Oct. 11, 2007 — The moon has always had an inexplicably loony, eccentric orbit, and now we know who to blame: Jupiter and Venus.
A new study of the moon's orbital history reveals that Earth's sister planet and the gas giant have taken turns tugging at old Luna on the rare occasions when even their very distant and therefore puny gravitational tugs could have an effect.
The idea that other planets could affect the moon would seem far-fetched since the gravity of those planets, at tens of millions of miles away, is miniscule.
However, Matija Cuk of the University of British Columbia has worked out the details of those times when the moon’s orbit and the orbits of Venus and Jupiter are in synch, 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," Cuk told Discovery News. His paper on the matter appears in the October 12 issue of the journal Science.
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"It’s a chaotic process," said Cuk of the orbital happenings in the solar system. "Things perturb each other weakly, but when you get resonance, the perturbations keep adding up."
A good way to visualize the process is to think of an adult pushing a child on a swing, explains another orbit investigator, Doug Hamilton of the University of Maryland.
If you stand behind the child and gently push at the right times and the child swings their legs in synch, they go higher and higher. But if you push from the side, in any direction with no rhythm, there is no cumulative effect, no swinging and you get a frustrated kid.
