We often talk about the Moon orbiting Earth, or the planets orbiting the Sun. And for everyday purposes, that’s a reasonable way to describe things. Technically, though, two gravitationally bound bodies actually orbit each other. They move around a common center of mass. And the location of that point depends on the relative masses of the two bodies.
Consider the Moon and the planet Jupiter, which put on a fine display tonight. They climb into good view by mid-evening, with brilliant Jupiter to the lower left of the Moon.
The Moon is a bit more than one percent as massive as Earth, and the two bodies are about a quarter of a million miles apart. When you do the math, you find that the center of mass for the Earth-Moon system is actually inside Earth itself — about a thousand miles below the planet’s surface. So in that respect, the Moon really does orbit Earth.
The same isn’t quite true for Jupiter and its orbital partner, the Sun. Jupiter’s only about one-thousandth as massive as the Sun. Because of the great distance between them, their center of mass is about 30,000 miles above the Sun’s surface. As a result, the Sun actually makes a small loop around that point in space as it responds to Jupiter’s gravitational pull.
Incidentally, Jupiter is the only planet for which the common center of mass is outside the body of the Sun. All the other planets are just too puny to exert that much pull on our star.
Script by Damond Benningfield, Copyright 2013
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