Transcript: Gravity keeps planets in their orbit of the Sun and keeps moons in their orbit of the planets. There is a second type of force that is important, however, in the solar system, tidal force. A tidal force is caused by the difference between the gravity force on one side of an object and the other side. It's essentially a stretching force. The size of a tidal force depends on the ratio between the front to back distance, or diameter, of an object like a moon or a planet and its distance from the object causing the gravity. If you work this out for the Earth, Sun, and the Moon you find out that although the Sun’s gravity pull on the Earth is much larger than the Earth’s gravity pull, the tidal force of the Moon exceeds that of the Sun by a factor of two. Thus, tides are caused primarily by the Moon but with a secondary contribution from the Sun. This explains why tides on the Earth are larger at new and full moons when the Sun and the Moon and the Earth all line up.