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Gravitational Forces
There are two bodies within our solar system that pull on the earth's surface: the sun and the moon. The magnitude of gravitational force between two objects is equal to G * (M1 * M2) / (r^2) where G is the gravitational constant 6.6726 * 10 ^ -11 N * m^2 / kg^2, M1 and M2 are the masses of the two bodies, and "r" is the distance between them. While the sun greatly outweighs the earth and the moon combined, the moon's proximity results in a much greater force.
Types of Tides
Since the sun and the moon both pull on the surface of the earth, there are two types of tides. When the sun and moon are approximately aligned and pulling in the same direction, it is known as a spring tide. This corresponds to a full moon or a new moon. Conversely, when the moon, earth and sun form an approximately right triangle, the high tide is called a neap tide. Neap tides occur when the moon is waning or waxing.
Proxigean Tides
A special spring tide is the proxigean tide, an event that happens when the moon is at its perigee, the point in its elliptical orbit when it is closest to the earth. When this perigee coincides with the moon's alignment with the sun for a high spring tide, along with certain other factors like the earth's inertia and the friction between the ocean water and the earth's floor working in symphony, extremely high tides will occur. This alignment happens every 31 years, with the last instance occurring in 1995.
Inclination
The earth does not rotate around its north-south axis, and the sun does not rotate around the earth's equator. This off axis rotation results in inclination, as the moon "inclines" above the rotational plane of the earth. This affects the upper and lower latitudes differently than the equator. North of the Tropic of Cancer and south of the Tropic of Capricorn, tides are diurnal, meaning they occur once a day. Around the equator, tides are semidiurnal, and they occur twice daily.