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Planetary scientists trying to work out why our moon has a bulge on one side think they have come up with an explanation based on classical mechanics, rather than a complex computer model.

The researchers, based at MIT, have invested much pencil lead in concluding that the very young moon must have orbited Earth in a very different path to the one it follows today.

The orbit would have had to be much more elliptical. Ian Garrick-Bethell and colleagues estimate that the moon's orbit (now nearly circular) could have had an eccentricity of as much as 0.61, compared to 0.05 today.

It may also have rotated much faster - possibly three times for every two orbits of Earth. Today, of course, it rotates once for each orbit, which is why we always see the same face, and never the far side.

This arrangement would allow for the still-molten moon to have been 'tugged' into a bulge when it was close to Earth. It would gradually have set, or frozen, into its current shape as it cooled over the millennia, PhysicsWorld reports.

The researchers also say the current form and orbit could have been created if the young moon rotated at the same rate as it does today, but even closer to the Earth (just 22 Earth radii away), if the orbit had an eccentricity of 0.49.

Working out how the moon's orbit was set up is notoriously difficult. Astronomers must take into account not only interaction of the Earth and the moon, but the effect of the sun on each body individually, and also as a pair.

The fact that the moon also has a bulge on its far side only makes it more difficult. ®

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