TIDES;

any of the cyclic deformations of one astronomical body caused by the gravitational forces exerted by others. The most familiar are the periodic variations in sea level on the Earth that  correspond to changes in the relative positions of the Moon and the Sun; the daily fluctuation of elevation of the water surfaces of oceans and seas and the larger lakes of the world.

Tide-generating forces:  The forces that cause the tides are called the tide-generating forces. A tide-generating force is the resultant force of the attracting force of the Moon or the Sun and the force of inertia (centrifugal force) that results from the orbital movement of the Earth around the common center of gravity of the Earth-Moon or Earth-Sun system.

At the surface of the Earth the gravitational force of the Moon is about 2.2 times greater than that of the Sun. The tide-producing action of the Moon arises from the variations in its gravitational field over the surface of the Earth as compared with its strength at the Earth's center. The effect is that the water tends to accumulate on the parts of the Earth's surface directly toward and directly opposite the Moon and to be depleted elsewhere. The regions of accumulation move over the surface as the position of the Moon varies relative to the Earth, mainly because of the Earth's rotation but also because of the Moon's orbital motion around the Earth. There are approximately two high and two low tides per day at any given place, but they occur at times that change from day to day; the average interval between consecutive high tides is 12 hours 25 minutes. The effect of the Sun is similar and additive to that of the Moon. Consequently, the tides of largest range or amplitude  (spring tides) occur at New Moon, when the Moon and the Sun are in the same direction, and at Full Moon, when they are in opposite directions; the tides of smallest range  (neap tides) occur at intermediate phases of the Moon.

 Although the observed tides possess the aforementioned broad features, this pattern does not correspond to a pair of bulges that move around the Earth. The inertia of the water, the existence of continents, and effects associated with the water depth result in much more complicated behavior.

For the main oceans, a combination of theory and observation indicates the existence of amphidromic points, at which the tidal rise and fall is zero: patterns of high and low tides rotate around these points (either clockwise or counterclockwise). Amplitudes are typically less than a meter.  Tides are most easily observed--and of greatest practical importance--along seacoasts, where the amplitudes are exaggerated. When tidal motions run into the shallow waters of the continental shelf, their rate of advance is reduced, energy accumulates in a smaller volume, and the rise and fall is amplified. The details of tidal motions in coastal waters, particularly in channels, gulfs, and estuaries, depend on the details of coastal geometry and water-depth variation. Tidal amplitudes, the contrast between spring and neap tides, and the variation of times of high and low tide all vary widely from place to place.

 For these reasons, purely theoretical calculation of the times and heights of tides at a particular station is quite impossible. Nevertheless, tides are quite successfully predicted on the basis of accumulated observations of the tides at the place concerned. The analysis of the observations relies on the fact that any tidal pattern (in time) is a superposition of variations associated with periodicities in the motions of the Moon and the Sun relative to the Earth. The periods involved are the same everywhere, ranging from about 12 hours to a year or more, but the relative sizes of their contributions are highly variable. Observations over a sufficient time make it possible to calculate which contributions are significant at a particular location and, thus, to forecast tidal times and heights. It is common that 40 components may be significant for practical calculations at one location.

In addition to tides in the oceans (and in large lakes, where similar processes occur with smaller amplitudes), there are analogous gravitational effects on the atmosphere and on the interior of the Earth.   Atmospheric tides are detectable meteorological phenomena but are a comparatively minor component in atmospheric motions.   An Earth tide (q.v.) differs from oceanic and atmospheric ones in that the response to it is an elastic deformation rather than a flow. Observations of Earth tides contribute to knowledge of the internal structure of the Earth.

Tidal processes can, of course, also occur on other members of the solar system. As just one example, it has been suggested that the volcanic activity of Jupiter's satellite  Io is the consequence of internal heating by frictional resistance to tidal deformation.

In contrast to the tides of the Atlantic--which are almost always semi-diurnal (i.e., twice-daily) occurrences--those of the Pacific include many instances of   diurnal (daily) and  mixed tides. In the diurnal type of tidal oscillation, only a single high water and a single low water occur each tidal day (which lasts for about 24 hours and 50 minutes). Tides of this type occur in the Gulfs of Tonkin and Thailand in Southeast Asia, the Java Sea in Indonesia, and Bismarck and Solomon seas north and east of the island of New Guinea. Mixed tides, in which both diurnal and semidiurnal oscillations appear, are characterized by large inequalities in successive high (or low) water heights. This type of tide is prevalent along much of the Pacific coast.

At certain places in the South Pacific the natural period of oscillation of the sea accentuates the    solar tidal oscillation. At these locations the time of the AM (or PM) high (or low) water, instead of getting later each day by about 50 minutes (as is generally the case), occurs at approximately the same time for several days in succession. The tide at  Tahiti, for example, follows the Sun and not the Moon--the time of high water occurring, day after day, at about midnight and noon, and that of low water at about 6 AM and 6 PM.

In general, tidal ranges within the Pacific are small. That at Tahiti is about one foot; at Honolulu it is about two feet; at Yokohama, it seldom exceeds five feet; and at Cape Horn it is never more than about six feet. In the upper reaches of the Gulf of California and Korea Bay, however, very large tidal ranges of more than 40 feet are quite common.

Considering the Earth-  Moon system, at any time the tide-generating force is directed vertically upward at the two places on the Earth where the Moon is in the vertical (on the same and on the opposite side of the Earth); it is directed vertically downward at all places (forming a circle) where the Moon is in the horizon at that moment. At all other places, the tide-generating force also has a horizontal component. Because this pattern of forces is coupled to the position of the Moon with respect to the Earth and because for any place on the Earth's surface the relative position of the Moon with respect to that place has, on the average, a periodicity of 24 hours 50 minutes, the tide-generating force felt at any place has that same periodicity. When the Moon is in the plane of the equator, the force runs through two identical cycles within this time interval because of the symmetry of the global pattern of forces described above. Consequently, the tidal period is 12 hours 25 minutes in this case; it is the period of the  semidiurnal lunar tide. The fact that the Moon is alternately to the north and to the south of the equator causes an inequality of the two successive cycles within the time interval of 24 hours 50 minutes. The effect of this inequality is formally described as the superposition of a partial tide called the  diurnal lunar tide, with the period of 24 hours 50 minutes, on the semidiurnal lunar tide.

 In the same manner, the Sun causes a semidiurnal solar tide, with a 12-hour period, and a diurnal solar tide, with a 24-hour period. In a complete description of the local variations of the tidal forces, still other partial tides play a role because of further inequalities in the orbital motions of the Moon and the Earth. The interference of the solar-tidal forces with the lunar-tidal forces (the lunar forces are about 2.2 times as strong) causes the regular variation of the tidal range between  spring tide, when it has its maximum, and   neap tide, when it has its minimum.

Although the tide-generating forces are very small in comparison with the Earth's force of gravity (the lunar tidal force at its maximum being only 1.14 [{times}] 10{sup -7} times the force of gravity), their effects upon the sea are considerable because of their horizontal component. Since the Earth is not surrounded by an uninterrupted envelope of water but rather shows a very irregular alternation of sea and land, the mechanism of the response of the oceans and seas to the tidal forces is extremely complex. A further complication is caused by the deflecting force of the Earth's rotation (the Coriolis force; see above).

 In enclosures formed by   gulfs and bays, the local tide is generated by interaction with the tides of the adjacent open ocean. Such a tide often takes the form of a running tidal wave that rotates within the confines of the enclosure. In some semi-enclosed seas, such as the Mediterranean, Black, and Baltic seas, a standing wave, or tidal seiche, may be generated by the local tide-raising forces.

In these seas, the tidal range of sea level is only on the order of centimeters. In the open ocean, it generally is on the order of tens of centimeters. In bays and adjacent seas, however, the tidal range may be much greater, because the shape of a bay or adjacent sea may favor the enhancement of the tide inside; in particular, there may be a resonance of the basin concerned with the tide. The largest known tides occur in the Bay of   Fundy, where spring tidal ranges up to 15 meters have been measured.