Tides And The Moon

Tides and the moon, what causes tides, the influence of the moon and sun, types, differences across the world, uses of tides, as well as information about tidal forces on other bodies in our Solar System.

Anyone who has sat by the edge of a pier, or taken a long walk by the sea-shore, must have noticed the way the level of the waters of the sea rise and fall periodically. This rising and falling of the level of the water is known as tides. But what is it that causes tides? School children all over the world have been told that the tides are caused by the gravitational force of the Moon. However if one points out that there are two low tides and two high tides daily, but the Moon appears above the Earth approximately once a day, it is difficult to see how the Moon causes tides. Careful consideration will also show that although the Sun is much further from the Earth as compared to the Moon, its mass is much more than that of the Moon, and consequently its gravitational pull on the Earth is much more than that of the Moon. Let us try to clear some of the confusion.

What causes tides?

For several centuries now, it was obvious that the tides were caused by the Moon, with Seneca, a Roman scholar, being the first to observe that the tides were especially high during the times that the Earth, Moon, and Sun were in the same line. However it was only in the seventeenth century that Sir Isaac Newton, put forward his theory of Gravitation in his book "Philosophiae Naturalis Principia Mathematica," and provided a mathematical explanation for the interaction between the Earth and the Moon. The tides are caused by the difference in the force of gravity between the end of the Earth closest to the Moon, and the opposite end, which is furthest away. While the Moon is pulling at the waters of the seas closest to it, causing a high tide, at the opposite end of the Earth, the pull is the least. This causes the waters to bulge away from the Moon, and therefore another high tide is formed on the side furthest away from the Moon. Somewhere in between these two points of high tides are two areas of low tides. This is what causes two high tides and two low tides daily.

Since the tides are caused by the difference in gravitational force between opposite ends of the Earth, it can be seen that although the Sun is massive as compared to the Moon, the difference in the Sun's gravitational force between one end of the Earth and the other is not as much as that of the Moon's.

The Moon takes about 27.3 days to rotate about its own axis, which is the same time it takes to complete one orbit around the Earth. This orbital period, is the true orbital period of the moon, and is called a Sidereal month, as it is measured against the stars. However as observed from the Earth, the Moon goes through its phases once every 29.5 days. This period is known as the Synodic month and the difference is caused by the fact that during the time taken by the Moon to complete its orbit, the Earth too has moved forward in its orbit around the Sun.

As the Earth rotates around its axis once every 24 hours, the Moon appears above the Earth approximately after an interval of 24 hours and 50 minutes. The 50 minute time delay is caused by the fact that while the Earth has completed one rotation within 24 hours, the Moon has also moved forward in its own orbit. This is the reason why the time interval between successive high tides or between successive low tides is 12 hours and 25 minutes. The time gap between one high tide and the following low tide is approximately 6 hours and 12.5 minutes.

Some common terms related to tides

Tides refer to the up and down motion of the water levels. The side to side or horizontal motion of the water is called currents, which flood in when the tides are high and ebb out when the tides are low. This is known as the flooding currents and ebbing currents respectively. During the period between high and low tides, there is a short period when there is no change in the level of the waters. This is known as the slack water period. The difference in the level of the high tide and the level of the low tide is called the range of the tide. Out in the open oceans the range of the tides is approximately 2½ feet. However in the coastal regions the actual levels of the tides depend on other factors like the winds, the depth of the waters, and the shape of the coast line.

Types of tides

The tidal force caused by the Sun on the Earth, is just 46% of the tidal force caused by the Moon on the Earth. Although it does not seem to be much, the effect is especially noticeable during Spring tides and Neap tides.

Spring tides are caused when the Moon and the Sun and the Earth are all in the same line, and the gravitational force of the Sun and the Moon both are acting in the same direction. This causes very high levels at High Tides and very low levels at Low tides. These are known as Spring tides, and could be caused even if the Moon and Sun are at opposite ends of the Earth. Spring tides occur approximately twice a month during the time of the Full Moon and the New Moon.

Approximately every 18 months during the Spring tide the Moon is at its closest distance to the Earth called its perigee. This gives rise to gravitational forces that are so strong, that they cause extremely high levels of tides. This is known as the Proxigean Spring Tide.

When the gravitational forces of the Sun and the Moon are perpendicular to each other, Neap tides occur. Since the gravitational force of the Sun is pulling in a direction opposite to that of the Moon, a part of the effect of the Moon's gravitational force is cancelled out. This gives rise to a very low level of High Tides and very high level of Low Tides. Neap tides usually occur during the quarter-moon phase of the Moon's cycle.

The relevance and uses of tides

Tides play an important part in Nature, with several marine organisms utilising the rising and falling of the water levels to obtain food. Birds and other animals living on the coastline also depend upon the tides to carry food in for them. In several parts of the world boats and ships can move in or out of harbour only when the tides rise, as only when the waters are deep enough, do the waterways become navigable. Certain fjords in Norway are so narrow, that when the tides are going up or down, it is very difficult if not impossible to navigate a vessel through. It is only between tides, during the slack water period that it is possible to pass through such fjords; at other times the water is too turbulent. The large waves caused by high tides are a source of recreational activities for surfers, wishing to ride the waves. Fishermen also have learned that they have a better chance of catching a fish when the tides are turning. For these reasons it is good to have a very clear idea of the timings of the tides, and how to predict routine changes from one day to the next, as well as information about the depth of the waters at various times.

Since the time of the Roman empire, man has attempted to harness the power of the tides for his own benefit. During the Roman occupation of England, several tide mills were built, which stored the incoming high tide waters behind a dam. During the periods when the tide waters were receding, the stored water was slowly released turning a mill, on its way out. These tide mills were used to grind grain and corn. The only costs involved were those incurred in building the mill as the energy of the tides was free and endless.

One of the most resourceful ideas related to tides is the use of the Tidal Energy to produce electricity. The renewable source of tidal energy used for producing electricity is one of the most environmentally friendly and clean methods of generating power. When the tides are rising, the water flowing in is used to drive a turbine, thus producing electricity. The incoming water is stored behind a temporary dam, and when the tides are falling, this stored water is released back into the ocean, again driving the turbines, and producing electricity. This technique is especially suited to places where the difference in levels between high and low tides is great. One such place is the Rance River, in France, where in 1966, infrastructure was installed to generate approximately a quarter Megawatt of power. The range of the tides here is approximately 25 feet. Another place where power is generated using tidal energy is the Annapolis Basin, Nova Scotia. Here about 20 Megawatts of power is generated meeting just 1% of the requirements of Nova Scotia.

Other places in the world where the range of the tides are higher than usual are the northern Coast of Australia, the English Channel, Ungava Bay and the Bay of Fundy, in Canada. The highest tides in the world occur in the Minas Basin, at the Bay of Fundy, where the range of the tides is around 45 feet. However this excellent source of renewable energy has not been exploited fully, although there was a proposal to erect a tidal power plant across Passamaquoddy Bay, one of the arms of the Bay of Fundy.

Tidal waves

Tidal waves are not caused by tides, or by the tidal effects of the Moon or Sun! Tidal waves are gigantic waves which come from the oceans and strike some coastal regions, causing great damage to homes, lives and belongings of people living there. These huge waves are caused by Earthquakes, volcanic eruptions, or landslides under the surface of the oceans. These tidal waves are also called tsunamis and some areas are more prone to them than others.

Other effects of the Moon

In addition to the effects of the Moon on the oceans of the Earth, the Moon also "˜pulls' other bodies of water, like that in lakes, bays, and coves. Since these are not as large as the oceans, the tidal effects are not usually discernible. However the levels have been seen to vary by several centimetres due to tidal effects. The tides in Lake Superior have been measured at around 2 inches. When these tidal effects are sometimes coupled with gusting winds, change in atmospheric pressure, or volcanic activity, the changes in the water levels can be pronounced. This effect is known as the seiche, and has been observed in Lake Geneva, Switzerland, and the California's San Francisco Bay, among other places.

Tidal effects are also observed in the Earth's atmosphere, as well as the land masses of the Earth. Relative to the centre of the Earth, the land and buildings may bulge by as much as 9 inches, depending on the latitude. This constant pulling on the land areas as well as the friction caused between the ocean's waters and the ocean floor, has led to a slowing down in the rotation of the Earth. This in turn has led to the lengthening of the day, by 0.002 seconds. This is why scientists in observatories who keep an accurate track of time, had to add a "˜leap second' to keep in time with the changes in the period of the rotation of the Earth. This concept is similar to that of the leap year where a day is added, approximately every four years. This constant slowing down of the rotation of the Earth, will over a few billion years lead to a situation when the Moon and Earth are "locked " together with the same side of the Earth and Moon facing each other.

Since the Earth's mass is several times greater than that of the Moon, the gravitational forces exerted by the Earth on the Moon is also greater. Although no oceans are present on the Moon today, the tidal forces are felt on the land causing the rotation of the Moon to slow down from its original speed, in a manner similar to the effect the Moon has on the Earth. Since the gravitational force of the Earth on the Moon is greater, than that of the Moon on the Earth, the slowing down of the Moon's rotation was more rapid, resulting in the present situation where the same side of the Moon always faces the Earth. Laser beams, along with reflectors placed on the Moon by astronauts, have helped in measuring accurately the distance between the Earth and the Moon. Repeated measurements have confirmed that the Moon is indeed moving away from the Earth at around 3.82 cm every year.

Since the distance between the Earth and Moon is slowly but surely increasing, the tidal forces on the Earth are constantly reducing by a corresponding degree. The ocean tides are consequently reducing as time goes by.

Although the tidal effect of the Moon on the land mass of the Earth is not much, on other planetary systems it is much more dramatic. Most notable in our Solar System is the interaction between the planet Jupiter and its moon Io. The mass of Jupiter is so large, that the tidal forces exerted by Jupiter on its moon Io is so great, that although there are no oceans on Io, the movement of the land masses is more than 5 times the ocean tides on earth. This large movement in the land generates a lot of heat in the core, which turns molten, and tries to escape from the surface resulting in volcanic activity. This is why among all the bodies in our Solar System, Io shows the largest amount of volcanic activity.

Although the tides are a beautiful thing to watch while lazing away a peaceful day, the power and violence that they can generate, is also awesome.

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