A new theory put forward by Harvard scientists suggests the Moon was once part of the Earth that spun off after a giant collision with another body.

In a paper published on Wednesday in the journal Science, Sarah Stewart and Matija Ćuk said their theory would explain why the Earth and Moon have similar composition and chemistry.

The Earth was spinning much faster at the time the Moon was formed, and a day lasted only two to three hours, they said.

With the Earth spinning so quickly, a giant impact could have launched enough of the Earth's material to form a moon, the scientists said in an explanation published on a Harvard website. www.fas.harvard.edu/~planets/sstewart/Moon.html

According to the new theory, the Earth later reached its current rate of spinning through gravitational interaction between its orbit around the Sun and the Moon's orbit around Earth.

The scientists noted that their proposition differed from the current leading theory, which holds that the Moon was created from material from a giant body that struck the Earth.

Stewart is a professor of earth and planetary sciences at Harvard, and Ćuk, an astronomer and an investigator at the SETI Institute, which supports research into the search for extraterrestrial life. The latter was conducting post-doctoral research at Harvard.