Tides in a Nutshell

By Dani Knod

The periodic, predictable rise and fall of the ocean, dubbed tides, has been a point of interest all through manís history. Occurring approximately an hour later each day, most shore areas worldwide experience tides, with the exception of the Mediterranean and the Baltic. It was mankindís realization of the similar hour-later-each-day rhythm of rising and falling embraced by the moon that led early thinkers to look toward the sky to find the explanation of this marine marvel.

Three heavenly bodies play a part in how tides work: the earth, the moon and the sun. Combine these with the force of gravity explained to us by Sir Isaac Newton and his apple long ago, and we have the four players in Sac-Beís Tides in a Nutshell.

Tides diagram 1


The moon and the sun pull upon the surface of the earth with their gravitational attraction. The moon is much closer to the earth than the sun, and its pull upon the earth is twice that of the sun.

The strength of this gravitational pull decreases as the distance from the moon increases. Since the solid parts of the earth (land masses and ocean floors) are rigid, the difference of gravitational pull cannot be seen. However, the waters of the oceans are much more easily movable and this gravitational pull from the moon leads to significantly noticeable tidal effects.

Tides diagram 2


The centrifugal force generated by the rotating earth and moon system places another kind of gravitational factor upon tides. A centrifugal force is one that acts in an outward direction: the force keeping water in a bucket swung in a complete vertical circle (see sketch 2). As the earth-moon system spins around its center of mass, there is an outward pull on the surfaces of the two bodies Ė for the earth; this includes the movable and non-movable surfaces.

Tides diagram 3


Now we put these ideas together and see how these two forces interact to create tides. The gravitational force is much stronger than the centrifugal force on the side of the earth facing the moon than it is on the side facing away from the moon. As a result, the side facing the moon has the water pulled into a bulge (high tide).

Tides diagram 4

On the opposite side of the earth, the gravitational pull from the moon will be at its least and the stronger centrifugal force will pull water into a bulge away from the earth (another high tide). Thus, we have two high tides each day and two low tides about halfway between the high tides.

Tides diagram 5


The effect of the sun is seen in spring and neap tides. Spring tides are the highest of high tides and the lowest of low tides. The great range is due to the sun and moon in direct alignment, thus combining their two gravitational forces into one stronger pull. Neap tides, on the other hand, occur when the moon and sun are at right angles to each other. These are tides of minimum range due to the gravitational forces of the moon and sun counteracting each other.

Of all the magical mysteries of the mother ocean, tides are very easy to observe but not so easy to explain. Though there are many more complicated realities behind this twice-a-day rise and fall of the ocean levels, Sac-Beís simplified version is a great start toward understanding and appreciating the beautiful complexity of tides.


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