Does the Earth follow a “squiggly” orbit?
I was watching a NOVA program about Isaac Newton and I started thinking. Is the elliptical orbit of the Earth around the Sun, as usually drawn, an “average” orbit? By this I mean, does the movement of the Moon around the Earth pull it faster, slower, inward and outward depending on the moon’s position? In that case, if the actual orbit could be rendered, wouldn’t it be (to a minute extent) a squiggly line? Reply Yes, the orbit would be “squiggly,” though the squiggles would be rather small. By the laws of celestial mechanics, what actually moves around the Sun in a (relatively) smooth ellipse is the center of gravity of the Earth-Moon system. The Earth is about 81 times heavier than the Moon, and their combined center of gravity is on the Earth-Moon line, 1/82 of the Moon-Earth distance from the center of Earth and 81/82 of that distance from the center of the Moon. Since the Earth-Moon distance is about 60 Earth radii, it follows that this point is always inside the solid Earth–about
