Tides and Gravity
When people talk about tides, they usually mean the regular rise and fall of large bodies of water which occur periodically and at predictable times that revolve around 12 hour cycles. However, the gravitational forces that act on the oceans and seas also influence the atmosphere and even the land itself--the ocean tides are simply the most noticeable result of the sun and moon's actions on our planet.
The more mass an object has the more gravitational effect it produces, and like all large bodies in space, the earth and the moon have sizable gravity fields of their own. The earth's gravity is what keeps the moon in a circling orbit around the planet, while the vast gravity of the sun keeps the earth and the moon in a circling pattern around it. The moon, while much too small to influence the movements of the earth in any noticeable fashion, does exert a gravity-based attraction on the earth's surface.
The moon's gravity pulls water--along with air--toward the moon, but the earth's gravity is by far sufficient to keep the water from leaving the atmosphere. The result is a bulge of water on the side of the earth currently facing the moon, as all the water in that particular location is drawn out toward it. On the opposite side of the earth a bulge also occurs, due to the earth's own rapid rotation and the absence of the moon's gravity. The parts of the earth on either side of the bulges have the least amount of water, since it is being drawn away. This creates a constantly cycling series of four tidal fluctuations--two bulges, known as crests, and two dips, known as troughs.
Of course, there is also the sun's gravity to consider. While the sun is much farther away and only has about 50 percent of the same tidal pull as the moon despite its size, it still creates tidal cycles of its own. These cycles overlap and interfere with the lunar tide produced by the moon, leading to two notable types of tides. The first occurs when the sun and moon combine forces, pulling on either side of the earth and creating very intense tides known as spring tides. The second happens when the sun and the moon work at right angles, canceling each other out in barely noticeable tides known as neap tides.