Tides

Introduction

Tides in the ocean are primarily concerned with the gravitational forces of the sun and moon. It may be pointed out that the earth rotates from west to east and revolves around the earth along an elliptical orbit. Similarly, the moon rotates from west to east and revolves around the earth along an elliptical orbit so that the distance between the moon and the earth changes at different times every month. The period of the farthest distance between the moon and the earth is called apogee while the period of the nearest distance is called perigee.

The earth’s outer surface is 3,77,600 km away from the outer surface of the moon. It is evident that the earth’s outer surface, which is opposite to that surface of the earth faces the moon 3,90,400 km away from the moon’s surface. The gravitational force of the moon will be maximum at the earth’s surface facing the moon. While it will be minimal at the opposite side of the earth. Consequently, the water of the earth’s surface facing the moon is attracted and pulled and high tide occurs. Low tide formed at the opposite side of the earth simultaneously because of the reactionary force of the gravitational force of the moon causing an outward bulge of water.

Theories of the Origin of Tides 

The equilibrium theory: According to this theory the diameter of the earth is about 8000 miles. Consequently, the side of the earth facing the moon is 4000 miles nearer to the moon than the centre, and 8000 miles nearer the opposite side. The moon, therefore attracts the near side of the earth more strongly than it attracts the Centre and the far side less strongly.

Apart from that, the earth is not in a fixed position and it yields to the attraction of the moon. The moon does not revolve around the earth, but both eartthe h and moon revolve around their common Centre of gravity. Owing to the much greater mass of the earth this common Centre is situated about 1000 miles beneath the surface of the earth.

Since the earth and moon neither approach nor receive from each other, it is evident that at the Centre of the earth, the centrifugal force is just equal to the attractive force. On the side nearest the moon the attractive force is greater than that of the centrifugal force and there is accordingly a surplus force pulling waters towards the moon. On the side farthest from   the moon the attractive force is less than at the Centre, and is, therefore, less than the centrifugal force, and there is a surplus force directed from the moon.

Thus the bulging of the water beneath the moons due to the excess of the centrifugal force about the attractive force.  In this way, according to the equilibrium theory the moon causes the water to be drawn towards the part of the earth facing it and also to the opposite side, and this produces high tide.

When the moon is at or near maximum declination the tides are no longer symmetrical about the equator and relative to a given parallel of latitude there will be one high water higher than usual and one lower. When the moon is a zero declination the tides will once again be symmetrical about the equator.

The Progressive Wave Theory: influence of the continental masses. From the preceding account, it might be inferred that at each place high tide should occur when the moon is on the meridian, and all places in the longitude should have their high tide at the same time. The movement of the tidal wave is also greatly influenced by the continental and other land masses.

The great land masses stretch from north to south and prevent its progress. Since in the Antarctic Ocean, there is an open and obstructed course, it was argued that is only in this southern sea that the world from east to west, like long but very low waves from theory it appears that the Atlantic tidal waves travel more rapidly in the deep water of mid-ocean than in the shallower water along the shore, and the crest become curved.

 

The stationary wave theory: The amount of tidal known ledge has increased rapidly in recent years, and it is now no longer possible to accept the simple explanations of many of the phenomena at first sight adequately dealt with by the progressive wave theory. According to that hypothesis, the tide travelled around the northern ocean and sent waves travelling northwards along the Pacific, Indian, and Atlantic Oceans.

Modern work on tides replaces a world phenomenon by regional or even local phenomena, and instead of assuming a progressive wave either for the world or for an individual ocean a stationary wave either for the world or for an individual ocean a stationary wave is postulated. This idea is simple and easily illustrated. If a fairly shallow rectangular tank is partly filled with water it is quite easy to set the water in it rocking so that it is high in the middle at the same time that it is quite easy to set the water in it rocking so that it is high in the middle at the same time that it is low at both ends.