If you’ve ever been taught about how Earth orbits around the Sun, you might well think our planet travels along an oval-shaped path that brings it much closer to the Sun at some times of the year than at others. You’d have a good reason to think that, too: it’s how most textbooks show things.
Consider the bike wheel In order to try to understand myself how circular the orbit of the Earth was and other planets, I decided to compare the shape of Earth’s orbit to an ordinary 26-inch bike wheel by scaling down the real dimensions to fit – and consulting my local bike shop about what the deviations would mean for a real wheel. I was very surprised at the result.
Mercury has the least circular of the orbits, with a deviation of 14mm, although this is still only 2%. However, in the real universe planets rarely go at just the right speed for a circle. Sometimes they travel a bit faster and sometimes slower, which can only be achieved with an elliptical orbit.