In ancient times Aristotle proposed that the planets travelled in orbits of perfect circles. No special force was needed to keep the planets on track because they were following simple perfect paths.

With the geocentric model, the earth is at the center of the universe, and the planets rotate around the earth. But from the outset, this model was flawed. Astronomers observed that the planets do not ...

The planets of our solar system move in ellipses. We've known this, so we are told, ever since Johannes Kepler devised his laws of planetary motion in the early 1600s. While it's true that orbits are ...