Consider what you know about Kepler's Laws, and the orbits of the planets around the Sun. What force keeps the planets in orbit? What determines the strength of this force?

Kepler's Third Law relates the orbital radius of a planet (how far away it lies from the Sun) with its orbital period (the amount of time it takes to travel once around the Sun). It applies to all of the planets and to the less significant bodies of the solar system, the big ones and the small ones, regardless of their innate characteristics. It says that if you know how far a planet lies from the Sun, you know the length of its year – or if instead you know how long it takes a planet to orbit the Sun, you know its average distance from the Sun. You don't need to know anything else to apply Kepler's Third Law.

• Imagine that planet Jupiter, far larger and more massive than Earth, was shifted inward from its current position to the same orbital path as the Earth. Would it take a year to travel once around the Sun, or would its larger mass and size produce a different orbital pattern?

• Does the Moon, both smaller and less massive than than the Earth, go once around the Sun each year as does the Earth, or does it take a different amount of time to do so?