Famous astronomer- He said each planet's orbit about the Sun is an ellipse and not a perfect circle. An ellipse is like the shape of an oval or egg.
He also said the farther you are as a planet, the the slower you revolve, and the closer you are, the quicker you revolve.
The speed of a planet's orbit depends on how close it is to the Sun. Mercury orbits the Sun in only 88 days. Whereas a planet that is further away, like Saturn, takes 10,759 days to do the same.