Earth's seas owe their regular tides to the orbit of the moon. Briefly, the Moon's gravity pulls the oceans slightly in its direction, so that water on the nearest coast rises noticeably while water on distant coasts lowers slightly.
The Moon orbits the Earth at a distance of about one-quarter of a million miles, and takes about 29.5 days to complete one orbit. However, as Earth's gravity pulls the Moon into this orbit, the Moon's gravity also has a smaller, subtler effect on the surface of the Earth. The Earth's surface is slightly closer to the Moon than the centre of the Earth is, so this results in a pronounced effect.
The rocky surface is more or less immovable, and the air in the atmosphere is compressible. Water, however, is neither compressible nor solidly anchored in place. Under the Moon's influence, large bodies of water (like the oceans) on the face of the Earth facing the moon tend to bulge outwards. On one side, they rise up the shore, pulled by lunar gravity. At the same time, on the opposite side of the ocean (which is farther away from the Moon), the water levels fall, slipping down to what we know as low tide. The effect on the world's oceans, then, is not unlike water sloshing around in a container.
Each day, this tidal cycle occurs twice at any given location on the surface. The Moon turns around the Earth in the same direction that the Earth itself rotates. However, from the perspective of a single observer on the surface, the Moon seems to lag behind slightly each day, so that it returns to the same position once every 24 hours, 50 minutes. Since tides occur twice a day, this means that they are at their strongest twice per lunar day cycle, or once every 12 hours, 25 minutes.
The precise positions of high and low tide fluctuate slightly over the course of the year because, just as the oceans are tugged by the Moon, they are also tugged by the Sun. The Sun is much farther away, but also much more massive. During full moon and new moon, the Earth, Sun, and Moon are aligned in space along a line. (This means that one could draw a straight line from the Sun through the Earth to the moon during a full moon, and from the Sun through the Moon to the Earth during a new moon.) During these periods, the influence of the Sun's gravity is added to that of the Moon's, so that low tides are somewhat lower and high tides are somewhat higher. During quarter moons, when the Sun's gravity well is not aligned with the moons, high and low tides are slightly closer together.
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