"Tides" is a generic term used to characterize the rising and falling ocean levels for the land, created by the Moon's gravitational attraction and the Sun.
To a much smaller degree, tides occur in large lakes, in the atmosphere, and within the Earth's solid crust, driven by the gravitational forces of the Moon and Sun.
Tidal Trivia
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The amplitude of gravitational tides in the deep mid-ocean is about 1 meter.
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Shoreline tides can be more than ten times as large as in mid-ocean.
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The amplitude of tides in the Earth's solid crust is about 20 cm.
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The Sun's gravitational force on Earth is 178 times as large as the Moon's force on Earth.
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The ratio of the Sun's or Moon's tidal forces on Earth is 0.465.b
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The tidal stretch of the human body (standing) changes its height by the fraction 10-16, an amount 1000 times smaller than the atom's diameter. By comparison, the body's weight stress causes a fractional change in body height of 10-2.
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Tidal friction causes Earth's days to extend 1.6 milliseconds/century. b
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The Earth's axial rotation angular velocity is 7.29 x 10-5 rad/s.
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The angular velocity of the Moon's revolution around the Earth is 2.67 x 10-6 rad/s.
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Earth's polar diameter is 12,710 km.
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Earth's mean radius is 6,371 km.
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Earth's equatorial diameter is 12,756 km.
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The difference between these diameters is 46 km.
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The difference between these radii is 23 km, or 0.4%.
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The thickness of the Earth's atmosphere is about 100 km.
Gravitational Forces and a Tidal Bulge
Newton's law of universal gravitation states that the magnitude of the gravitational force between two bodies depends on their masses and the distance between them. This law also states that the direction of the gravitational force is toward the center of mass of the two bodies.
The Earth and Moon are held together by gravitational attraction, which is balanced at the center of mass by an equal and opposite centrifugal force.
As the Earth's separation from the Moon changes, the distribution of gravitational forces shifts away from the center of gravity. Nonetheless, the centrifugal force remains constant everywhere on Earth.
Subsequently, aside from the center of gravity, the gravitational and centrifugal forces are not equal, and this difference creates a residual force. These three forces are portrayed in the diagram below.
If an extremely deep sea covered the Earth, this remaining force would follow up on the water and produce two bulges, one on the side facing the Moon and one on the Earth's contrary side.
The next graph shows the Earth and Moon from over the North Pole. An observer underwater on Earth at position 1 is beneath one of the water bulges and would experience a high tide.
A quarter of a revolution later, at position 2, where the original water level has been depressed, the low tide would be seen. The high-tide, low-tide sequence repeats as the observer moves through points 3 and 4.
The Earth-Sun system also depends on similar gravitational and centrifugal forces; however, because of the Sun's distance, these forces are not nearly as strong as the lunar-related forces. As a result, the sun-related residual forces and coming-about bulges are correspondingly smaller.
Shouldn't something be said about the planets?
Venus exerts the strongest gravitational pull on Earth among the apparent multitude of planets, yet it is only 0.0054% of the Moon's pull, establishing no genuine connection.
Despite being a giant planet, Jupiter's more distant impact suggests it is multiple times smaller than Venus's. So, the Moon and Sun are the main heavenly bodies with no significant gravitational effect on the Earth.
More Tidal Bulges
The orbital motions of the Moon and Earth cause the directions of the Moon and the Sun to change independently over a month and a year.
The tilt of the Earth's axis likewise causes the directions of the Moon and Sun above and below the Earth's equatorial plane to change over a portion of their respective orbital periods.
Moreover, because their orbits are elliptical, the distances to the Moon and Sun also fluctuate throughout their month- and year-long cycles.
The gravitational impact of each of these astronomical cycles can be characterized. It is helpful to envision each type of gravity producing its own tidal bulge.
The period of each bulge is identical to that of the gravitational force that made it. These periods, by definition, don't change and are equivalent wherever on Earth.
The size, or amplitude, of these bulges is small—the biggest is merely a couple of tenths of a meter high.
These tidal bulges are known as constituent tides.
Tidal Bulges Changing Size
As the Moon's orbit around the Earth and the Earth's orbit around the Sun are not circular, the distances to each of these bodies vary.
Since the strength of gravitational attraction is resolved, to some extent, by the distance between the objects, as the distances change, so too does the strength of the tide-raising forces.
The lunar distance changes for 27.56 days, which is too near. Yet, it is not equivalent to the Moon's orbital period. As an additional complication, the purpose of the lunar orbit nearest Earth doesn't remain in the same spot. Rather, it moves gradually around the Moon's orbit, which takes 8.85 years.
The distance-related solar-tide-generated force varies over 365.26 days. As with the Moon and the Earth, where the Earth is nearest to the Sun, it likewise moves around the Earth's orbit. However, this is delayed because it takes 20,930 years to complete the circuit.
The Impact of this Present Reality on the Tidal Bulges
The arrangement of the continents has created the world's three major water basins: the Pacific, Atlantic, and Indian Oceans. Because the sections of connecting water are generally small, the movement of the tides is comprehensively limited within individual sea basins.
As the equilibrium theory evolved, the tidal bulge should now be considered an exceptionally long wave caught inside every basin.
Everybody in water has a natural oscillation period, which affects its response to the tide-raising forces. As a rule, the Pacific Ocean's oscillation time frame is 25 hours; thus, many tides in the Pacific are diurnal.
The standard oscillation time frame in the Atlantic Ocean is about 12½ hours, so the waves are predominantly semi-diurnal. Obviously, there are numerous places where the tidal system is shaped by the local configuration of land and water depth rather than by enormous oceanic forces.
If you move a water container from side to side, the water level at each end will rise and fall, while the water level over the center of the container will remain constant. This misrepresented model depicts the idea of the tides in enclosed waters, for example, the Persian Gulf, Red, and Mediterranean Seas.
Now, move the container about in a circular manner. The water level will rise and fall around the edge of the container.
The area where the water level does not change is reduced to a point at the center of the container. This example of wave behavior is similar to what occurs in the sea basins as the Earth rotates.
In the equator's northern hemisphere, the long tide wave goes anticlockwise, while the surge in the southern hemisphere goes clockwise. The center of Coriolis rotation is known as an amphidromic point, a location where there is no tide.
The speed at which a wave travels is constrained by water depth. Those parts of the wave passing over continental shelf areas and shallow coastal regions will back off, implying that the wavefront no longer remains straight.
The wave's slowing also increases its height. According to the equilibrium tide theory, the tide wave height is not exactly a large portion of a meter; slowing back the wave in shallow water produces observed tide heights that are ordinarily worth it.
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