TIDE, an alternating relative motion of the matter of a planet, satellite or star, which is attributed to the gravitational actions of external bodies. Such motions are known to occur in all three principal divisions of the earth. The alternating slight changes of shape of the solid body or lithosphere of the earth attributed to the gravitational action of the moon and sun are known as bodily tides. Similarly, there are the atmospheric tides in the atmosphere, and the familiar ordinary ocean tides in the hydrosphere. A still wider use of the word is based on descriptive instead of on causal and astronomical ideas, and takes for its criterion that the motions in question have certain features in common with ocean tides. This extension includes those long waves of the sea, or tsunamis, which are sometimes caused by earthquakes, as well as effects on the sea of variable atmospheric pressure and wind, known as meteorological tides.

General Description of Phenomena-At most seaside places the water reaches its highest level approximately twice a day, the average interval between two successive high waters being 12 hr. 25 min., though this interval varies considerably during the course of a week. At certain places in the East Indian seas two successive high waters are separated by an interval of 12 hr., while at certain places in the Qhifi-asea the interval is often more than 24 hr. At places on the shores,of the oceans the time taken by the tide in rising is about equal to the time taken in falling on the same day, but in estuaries the tide usually rises more quickly than it falls. At certain places, such as Southampton, Eng., the high waters are often doubled; i.e., the water reaches a maximum height, falls a little and then rises to a maximum again. At other places the low waters are often doubled. As one goes along any stretch of coast the time of high water generally becomes progressively earlier or later, while as one goes up an estuary from the sea the time of high water always becomes progressively later.

At most places, on the average, a high water is about as much above the mean level of the sea as the succeeding low water is below it. The difference in level between successive high and low waters is called the range of tide. The range of tide at any place may vary much from day to day. At most places it reaches a maximum once a fortnight, and a minimum at times midway between two successive maxima. At London bridge the greatest range of the fortnight has an average value of 21 ft. while the least range of the fortnight has an average value of 15 ft. At the head of the Bay of Fundy the range of tide reaches 50 ft. while at certain islands of the Pacific and over most of the Mediterranean the range never exceeds 2 ft. At many places outside the Atlantic the heights of two successive high or low waters are markedly different, a phenomenon known as the diurnal inequality.

At a place in a strait or narrow sea the tidal current usually flows for about 6 hr. 12 min. in one direction, and then for about the same time in the opposite direction. At the reversal of such a current there is the state of rest usually called slack water. In estuaries the current generally flows downstream for a greater length of time than it flows upstream. At a place in the open sea the direction of the current often takes all points of the compass, making the compl~te revolution in the tidal period. During this period there are usually two times of maximum current and two intermediate times of minimum current. The times of maximum current are separated by about half the tidal period and the directions of maximum current are nearly directly opposite; similarly for the two minimum currents. There is no fixed general relation between the time of high water and the time of maximum current, If the current flows directly in and out of a bay it will reach its inward maximum nearly a quarter period before high water at the head of the bay, so that slack water is simultaneous with high water. On the other hand, in an estuary the current continues to flow upstream for some considerable time after high water and to run downstream similarly after low water. When the current is directed toward land or up an estuary it is called the flood current: when it runs away from land or down an estuary it is called the ebb current. The speeds of tidal currents vary greatly from place to place; e.g., in the Seymour Narrows, British Columbia, the maximum current reaches ten knots; in the North sea it rarely exceeds one. At some distance up certain rivers-as, for example, where the Colorado river meets the tide-a wave ten or more feet high travels up the river almost like a wall of water. This phenomenon is called a bore. Near a headland separating two bays there is sometimes a swift current termed a race.

Relation to Motions of Moon and Sun.--The times of high water bear an intimate relation to the positions of the moon and sun. The period of 12 hr. 25 min. is half that of the moon's apparent revolution around the earth. The length of time between the moon's crossing of the meridian of a place and the next high water at that place is known as the lunitidal interval, or the high,water interval, for the place. Similarly, the length of time between the moon's crossing of the meridian and the next low water is called the low-water interval. For Philadelphia, the lunitidal average is i hr. 30 min. In many cases, including those of British waters, the chief variation in the lunitidal interval is associated with the phase of the moon. The average value of the lunitidal interval on the days of new and full moon is known as the establishment of the port.  For London bridge this is 1 hr. 58 min.

The range of tide may be similarly correlated. In British waters it reaches its maximum a day or so after new and full moon and its minimum a day or so after the quarters. In these circumstances the maximum tides are known as spring tides and the minimum tides as neap tides. About the time of the equinoxes spring tides are generally larger and about the time of the solstices they are generally smaller than usual. The average interval between new or full moon and the next following spring tide is known as the age of the tide at the place in question. At certain places in Canadian waters the chief variation in the range of tide is associated with the varying distance of the moon from the earth, while at others it is associated with the varying declination of the moon.  The diurnal inequality is always associated with the declination oi the moon or sun. But the most complete correlation between the tides and astronomical variables is provided by the harmonic methods.