Entry

Originally posted by Wikipedia:

Principal lunar semi-diurnal constituent

In most locations, the largest constituent is the "principal lunar semi-diurnal", or M2 tidal constituent.

About 12 hours and 25.2 minutes, exactly half a tidal lunar day.

The moon orbits the Earth in the same direction as the Earth rotates. It takes slightly more than a day (+50 minutes) the same location in the sky.

In many places the period of strongest tidal forcing is the above mentioned, about 12 hours and 25 minutes. BUT not necessarily when the moon is nearest to zenith or nadir, but the period of the forcing still determines the time between high tides:

This is a plot of the earth's own field of gravity: It isn't a striking as the one for the moon but it is an whole lot nearer. Which over-rides what the Wikipedia then goes on to say:

Because the gravitational field created by the moon weakens with distance from the moon, it exerts a slightly stronger force on the side of the Earth facing the moon than average…

…and a slightly weaker force on the opposite side.

The moon thus tends to "stretch" the Earth slightly along the line connecting the two bodies. The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally.

As the Earth rotates, the magnitude and direction of the tidal force at any particular point on the Earth's surface change constantly; although the ocean never reaches equilibrium.

There is never time for the fluid to "catch up" to the state it would eventually reach if the tidal force were constant—the changing tidal force nonetheless causes rhythmic changes in sea surface height.

http://en.wikipedia.org/wiki/Tide#Principal_lunar_semi-diurnal_constituent

And you thought that astrology and magic were dead?

If you were a drop of water confronted with a small planet only 1/4 of a million miles away would you take you time?

Sad, isn't it?

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