When a navigator is preparing to take his vessel into port, one of his concerns is the depth of the water at the time of arrival. In nearly every port, that depth will vary from time to time as a result of a natural phenomenon known as tide. Knowledge of tidal action and how the height of tide-which in turn relates to depth of water-can be predicted and calculated is a part of the qualifications of anyone who would call himself "navigator."
Concern over adequate depths of water in channels, across bars, alongside piers and wharves, ete., there may in some locations be concern as to sufficient vertical clearance beneath a fixed bridge. Tides play a major role, for as depths increase for safety, vertical clearances decrease by the same amount.
The vertical rise and fall of the ocean level due to gravitational and centrifugal forces between the earth and the moon, is called tide. Local conditions cause considerable variations in tidal phenomena from place to place, but most places on the earth's oceans and connecting waters experience two high tides and two low tides each lunar day. High tide or high water is the highest level reached by an ascending tide. From high tide the level of the water decreases until it reaches a minimum level called low tide or low water. At high water and low water there is a period when no change in the water level can be detected. This period is called stand. The total rise or fall from low water to high water, or vice versa, is called the range of the tide. Mean sea level is the average height of the surface of the sea for all stages of tide.
The earth, and especially the oceans on it, are affected by the gravitional attraction between the earth and the moon, and by the centrifugal forces resulting from their revolution around a common center point located within the earth about 810 miles beneath the surface. The gravitational and centrifugal forces are in balance, and as a result, the earth and moon neither collide nor flyaway from each other in space. Although the earth-moon system as a whole is in equilibrium. The centrifugal force is the same everywhere, since all points on the earth's surface describe the same motion around the common center of mass; these forces are all parallel to each other and to a line joining the center of the earth and moon. The gravitational force is not everywhere the same; particles nearer the moon feel a greater force than on those on the far side of the earth; these forces are not parallel, each being in the direction from that to the center of the moon. There are a series of forces that will cause the surface water to flow toward the points on the earth's surface that are nearest and farthest from the moon. This flow causes higher than normal levels of water at these points, and lower than normal levels at the areas from which the flow comes. Although at the nearest and farthest points there is an indicated outward force, this is very slight and not nearly enough to cause an appreciable tide; the true tide results from the near-horizon forces causing the flow.
As the earth rotates each day on its axis, the line of direction toward the moon changes, and so each point has two highs and two lows. As result of the tilt of the earth's axis, the highs and lows are not normal of equal levels.
The tides that occur on earth are the result of the lunar and solar influences. When these two bodies are in line with the earth as at both new and full moon the two influences act together and the result is higher than average high tides and lower than normal low tides; these are called spring tides. This is true regardless of whether the moon is between the earth and the sun or on the opposite side.
When the directions of the sun and moon are 90° apart as at both first and third quarter moons the effect of the sun is to partially counteract the moon's influence. At these times, the high tides and low tides are lower than normal; these are neap tides.
As the moon revolves about the earth once each lunar month of roughly 28 days, its transit of any meridian on earth occurs approximately every 24 hours and 50 minutes. This is the period for two high waters and two low waters and is called a tidal day; the period for one high and one low is sometimes referred to as a tidal cycle. The daily rotation of the earth on its axis has a frictional effect on the tides so that high tides normally lag the time of the moon's transit across the meridian of any location.
Tides in the open oceans are only one to two feet high (with tides on the far side being about 5 percent less than those on the near side). Actual coastal tides are often much greater, in some places as much as 40 or 50 feet or more. This is the result of large land masses restricting the flow of water.
