Venus and Jupiter Helped Shape Moon's Orbit

By Ker Than, Staff Writer

posted: 11 October 2007 02:00 pm ET

Ancient gravitational interactions with Venus and Jupiter could help explain some quirks about our moon's orbit, according to a new computer model.

Scientists think Earth's only natural satellite was born when a rogue Mars-sized object struck a young and still molten Earth. The collision created a disk of debris around Earth that eventually coalesced to form our moon about 4.5 billion years ago.

Today, the moon's rotation is unusual in that its spin axis is tilted, and it travels along a particularly elongated oval-shaped path around the Earth.

Matija Cuk, an astronomer at the University of British Columbia in Canada, developed a 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 our moon's orbit slightly through a phenomenon called gravitational resonance.

"Resonance can sometimes come across as a spooky action at a distance," Cuk told SPACE.com. "It's just like a diva singing and at some frequency the glasses start breaking because her voice gets into a resonance with the vibrations of the glass."

But instead of sound waves, Venus and Jupiter affected the moon via their gravities, which exerted a strong tug on the moon at some time in our solar system's past when the two planets' orbital periods were aligned with the moon's orbit.

The gravitational resonance occurred because not only does the moon orbit the Earth in an elliptical path, the moon itself 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, our 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.