The Motion of the Ocean

This topic submitted by Oliver Dunn ( Dunnod@miamioh.edu) at 2:01 PM on 6/7/02.

The "Three Amigos" pose in the Bahamas

Tropical Field Courses -Western Program-Miami University


Oliver Dunn
The sheer power and force of the ocean has been something that has amazed me for as long as I can remember. I remember going on family trips to places like Captiva, Florida and looking out onto the waters in awe that there is something so big and mysterious on this earth. I was curious as to why the water was high at some time and low at others. I would wake up at 5 in the morning in my early years to catch the water when it was high; this would be the time to find the coolest things like living horseshoe crabs, sand dollars and puffer fish. This would be the highlight of my trips to Captiva and every year we went it would be the same thing. Since high school our family has not made our yearly journey to Florida, but I feel that part of me is still there. When it came to thinking about tides, later in life, I realized that it had something to do with gravity and that is about as much attention that I ever paid to it. When most people think of tides an image of annoyance comes to mind in thinking about how it washes beach chairs away and ruins sandcastles. In reality tides are amazing things that have a cyclical beauty all of their own. In this essay I will discuss tides, types and the effects of the gravitational pull of the sun and moon.
The sun and moon play an integral role when it comes to the effects of tide. Gravity is the chief player in the motion of the ocean. Something that was surprising to me was that even though the sun is many times larger then the moon, it has less of a gravitational pull on the earth. The moon is the largest influence on the ebb and flow of the oceans. The exact role of the sun and moon will be gotten to later while discussing the types of tides.
Knowledge of tides have been around for thousands of years, pretty much since the oceans had been used as major highways. In 325 BC Greece an explorer by the name of Pytheas connected tides with the phases of the moon, but did not understand why. Years later people got wise and invented theories as to why this occurred, some such theories pertained to a great whale breathing and this is what caused it. Others thought that to be ridiculous and it has to be God’s foot coming in and out of the oceans and it must just be water displacement. Finally it took Sir Isaac Newton to come up with something that wasn’t moronic. He found that every piece of matter is attracted by the gravitational pull of the moon. To understand why tides vary in height and frequency in different parts of the world, imagine a smooth- surfaced globe completely covered by water around which the moon revolves every 28 days. Each particle of water is attracted toward the center of the moon, but the farther away it is from the center, the weaker the attraction. So the waters nearest the moon is more strongly attracted to it and consequently is drawn out from the Earth’s surface in a bulge. Because the Earth is rotating, centrifugal force is exerted. This opposes gravity, and this opposition keeps everything in balance. On the other side of the earth farthest away from the moon, another bulge occurs. This happens because there is a weaker gravitational pull and the water tries to move away from the surface of the earth.
The tidal pattern so established on the entirely smooth globe can be perfectly seen to bring about two high tides each lunar day, one for the gravity-derived bulge toward the moon, the other for the bulge away from the earth. Two low tides occur in those parts of the globe at right angles to the moon. The evening high tides on successive days tend to be more similar than the tides occurring on the morning and evening of the same day.
With something as simple as a tide there are actually different types. The moon and sun as have been noted are the chief players when it comes to the gravitational pull of the oceans. The tide raising power of the moon is 2.2 times greater then the influence of the sun. It is estimated that the attraction the moon exerts on a molecule of water on Earth is six million times smaller then the attraction from gravity, the sun has even less then that. Spring and Neap tides are the types of tides, which occur in oceans.
A spring tide occurs when the sun and the moon are in line with each other. This can either be that both are on the same side, also if they are on opposite sides. The spring tide is the stronger of the two with good reason. With this type both the sun and the moon are working in unison to create the most gravity possible. Below is a diagram of this. The innermost circle is the earth, which is surrounded by the elliptical waters. The orbit of the moon and the sun creates the elliptical water, which is the outer circle.

The other type of tide is the neap tide. The moon and the sun being at right angles of each other cause this tide. A neap tide is the weaker of the two because the sun and the moon are not working together but rather in opposing directions. There is an effect of gravitational pulls canceling each other out during a neap tide. As can be seen below the sun and moon are at right angles changing the elliptical shape of the waters.

The rotation of the Earth influences the times of tides. If the moon revolved round the Earth at the same speed as the Earth’s own rotation, a high tide would occur at a fixed point on the surface every 12 hours. However, although the moon moves around the Earth in the same direction as the Earth is rotating, it moves just slightly slower. So the earth has to rotate for an extra 50 minutes each day to bring the same point under the moon. This means that each of the two spring tides will be 25 minutes later each day.
The moon complicates things still further by not moving around the Earth in exact orbit. It swings elliptically backward and forward in relation to the equator, setting the oceans oscillating in two types of tidal rhythm. In some areas the result is the pattern with which Europeans are familiar, two high and two low waters a day at roughly similar heights. These are called semidiurnal oscillations. Elsewhere, such as in the Gulf of Mexico, there is only one high and one low tide each 24 hours and 50 minutes. This is called a diurnal oscillation. In still other regions such as the Pacific and Indian Oceans, there are two high and two low waters a day, but the difference in heights of the morning and afternoon high waters is considerable. These are called mixed tides.
When the sun and moon pull on the world oceans to create a bulge on either side of the Earth, it is not the bulge that is the tide. This can be proved by checking the time of a particular high tide against the position of the moon. If the bulge were causing the tide, the moon would be directly overhead. But the time difference between the passage of the moon and high water can be anything up to 12 hours and 25 minutes. This is not to say that high water will never occur when the moon is overhead but only that if it did, it would be purely coincidental.
What causes tidal flow, then, is not the bulge but the flow of water horizontal to the Earth’s surface as it moves to make the bulge. When a body of water is subjected to a regular disturbance it starts a rocking motion from side to side along a virtually motionless central line. The time taken for each rocking motion is determined by the shape of the coastlines surrounding the body of water, its depth, atmospheric conditions, and any submarine features such as mountains and trenches. The time the rock takes is called the natural period of oscillation.
There are many animals other then humans who depend on the tide to help them. When it comes to humans we need to know that our ships will not become beached if left overnight. Animals such as the grunion on the West Coast depend on high tide for reproduction. It is very well timed mating ritual they have developed; the eggs are released, fertilized and buried all in one ebb of the tide. The sea anomie in the tide-pools of Monterey Bay, California depend on the tide to bring food to them, otherwise they would die. There are many forms of life that depend on the tide. It is amazing the sheer power and simplicity of this great force.


References

Barton, Robert. 1980, “The Oceans”
Day, Trevor. 1999, “Ecosystem: Oceans”
Erickson, Jon. 1996, “Marine Geology: Undersea Landforms and Life Forms”
Fundamentals of Physical Geography, http://www.geog.ouc.bc.ca/physgeog/contents/8r.html
High Tide in Ocean Modeling, psc.edu/science/okeefe/okeefe.html
Lunar Tides, http://csep10.phys.utk.edu/asrt161/lect/time/tides.html
Svarney, Thomas E. 2000, “The Handy Ocean Answer Book”


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