Jim Kaler

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Low tide High tide
Low and high tides at Jekyll Island, Georgia, shortly before the new Moon of March, 2012.

Low tide High tide
Low and high tides at Jekyll Island, Georgia, as seen from a higher position on the shore.


Tides The Earth orbits the Sun and the Moon the Earth because of the mutual gravitational attractions between the bodies. Ocean tides are caused by the differences in the gravitational pulls of the Moon and Sun across the body of the Earth (gravity dropping off with the inverse square of distance). To the left, we look only at the effect of the Moon. The depth of the ocean and relative sizes of Earth and Moon are greatly exaggerated (the Moon actually a quarter the size of Earth and 30 Earth diameters away). At the top (a), the arrows symbolically show the gravitational force between the Moon and Earth at different distances from the Moon. At center (b), the central force is subtracted from all the arrows. What remains is the differential force, which causes the ocean waters to flow toward the line between the Earth and Moon. As the Earth rotates, we therefore pass under high water, then low, then high again, making the waters rise and fall at the coasts. As a result of the Earth's rotation and the time it takes the water to flow, the tidal bulge leads the Moon (c). Because the Moon is moving counterclockwise around the Earth with a period relative to the Sun of 29.5 days (the phase period), high tides are separated not by 12 hours, but by an average of 12 hours 24 minutes. Though the Sun is much farther away, it is also much more massive, so it produces a tide as well, of about 45 percent the strength of the lunar tide. The Sun thus contributes about a third toward the total of the two when they line up to produce the highest spring tides at new and full Moons. At the lunar quarters, when the Moon and Sun are at right angles to each other, the solar tide fills in the lunar, resulting in much smaller neap tides. The height of the tide is also influenced by the distances of the Earth from the Moon and Sun, which change because of the eccentricity of the lunar and terrestrial orbits. The strongest tide will generally occur when new or full Moon coincides with lunar perigee, where the Moon is typically 5.5 percent closer to the Earth than average. The Moon and Sun pull back on the tidal bulge to slow the Earth's rotation and increase the length of the day by about 0.0014 seconds per century. The effect must be taken into account in timekeeping. In reality, the situation is much more complex, involving shore lines, the position of the Moon relative to the celestial equator, the time of year, the latitude of the observer, and various other factors. (Image and some text from The Ever-Changing Sky, J. B. Kaler, Cambridge University Press, 1996, 2002.)
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