ELI5: What is Kepler's third law

Kepler's third law means that the farther a planet is from the sun, the longer it takes to complete a full trip around it.

Imagine you and your friend are at a running track.

• You run on the inside lane, which is very close to the center.
• Your friend runs on the outside lane, which is far away from the center.

In our solar system, the Sun sits in the middle of the track, and the planets are the runners.

• Mercury runs on the closest inside lane, so it finishes a full trip, or year, in just three months.
• Earth runs in a middle lane, taking twelve months to make one circle.
• Neptune is on the farthest outside lane, so its circle is so huge that one single lap takes about one hundred and sixty-five Earth years.

You can see this same rule in everyday life:

• Merry-go-rounds: If you sit near the middle pole, you make a quick, tight spin. If you sit on the very outside edge, each turn feels like a much longer ride.
• Walking a dog on a leash: If you hold the leash short, your dog can run circles around you in seconds. If you give the dog a very long leash, it takes much longer for it to walk a full circle around you.