TIE A WEIGHT like a key chain or small stuffed animal to the end of a string that is about a yard (three feet) long. Swing the object in a circle over your head. Swing it so the object is moving as slowly as possible but not falling down.
Now slowly shorten the length of the swinging part of the string. What happens to the speed of the moving object? You will notice that as you shorten the length of the string, the speed of the object increases.
This is what happens to planets. Planets that are close to the sun move fast in their orbit, and those planets that are farther away from the sun move more slowly! It is the sun’s gravity that makes planets that are closer move faster.
-By Harriet Vender, 7th and 8th grade science teacher