The moon is believed to have formed out of a giant cataclysmic collision early in the history of the solar system when an interplanetary interloper the size of Mars slammed into Earth and lofted a ring of debris circling over the Equator. The debris coalesced into the moon.

At its birth, the moon was quite close to the Earth, probably within 20,000 miles. Because of the tidal pulls between the Earth and moon, the moon ' s orbit has slowly been spiraling outward ever since, and as it does, Earth ' s pull diminishes, and the pull of the sun becomes more dominant. By now, with the moon a quarter million miles from Earth, the sun ' s gravity should have tipped the moon ' s orbit to lie in the same plane as the orbits of the planets. But it has not. The moon ' s orbit is about 5 degrees askew. The moon did indeed form in the Earth ' s equatorial plane, the scientists said, but then a few large objects, perhaps as large as the moon, zipping through the inner solar system repeatedly passed nearby over a few tens of millions of years and tipped the moon ' s orbit. "This mechanism works for a broad range of physical conditions," Pahlevan said. Long run the crisscrossing mini-planets would have been tossed out of the solar system, swallowed by the sun, or slammed into the Earth or the other planets. Robin M. Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who wrote an accompanying commentary in Nature, said the thousands of close passes that typically occur before an impact were a "really new realization" by Pahlevan and Morbidelli.