Pam Sanders, age 12, of Ulmers, South Carolina, for her question:
Why cannot the sun orbit around the earth?
The sun seems to orbit around the earth once every 24 hours and for ages people thought it really did. Later they discovered that the earth really orbits around the sun. Now we know that the earth and the sun actually move together in sort of double orbital pattern.
The earth makes its yearly orbit around the sun because of the universal laws of gravitation. The sun's pull of gravity stops us from sailing off into space and the orbital speed of the earth prevents us from falling into the sun. Universal laws, of course, apply to everything. The sun tugs at the earth and the earth tugs back. It is logical to wonder why the earth's pull does not make the sun orbit around us. Actually it does not pull hard enough to do this. But it does pull hard enough to drag the sun slightly out of line.
Orbits always come in pairs, though one may be immense and the other almost too small to measure. The earth and the sun are dancing partners, moving around together in a heavenly hoedown. The pattern of the dance depends upon the gravitational strength of each partner and this built in pulling power depends upon mass. The sun is 332,000 times more massive than the earth and the pull of the earth is therefore 332,000 times weaker.
The gravitational force from each heavenly body seems to come from a point at the very center of its mass. This gravity center is deep, deep below the surface. Its location is very important in the orbiting partnership. The two partners act as if they were joined together by a straight line between their two centers of gravity. If it were a see saw, the sun and the earth could tip each other up and down. But to do so, the massive sun would have to be near the middle of the line and the small earth way out at the opposite end.
Instead of tipping up and down, the line of force between the earth and the sun swings around with one of the dancing partners at each end. Like a see saw, however, it must be balanced. In order to do this, the masses of the sun and the earth must be totaled together and divided in half. The balance point separates this total mass into two equal parts. And since the sun is so much more massive, it is much closer to the balancing point. This is the point around which the two partners move.
The earth is some 90 million miles from the mass center and so it swings around in a wide orbit. At the other end, the mass center is miles below the surface of the sun. At this end, the sun cannot swing around in a wide orbit. But nevertheless, the sun moves with its dancing partner. The earth is not massive enough to make the sun orbit around it. But it has enough strength to make the sun move slightly. The other planets also are partners of the sun. Each pulls the sun a little~.out of line as it swings around its orbital path.
The earth and the moon also are orbital partners. The earth is 83 times more massive than the moon and swings the moon around its orbit. The moon is not strong enough to swing the earth around, but it does force the earth to weave from side to side as it travels around the sun. This motion is called "perturbation." The earth cannot swing the sun into an orbit, but it can and it does cause a motion of perturbation in the sun.