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Computer model explains moon's loopy orbit
2007-10-23 04:49:29 Xinhua English

BEIJING, Oct. 23 (Xinhuanet) -- A new computer model, based on gravitational interaction with Jupiter and Venus, may explain the moon's eccentric orbit as it travels along an elongated oval-shaped path around Earth.

Matija Cuk, an astronomer at the University of British Columbia in Canada, developed the computer model in an attempt to explain these lunar eccentricities.

"I asked myself-was the orbit of the moon early on more circular than now, or was it just like now, or more eccentric?" Cuk said.

His model, detailed in the Sept. 12 issue of the journal Science, suggests that Venus and Jupiter have elongated the moon's orbit slightly through a phenomenon called gravitational resonance. The planets affected the moon with their gravities, exerting a strong pull on the moon at some time during our solar system's past when the two planets' orbital periods were aligned with the moon's orbit.

The gravitational resonance occurred because the moon orbits the Earth in an elliptical path, and the moon rotates counterclockwise as it treks around the Earth on a nine-year cycle as a result of interactions with the sun. At some point in the past, however, the duration of this "lunar precession" was about 12 years, which is approximately equal to the time it takes Jupiter to go once around the sun.

When the lunar precession and Jupiter's orbital period were equal, about 1 billion years ago according to the new model, Jupiter's small gravitational tug on the moon became amplified with each cycle, Cuk said.

According to the model, the moon underwent the same thing with Venus about 2 to 3 billion years ago. If not for those interactions, the moon's orbit would be closer to circular, Cuk said, and it would not seem to shrink and grow, or speed up and slow down, so much as it traverses the night sky.

(Agencies)

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