Tides of Planet Earth

As we know, planet earth’s surface is mostly covered in water, about 70% of it. The oceans have a tidal system which consists of high tides and low tides alternating throughout a twenty four hour period. You can see the tides mostly at shore areas where the sea level rises and falls very regularly. In some part of the world they can move vertically by less than a meter, for example the Mediterranean and around Jamaica (2). But in the Bay of Fundy and Canada they have a tidal range of up to 15 meters (2). So why do the oceans appear to disappear and come back, what could cause such an effect and what effect does it have on our world?

The three major forces affecting the tides are the moon, sun and the earth’s rotation. Gravity is the force we know well, without it we would all be floating in space. But it also keeps order in our solar system. Gravity is the force responsible for keeping all the planets in our solar system in the orbits that we know today. It is also the same force that keeps the moon circling and orbiting the earth. But it is not limited to managing the dynamics of the planets, it also has a much more observable influence on the earth. The moon is approximately 248,000 miles away from the earth and has the biggest effect on the tides. Although the strength of the suns gravity is 179 times that of the moons, the sun is much further away at approx. 93 million miles away. This means that the sun is responsible for only 44% of the tidal forces on earth whereas the moon is responsible for 56% (5).

Due to the cyclic rotation of the earth and moon, the tide cycle, two low tides and two high tides, is 24 hours and 52 minutes (5). During those hours, any one point on the earth surface, cycles through two high tides and two low tides.  To understand how, you have to understand the motion of the earth and moon. Both are always moving through space and as the earth spins on its own axis, that motion keeps the water balanced on all sides because of centrifugal force. But a ‘bulge’ is caused by the gravitational pull of the moon. A ‘bulge’ also occurs on the opposite side of the earth due to earth’s rotation. The two sides where the bulges appear are at high tide. The other two sides are at low tide. As the moon moves around the earth, then the force moves with it as well, causing the tidal effects to move around the earth.

The tidal range (the highest point of the tide to the lowest) can vary over the course of a month. The moon is not always the same distance from the earth, the moons orbit brings it closer in. This part of the orbit is called perigee(4). Its gravitational pull can be 50% stronger at this point and leads to higher tides(4).  The opposite of this is called apogee (4), when the moon is furthest away from the earth. Meaning the tides are smaller.

But all the forces from the moon are also affected by the sun, depending on the suns position. If the sun and moon are at right angles to each other, not lining up, then the high tides are not as high as they usually would be. This is because, despite being very far away, the sun still has an effect on the oceans. This is enough force to lessen the height of high tide by pulling the water in the low tide areas. This is called a neap tide. Likewise if the sun and moon are lined up either on the same side of the earth or at opposites, then the suns gravitational pull effectively adds more pull to the moons force.  This creates higher, high tides.  These are known as spring tides, not after the season, but because the water ‘springs’ higher than normal. The variance in the height of tides can also depend on the local geography of the coastline and the topography of the ocean floor.

Tides can be said to be regular in that they occur twice a day. But the timings don’t work on the same 24 hour day that we use for our calendar. Meaning the high tides are staggered through the course of the month, each tide cycle starts 24 hours and 52 minutes after the one before it. This is because the moon takes slightly longer, 52 minutes, to get back to the same position on the earth it started from.

So the moon, sun and their orbits are what cause the tides to go in and out on planet earth, literally pulling the water away from the surface because of gravitational force. But do the tides have any direct effect on the earth other than merely inconveniencing the seaweed that awaits the waters return?

While the moons gravity is pulling the water away from the earth, it is also stretching the rocks and solid structures by a tiny fraction. This means that in one complete rotation of the earth, the planets solid mass is being squeezed and relaxed just like the water is as well. This movement of materials generates heat. Generating that heat uses energy and this is taken from the earth’s rotation, meaning it is slowing the earth down. Every 100 hundred years that pass, hour 24 hour day increases by Approx. 2.3 milliseconds (3). This may seem a very small amount of time. But earth is estimated to be 4.55 billion years old (6). This means that, if we had invented the time system for a day back then, our days would now be 45.5 hours longer (1). A considerable amount of time for a tiny frictional ‘braking’ force caused because our planet is stretched by the moon.  Imagine what effects all the other, much larger, forces of the universe are having on planet earth.

 

References

 1. Crawford, J. (1994). Tidal Friction. Available: http://www.physics.mcgill.ca/~crawford/PSG/PSG11/204_97_L11.9_tidfric.html . Last accessed 16th Oct 2012.

2.Meadows, P.S. (1988). Tides. In: Campbell J.I. An Introduction to Marine Science 2nd edition. 2nd ed. Glasgow & London: John Wiley and Sons. 33-35.

3. Ray, R. (2001). Ocean Tides and the Earth’s Rotation. Available: http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html. Last accessed 16th Oct 2012.

4. Rose, D. (2000). The Oceans Tides Explained. Available: http://www.moonconnection.com/tides.phtml. Last accessed 15th Oct 2012.

5. Rosenberg, M. (2012). Tides. Available: http://geography.about.com/od/physicalgeography/a/tides.htm. Last accessed 16th Oct 2012.

6. Stassen, C. (2005). How Old is the Earth, and How Do We Know. Available: http://www.talkorigins.org/faqs/faq-age-of-earth.html. Last accessed 16th Oct 2012.

 

6 years ago

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