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Tides and Their Causes
The tides are a natural phenomenon involving the alternating rise and fall in the large fluid bodies of the earth caused by the combined gravitational attraction of the sun and moon.
The combination of these two variable force influences produce the complex recurrent cycle of the tides. Tides may occur in both oceans and seas, to a limited extent in large lakes, the atmosphere, and, to a very minute degree, in the earth itself. The period between succeeding tides varies as the result of many factors and force influences.
The tide-generating force represents the difference between (1) the centrifugal force produced by the revolution of the earth around the common center-of-gravity of the earth-moon system and (2) the gravitational attraction of the moon acting upon the earth's overlying waters. Since, on the average, the moon is only 238,852 miles from the earth compared with the sun's greater distance of 92,956,000 miles, this closer distance outranks the much smaller mass of the moon compared to the sun, and the moon;s tide raising force is, accordingly, 2 1/2 times that of the sun.
The effect of the tide-generating forces of the moon and sun acting tangentially to the earth's surface (the so-called "tractive force") tends to cause a maximum accumulation of the waters of the oceans at two diametrically opposite positions on the surface of the earth and to withdraw compensating amounts of water from all points 90 degrees removed from the positions of these tidal bulges. As the earth rotates beneath the maxima and minima of these tide-generating forces, a sequence of two high tides, separated by two low tides, ideally is produced each day.
Twice in each lunar month, when the sun, moon, and earth are directly aligned, with the moon between the earth and sun (at new moon) or on the opposite of the earth from the sun (at full moon), the sun and the moon exert their gravitational force in a mutual or additive fashion. Higher high tides and lower low tides are produced. These are called spring tides .
At two positions 90 degrees in between, the gravitational forces of the moon and sun - imposed at right angles - then to counteract each other to the greatest extent, and the range between high and low tides is reduced. These are called neap tides.This semi-monthly variation between the spring and neap tides is called the phase inequality.
The inclination of the moon's orbit to the equator also produces a difference in the height of succeeding high tides and in the depression of succeeding low tides which is known as the diurnal inequality. In extreme cases this phenomenon can result in only one high tide and one low tide each day.
The actual range of tide in the waters of the open ocean may amount to only one or two feet, However, as this tide approaches shoal waters and its effects are augmented the tidal range may be greatly increased. In Nova Scotia along the narrow channel of the Bay of Fundy, the range of tides may reach 43 feet or more (under spring tide conditions) due to resonant amplification.
In every case, actual high or low tide can vary considerably from the average due to weather conditions such as strong winds, abrupt barometric pressure changes, or prolonged periods of extreme high or low pressure.
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