What is orbital rotation? The basic picture is clear enough: One body is at rest, while the other follows some circular or elliptical path around it. The trouble is just to figure out which body is which. If you’re standing on the surface of the earth, it appears that the sun slowly orbits around you once per year. (Of course, it also looks as if the entire sky rotates around you once per day, because the earth rotates on its axis. But we are thinking about the motion of the sun relative to the rest of the sky, which happens on a yearly cycle.) But on the surface of the sun, it would presumably also look as if the earth orbits around you once per year. Figuring out which of these reflected the true motions of the solar system, as opposed to merely apparent motions, was once a matter of heated controversy.

Seventeenth-century physicist Galileo