Epicycle

Imagine you are riding a bicycle around a big circle. But, instead of going straight, you are making little circles at the same time. These little circles are called epicycles.

A long time ago, people thought that planets moved in circles around the Earth. But they noticed that sometimes, the planets appeared to go backwards in the sky for a little while before continuing on their path. To explain this, they came up with the idea of epicycles.

Think of a planet like a bicycle rider going around the Earth. But, instead of just going in a simple circle, the planet also goes in a smaller circle at the same time. This little circle is the epicycle.

The idea was that the planet was moving around in a circle, but also had a little wiggle that caused it to sometimes appear to go backwards. The epicycle helped explain why the planets moved the way they did.

Nowadays, we know that planets move in ellipses around the Sun, not in circles around the Earth. But the idea of epicycles helped scientists understand the motions of the planets long ago, and was an important step in the development of astronomy.