope ratios, suggesting that they formed from the same pool of raw material.
NEXT: Spinning Off
Most scientists discount the fission hypothesis, saying that Earth could not have been spinning fast enough to expel a huge blob of rock. But one 2010 study suggested that a natural nuclear explosion, set up by the superconcentration of radioactive elements, may have provided the kick to dislodge a moon-size piece of the early Earth into orbit.
NEXT: Forming With Earth
While this hypothesis can account for the isotopic similarities between the Earth and moon, it falls short in other ways. It cannot explain the high angular momentum of the Earth-moon system, for example, or why the moon has such a small iron core compared to that of our planet.
NEXT: Asteroid Rubble
Little evidence supports this theory, which also cannot explain the geochemical similarities between the Earth and its natural satellite.
NEXT: A Huge Smash-Up
One variant of this idea holds that the impactor, dubbed "Theia," was about the size of Mars. Another version, introduced in 2012, suggests that both the impactor and the target — the proto-Earth — were about 50 percent as massive as Earth is today.
While the giant-impact hypothesis continues to be tweaked and refined, it does the best job of explaining the moon's composition and orbit, most scientists say. For example, the theory predicts a small iron core for the moon, since it would have formed primarily from the mantles of the impactor and early Earth (both of which lacked iron, which had already been concentrated deep in the core).