Tides are deformations in the shape of a body caused by the gravitational force of one or more other bodies. At least in theory, any two bodies in the Universe exert such a force on each other, although obvious tidal effects are generally too small to observe. By far the most important examples of tidal forces as far as humans are concerned are ocean tides that occur on Earth as a result of the Moon and Sun's gravitational attraction.
The side of Earth facing the Moon, due to the Moon's proximity, experiences a larger gravitational pull, or force, than other areas. This force causes ocean water, since it is able to flow, to form a slight bulge, making the water in that area slightly deeper. At the same time, another bulge forms on the opposing side of the Earth. This second bulge, which is perhaps a bit harder to understand, forms due to centrifugal force. Contrary to popular belief, the Moon does not revolve around the Earth, but rather the Earth and Moon revolve about a common point that is within the Earth, but nowhere near its center (2880 miles or 4640 km away). When you twirl a ball above your head at the end of a piece of string, the ball pulls against the string. This pull is known as centrifugal force.
When the Earth-Moon system revolves around its common axis, the side of Earth that is farthest from the Moon experiences a centrifugal force, like a ball spinning at the end of a string. This force causes a second tidal bulge to form, which is the same size as the first. The result is that two lunar tidal bulges exist on Earth at all timesne on the side of the Earth facing the Moon and another directly opposite to it. These bulges account for the phenomenon known as high tide.
The formation of these two high tide bulges causes a belt of low water to form at 90° to the high tide bulges. This belt, which completely encircles the Earth, produces the phenomenon known as low tide.
As Earth rotates on its axis, land areas slide underneath the bulges, forcing the oceans up over some coastlines and beneath the low tide belt, forcing water out away from other coastlines. In a sense, as Earth rotates on its axis, the high tide bulges and the low tide belt remains stationary and the continents and ocean basins move beneath them. As a result, most coastal areas experience two high tides and two low tides each day.
In addition to the lunar bulges, the Sun forms its own tidal bulges, one due to gravitational force and the other due to centrifugal force. However, due to the Sun's much greater distance from the Earth, its tidal effect is approximately one half that of the Moon.
When the Moon and Sun are in line with each other (new Moon and full Moon), their gravitational, or tidal forces, combine to produce a maximum pull. The tides produced in such cases are known as spring tides. The spring high tide produces the highest high tide and the spring low tide produces the lowest low tide of the fortnight. This is the same as saying the spring tides have the greatest tidal range, which is the vertical difference between high tide and low tide.
When the Moon and Sun are at right angles to each other (first and third quarter Moon), the two forces act in opposition to each other to produce a minimum pull on the oceans. The tides in this case are known as neap tides. The neap high tide produces the lowest high tide and the neap low tide produces the highest low tide, or the smallest tidal range, of the fortnight.
It is now possible to write very precise mathematical equations that describe the gravitational effects of the Moon and the Sun. In theory, it should be possible to make very precise predictions of the time, size, and occurrence of tides. In fact, however, such predictions are not possible because a large number of factors contribute to the height of the oceans at high and low tide at a particular location. Primary among these is that the shape of ocean basins is so irregular that water does not behave in the "ideal" way that mathematical equations would predict. However, a number of other variables also complicate
the situation. These include variations in the Earth's axial rotation, and variations in Earth-Moon-Sun positioning, including variations in orbital distance and inclination.
Scientists continue to improve their predictions of tidal variations using mathematical models based on the equilibrium theory of tides. However, for the present, estimates of tidal behavior are still based on previous tidal observations, continuous monitoring of coastal water levels, and astronomical tables. This more practical approach is referred to as the dynamical theory of tides, which is based on observation rather than mathematical equations.
The accumulated information about tidal patterns in various parts of the world is used to produce tide tables. Tide tables are constructed by looking back over past records to find out for any given location the times at which tides have occurred for many years in the past and the height to which those tides have reached at maximum and minimum levels. These past records are then used to predict the most likely times and heights to be expected for tides at various times in the future for the same locations. Because of differences in ocean bottoms, coastline, and other factors, unique tide tables must be constructed for each specific coastline every place in the world. They can then be used by fishermen, those on ocean liners, and others who need to know about tidal actions.
In most places, tides are semidiurnal, that is, there are two tidal cycles (high and low tides) each day. In other words, during a typical day, the tides reach their highest point along the shore and their lowest point twice each day. The high water level reached during one of the high tide stages is usually greater than the other high point, and the low water level reached during one of the low tide stages is usually less than the other low tide point. This consistent difference is called the diurnal inequality of the tides.
In a few locations, tides occur only once a day, with a single high tide stage and a single low tide stage. These are known as diurnal tides. In both diurnal and semidiurnal settings, when the tide is rising, it is called the flood tide. When the tide is falling, it is the ebb tide. The point when the water reaches its highest point at high tide, or its lowest point at low tide, is called the slack tide, since the water level is static, neither rising nor falling, at least for a short time.
As the Moon revolves around the Earth, the Earth also rotates on its axis. Consequently, the Earth must rotate on its axis for 24 hours, 50 minutes, known as a lunar day, to return to the same position relative to the Moon above. The additional 50 minutes allows Earth to "catch up" to the Moon, so to speak. In other words, if the Moon was directly overhead at Boston, Massachusetts, at noon yesterday, it will again be above Boston at 12:50 PM today. As a result, on a coast with diurnal tides, each day the high tide (or low tide) will occur 50 minutes later than the day before. Whereas, on a semidiurnal coast, each high tide (or low tide) will occur 12 hours, 25 minutes later than the previous high one.
The movement of ocean water as a result of tidal action is known as a tidal current. In open water, tidal currents are relatively weak and tend to change direction slowly and regularly throughout the day. They form, therefore, a kind of rotary current that sweeps around the ocean like the minute hand on a clock. Closer to land, however, tidal currents tend to change direction rather quickly, flowing toward land during high tide and away from land during low tide. In many cases, this onshore and offshore tidal current flows up the mouth of a river or some other narrow opening. The tidal current may then attain velocities as great as 9 mi (15 km) an hour with crests as high as 10 ft (3 m) or more.
Most tides attain less than 10 ft in size; 30 ft (1 m) is common. In some locations, however, the tides may be much greater. These locations are characterized by ocean bottoms that act as funnels through which ocean waters rush upward towards or downward away from the shore at very rapid speeds. In the Bay of Fundy, between Nova Scotia and New Brunswick, for example, the difference between high and low tides, the tidal range, may be as great as 46 ft (14 m). In comparison, some large bodies of water, such as the Mediterranean, Baltic, and Caribbean Seas, have areas with tides of less than 1 ft (0.3 m). All coastal locations (as well as very large lakes) experience some variation in tidal range during a fortnight due to the affects of neap versus spring tides.
See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Gravity and the gravitational field; Marine transgression and marine regression
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