A waxing crescent moon is visible in the night sky during the National Capital Astronomers Club monthly outreach event at Rock Creek Park in Northwest Washington. October, 20, 2012 in Washington, DC. (Craig Hudson/FOR THE WASHINGTON POST)

Two scientists are theorizing that the moon was once part of the Earth and that it formed after the Earth collided with another body.

In a paper published last week in the journal Science, Sarah Stewart and Matija Cuk said their theory would explain why the Earth and the moon have similar composition and chemistry.

The Earth was spinning much faster at the time the moon was formed, they said, with a day lasting only two to three hours.

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 a separate piece published on a Harvard Web site.

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 differs 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. Cuk is an astronomer and an investigator at the SETI Institute, which supports research into the search for extraterrestrial life.

— Reuters