Friday, October 12, 2007

Looney Moon orbit mystery solved

The Moon was pulled out of its circular orbit around Earth by the tug of gravity from Jupiter and Venus, a new study suggests.

The idea that other planets can affect the Moon would seem far-fetched, since the gravity of those planets, at tens of millions of kilometres away, is miniscule.

But Cuk has worked out the details of those times when the Moon's orbit and the orbits of Venus and Jupiter are in sync, and found that over the eons with repeated tugs, the two planets can have cumulative effects.

These "resonance" effects have pulled the Moon out of its circular orbit and elongated it.

This is the first time that anyone has shown that the Moon is affected by other planets in major ways.

It's a chaotic process, of the orbital happenings in the solar system.

Things perturb each other weakly, but when you get resonance, the perturbations keep adding up.

Elongating a circular orbit

What brought the Moon into sync with Jupiter and Venus, at different times, is nothing less than Earth's oceans.

The constant tidal tug-of-war between the oceans and the Moon has caused the Moon to recede further from Earth and gradually increased the time it takes to complete an orbit. It also made the Moon's orbit more circular.

But as the lengthening time period of a lunar orbit increased, it crossed into long episodes of resonance with Jupiter and then Venus.

The tiny pushes upset the Moon's neat circular path, making it the elongated orbit we see today.

The most visible result of this eccentric orbit, is that during some solar eclipses the Moon is on the more distant part of its non-circular orbit and it doesn't quite cover the entire Sun - this is called an annular eclipse.

At closer approaches, the Moon covers the Sun entirely during eclipses.

This has been a mystery for a long time, of the Moon's eccentric orbit.