Tides: An effect that happens when a large object is moving in an orbit in a gravitational field. The object behaves, as far as the field is concerned, as if it were concentrated at a single point, the center of mass. So the center of mass moves in exactly the right orbit. But every part of the object that isn't at the center of mass is in a “wrong” orbit. If the object is a planet orbiting the Sun, for example, bits of the planet that are farther out from the Sun than the center of mass is, are being dragged around faster than their correct orbital speed for their distance from the Sun, and bits of the planet that are closer to the Sun than the center of mass is, are being held back from their correct orbital speeds. The result is tidal forces, which stretch the planet in both directions outward from the center of mass along a line joining the center of mass to the Sun. Tidal forces produce tides even in the solid surface of a world. But they have a much bigger effect on the oceans. Consequently, there are two bulges of water in the oceans, one on the side of Earth facing the Sun and one on the opposite side. The Moon has double the tide-raising effect of the Sun because it is so much closer. These effects combine at New Moon and at Full Moon giving rise to the spring tides.