What causes ocean tides?

Anyone who has spent time at the coast knows that the sea does not stay at one level. Twice a day, in most places, the water slowly climbs up the shore and then slowly retreats again. This regular rise and fall of the sea is called the tides. High tide and low tide arrive on a dependable schedule, year after year. But what could possibly move such an immense amount of water so reliably? The answer reaches all the way out to the Moon.

The main cause of the tides is gravity, specifically the gravitational pull of the Moon. Gravity is a force of attraction between objects, and the Moon, though far away, is large and close enough to tug gently on the Earth. While the Moon's gravity pulls on the whole planet, water is free to move and respond, so the oceans react the most. The Moon's pull draws ocean water into a slight bulge, a region where the sea is raised a little higher.

Here is a part that surprises many people. There is a tidal bulge on the side of the Earth facing the Moon, but there is also a second bulge on the opposite side, facing away from the Moon. This happens because the tides come from differences in the Moon's pull across the Earth. The near side is pulled most strongly, the far side least strongly, and this difference leaves water bulging out on both sides. As the Earth rotates each day, places pass through these bulges, giving most coasts two high tides and two low tides daily.

The Moon is the main player, but the Sun also has a role. The Sun is far more massive than the Moon, but it is also vastly farther away, so its tide-raising effect is roughly half as strong as the Moon's. When the Sun and Moon line up, their pulls combine to produce especially large tides. When they are at right angles, their effects partly cancel, producing smaller tides. So the tides are really a steady dance between the Earth, the Moon, and the Sun.

This article was written using information from Wikipedia.