Around the Sun

We all know the planets go around the Sun, have known since grade school, and have all seen models of the solar system, with a family of nine (or eight) planets circling a parent Sun. But how do we know this? For the longest time, it made perfect sense to everyone that the Sun circled the Earth. That's what we all see; we still speak of "sunrise" and "sunset" as if it were the Sun that was moving, not us.

Even in the ancient world, though, there were some who believed that the Earth circled the Sun. The Greek astronomer Aristarchus proposed just such a theory in 250 B.C., when he calculated that the Sun was much larger than the Earth, and so must be the center. But most still believed that the Sun traveled around the Earth.

Then, in 1547, Copernicus proposed that, not only did the Earth circle the Sun, but that it also rotated once a day, giving us day and night. His system was much more elegant than the ancient Earth-centered system, so was believed by many, even though Copernicus offered no proof that it was true.

About a hundred years later, Galileo proved the law of inertia, that a moving body would move forever in the same direction, and with the same speed, unless it were stopped. When we try to stop our can on glare ice, the car continues to move, because, of course, there's no way to stop it.

And then, in 1687, Newton gave us this explanation; one we all could accept, an explanation that is the real reason to believe that the planets move around the Sun. All the planets are in motion, and their inertia would keep them flying into the darkness of space, were it not for one thing, and that is gravity. The gravity of the Sun tugs on the Earth ever so slightly, to hold it back. The Sun pulls the Earth only a fraction of an inch each second, but this is enough to keep it in its orbit.

(05/14/08)