Wanderers in the Sky

Look up at the Sun as it moves slowly across the bright blue sky, or watch the Moon's nightly voyage. On dark, moonless nights, you also might notice a planet such as Mars or Jupiter traveling against the stars.

Ancient astronomers thought that the Moon, Sun, and planets all moved in circles, forever wheeling around the central, unmoving Earth. After all, a wheel moves easily across the ground because it is round; because the circumference of a circle has no beginning or end, the cosmic motions could continue forever.

The Moon does indeed revolve about the Earth but our home planet is no longer considered to be the center, focus, and fulcrum of all things. The Earth and other planets revolve about the Sun, which is an important lesson in perspective. Motion is relative, perceived only in relation to something else, by comparison with another object that is either at rest or moving in a different way. It was once thought that the Earth was still and that the Sun revolved around it, but no, it is the other way around.

The earliest theories of planetary motion around the Sun had one fatal flaw. Like the mistaken Earth-centered interpretation, the Sun-centered one also initially assumed that the planets move in circular orbits. This explanation could not be reconciled with careful observations of the changing positions of the planets in the sky, meticulously carried out by the Danish astronomer, Tycho Brahe. As discovered by Johannes Kepler, Brahe’s assistant and eventual successor, the architecture of the solar system had to be described by noncircular shapes.