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Lunar Tides
Due to the gravitational attraction that typically exists between celestial bodies, the moon pulls on the Earth -- but not just on the Earth's center. It also pulls on the oceans covering much of the planet. Essentially, the oceans bulge out from the Earth's surface in two constantly shifting locations on opposite sides of the planet: at the point closest to the moon and at the point farthest from the moon. Gravitational pull directly causes the bulge that occurs closest to the moon, while this pull plus centrifugal force cause the bulge at the point farthest from the moon. At points 90 degrees from these two bulges, locations experience low tides as waters surge away from the areas to collect in the bulges.
Lunar Day
High tides and low tides occur twice every lunar day -- which is slightly longer than the solar day on which people base their clocks and calendars. A solar day refers to the length of time it takes for one location on the Earth to rotate from its position under the sun to that same point under the sun. Similarly, a lunar day refers to the time it takes the location to rotate from its position under the moon to the same position under the moon. A lunar day lasts 24 hours and 50 minutes -- nearly an hour longer than the solar day. This means a location's first daily high tide occurs almost one hour later each day -- and that a 24-hour-long solar day lasting from 12 a.m. to 12 a.m. usually will have two tides but very occasionally will include only one high tide.
Earth's Surface
Due to the interference of continents, which block the free passage of water bulges, some areas habitually experience only one high tide per day. The Gulf of Mexico, for example, experiences only one high tide and one low tide each lunar day. In addition, the shape of a coastline may affect the height of high tides. When an ocean bulge hits a wide continental coastline, the height of the tide may be magnified, whereas that same bulge colliding with the coast of a small island will produce a lesser effect as some water moves around the island instead of merely crashing into it. Also, localized weather, such as high winds or low or high pressure systems, may magnify or lessen the height of tides.
Solar Influence
Although larger than the moon, the sun's gravitational pull on the Earth does not affect the tides as strongly as the moon's does because the moon is closer to the Earth than the sun is. However, the sun's pull does influence the height of lunar tides. When the moon is positioned directly between the sun and the Earth, the sun's gravitational pull coincides with -- and therefore exaggerates -- the moon's pull on the oceans, producing a higher high tide, called a "spring tide." Neap tide occurs when the sun and moon are at 90 degree angles relative to the Earth. Their pulls diminish each other, producing a lower high tide.
Elliptical Orbits
The Earth orbits the sun and the moon orbits the Earth along elliptical -- or oblong -- paths. This means at some parts of the year, the Earth is positioned closer to the sun than at other parts of the year. Likewise, the moon sometimes lies closer to the Earth than at other times. Whenever the moon is closest to the Earth, at "perigee," its gravitational pull affects the tides more strongly, producing higher high tides and lower low tides. This happens once each month.
Whenever the moon is farthest from the Earth, at "apogee," the difference between high and low tides is much less. A similar situation occurs annually as tidal differences increase when the Earth moves closer to the sun, around January 2, and decrease as the Earth reaches its furthest point from the sun, around July 2.