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What Causes Tides?

John M. Regan

             The simple answer is, of course, the gravitational pull of the moon.  Like most things in nature, however, the entire story is a tad more complex.  Although the moon rightly gets the starring role (please pardon pun) she's not in this alone.  Our lunar friend has help from the sun, the surface of the earth, and the gravitational pull of the earth on the moon - and that is just to name a few. 

            Tides occur every 12 hours and 25 minutes.  The lowest tides are along the equator and become progressively higher to the north and south.   Tides in Hawaii are nothing compared to our Puget Sound comings and goings.  Puget Sound tides are pretty high at four to five meters, but nowhere near the highest known tides that occur in a place called Wolfville, Nova Scotia.  In Wolfville the difference between low tide and high tide averages 12 meters, but can be as high as 16 meters!

            The gross explanation of tide mechanics is that the moon pulls on the earth due to gravitational attraction.  This causes an oceanic bulge - a rise in the ocean height relative to the earth - to form on that side of the earth closest to the moon; the ocean is literally pulled up.  And this does pretty much account for oceanic tides.  Why then is there a difference in the height of the tides at different times of the day and year?  Why don't tides occur at the same time every day?  Why aren't the tides all the same latitude all the same height throughout the earth?  Well here is where the story gets more complicated.  The moon by itself doesn't provide a complete explanation.

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           To begin with you have to forget about perfect circles.  The earth is not a perfect circle.  Our constant rotation (spinning on axis) there is a bulge at the equator due to the centrifugal force.  The earth does not revolve around the sun in a perfect circle, either.  The earth's orbit around the sun, like the moon's orbit around the earth is an ellipse, an oval shape to put it simply.  Add to this the fact that the surface of the earth is highly irregular and the fact that the sun exerts an enormous gravitational pull on the earth as well as the moon, and we begin to see a confluence of factors that affect the tide.

            So let's suppose that the moon is on our side of the earth and pulling at our side of the ocean.  The ocean bulges out causing a high tide.  Next imagine that sun is also on that side of the earth.  Now we have the sun and the moon both pulling on the ocean from the same side.  The result is a higher tide.  Take this one step further (or closer) and imagine that the moon is in its particular part of its elliptical revolution around the earth where it happens to be in a position especially close to earth.  Now we have the sun and the moon both tugging on the earth in the same direction coupled with the moon at its closest point to the earth.  The result is an even higher tide.  When these three factors are combined with the irregular contours of the ocean floor, and the surrounding earth surface, and the way in which the earth tilts toward the sun, and the slight wobble of the earth as it rotates...the simple concept of high and low tide becomes a thought provoking phenomena.

            Then we have the timing of the tides.  Tides occur every 12 hours and 25 minutes (12 hours, 25.2 minutes to be more exact).  Therefore tides happen about one hour later every day.  Why?  Because the moon's lunar revolution around the earth takes 24 hours and 50 minutes.  When the moon is on our side of the earth it pulls the ocean into a bulge.  As it continues on its revolution around the earth the water level returns to normal as the bulge is pulled along with it.  Half way through its complete revolution, 12 hours and 25 minutes later, the moon is on the other side of the earth from us and hence exerts the least amount of pull on our ocean.  Now we have a high tide.  (Keep in mind that the earth rotates on its axis in the same direction that the moon revolves around the earth.)

Spring Tides - Occur during new moon phase and full moon phase when the sun and moon are aligned.  Produce the highest tides of the year, but actually occur at times other than spring.

·         Perigee tides:              Twice per year as the moon comes closest to earth and produce the highest tides

Neap Tides - Occur during the first and third quarters of the moon.  The moon and sun are at right angles to each other and therefore pulling against each other.  This alignment produces the lowest tides.         

·         Apogee tides:  Twice per year when the moon is farthest away from earth and produces the lowest tides

            Think about it.  When you see a tide rise you are actually witnessing a giant tidal wave on its way around the earth.  You are seeing the ocean being pulled along, mainly by the moon, but also by the sun, the shape of the earth and the rotation and revolution of earth and moon.  And think about one other thing.  Behind all of this is the most enormous, yet least understood power in the universe.  It's a force we are all intimately familiar with.  We call it gravity.    But that's a topic for another essay.

HOOAH

Jack