Tides

Tides are distortions that occur in the shape of a celestial body. They are caused by the gravitational force of one or more other celestial bodies on that first body. In theory, any two bodies in the universe exert a gravitational force on each other. The most important examples of tidal forces on Earth are ocean tides, which result from the mutual attraction of the Moon and the Sun.

Greek geographer Pytheas (c. 380 b.c.–c. 300 b.c.) was perhaps the first careful observer of ocean tides. In about the third century b.c., he traveled outside the Strait of Gibraltar and observed tidal action in the Atlantic Ocean. Pytheas suggested that the pull of the Moon on Earth's oceans caused the tides. Although largely correct, his explanation was not widely accepted by scientists until the eighteenth century, when English physicist and mathematician Isaac Newton (1642–1727) first succeeded in mathematically describing the tides and what cause them.

Theories of tidal action

Although the Sun is larger than the Moon, the Moon is closer to Earth and, therefore, has a greater influence on Earth's ocean tides. The Moon's gravity pulls on the ocean water on the near side of Earth. This force causes the water, since it is able to flow, to form a slight bulge outward, making the water in that area slightly deeper.

At the same time, on the opposing side of Earth, a second tidal bulge occurs that is the same size as the first. This second bulge forms because the force of the Moon's gravity pulls the solid body of Earth away from the water on Earth's far side. The result is that two lunar tidal bulges exist on Earth at all times—one on the side of 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-degree angles to the high tide bulges. This belt, which completely encircles Earth, produces the phenomenon known as low tide.

In addition to the lunar bulges, the Sun forms its own tidal bulges. However, due to the Sun's much greater distance from Earth, its tidal effect is approximately one-half that of the Moon.

Words to Know

Diurnal: Occurring once every day.

Ebb tide: Period when the water level is falling; the period after high tide and before low tide.

Flood tide: The period when the water level is rising; the period after low tide and before high tide.

High tide: The event corresponding to the largest increase in water level in an area that is acted upon by tidal forces.

Low tide: The event corresponding to the largest decrease in water level in an area that is acted upon by tidal forces.

Neap tides: Period of minimum tidal range that occurs about every two weeks when the Moon and Sun are at 90-degree angles to each other (the first and third quarter moons).

Semidiurnal: Occurring twice every day.

Slack tide: Period during which the water level is neither rising nor falling.

Spring tides: Period of maximum tidal range that occurs about every two weeks when the Moon and Sun are in line with each other (at the new and full moons).

Tidal current: Horizontal movements of water due to tidal action.

Tidal range: Vertical distance in sea level between high tide and low tide during a single tidal cycle.

Every 14 days, the Moon and Sun are in line with each other (new moon and full moon). Their gravitational forces combine to produce a maximum pull on Earth. 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.

Seven days later, 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.

The nature of tides

In most places, tides are semidiurnal, meaning there are two tidal cycles each day (a tidal cycle is one high and one low tide). The high water level reached during one of the high tide stages is usually greater than the other high tide 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, a rising tide is called the flood tide. A falling tide is called 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. At this point the water level is static, neither rising nor falling, at least for a short time.

As the Moon revolves around Earth, Earth also rotates on its axis. Consequently, in order to return to the same position relative to the Moon above, Earth must rotate on its axis for 24 hours and 50 minutes (a period known as a lunar day). The additional 50 minutes allows Earth to "catch up" to the Moon. 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. On a semidiurnal coast, each high tide (or low tide) will occur 12 hours and 25 minutes later than the previous high tide (or low tide).

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. 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. When this occurs, the tidal current may then reach speeds as great as 9 miles (15 kilometers) an hour with crests as high as 10 feet (3 meters) or more.

Most tides rise and fall between 3 and 10 feet (1 and 3 meters). 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, the difference between high and low tides (the tidal range) may be as great as 46 feet (14 meters). In comparison, some large bodies of water, such as the Mediterranean, Baltic, and Caribbean Seas, have areas with tides of less than a foot (0.3 meter). All coastal locations (as well as very large lakes) experience some variation in tidal range due to the affects of neap versus spring tides.

[See also Celestial mechanics; Gravity and gravitation; Moon; Ocean ]

tides are shallow-water waves generated by the gravitational forces of the moon and sun acting on the oceans. The earth's rotation results in high tides occurring both on the side nearest to the moon and on the opposite side. When the moon and sun are most closely aligned, at full and new moon, their gravitational pulls work in concert to produce spring tides, the highest tides of the cycle. Neap tides, the lowest-ranging tides, occur at the first and third quarters of the moon's cycle when the sun and moon are unaligned. The earth rotates every 24 hours 50 minutes relative to the moon, so the two daily tides are often of different heights. The earth spins too quickly for the tide's wave to keep up with the maximum gravitational pull of the moon, so it lags behind. This lag acts as a form of friction, which is slowing the earth's rotation.

Tidal ranges are also affected by the shapes of ocean basins. The tidal wave (not to be confused with tsunamis, also colloquially known as tidal waves) circulates within a basin around an amphidromic point, which is a point at which the tidal range is zero. Islands also affect tides. In the Solent in southern Britain the tidal wave, which comes from the west, arrives earlier along the shorter route round the north of the Isle of Wight than along the longer route around the south and east of the island. The two peaks of high tide arrive separately generating high- and low-water stands; periods when the sea neither rises nor falls.

The tidal heights are also affected by weather conditions. When atmospheric pressure is low the water can rise higher than expected, and if in addition there are stronger winds blowing from the sea even more water can pile inshore. Such events are known as storm surges. During tropical storms in coastal regions, more people are drowned by the storm surge than by the direct effects of the wind's violence. In 1953 a 3-metre (10-ft) storm surge in the southern North Sea killed 300 people in Britain and 1,800 in Holland, and resulted in the building of the Thames Barrier and the Delta project across the Eastern Scheldt. See also tidal power.

M. V. Angel

tides The rise and fall of the Earth's oceans, caused by the gravitational attractions of the Moon and Sun. At the Earth's centre, centrifugal force due to the motion of the Earth about the barycentre of the Earth–Moon system is balanced by gravitational force. On the side of the Earth nearest the Moon, the Moon's gravitational pull is greater than centrifugal force, while on the opposite side the centrifugal force is greater. In both cases a tidal bulge is produced in the oceans. The Sun's tide-raising power is about one-third that of the Moon, as a result of its greater distance from the Earth. The Sun and Moon act together at new or full Moon to produce extra-high and extra-low tides; these are the so-called spring tides. When the Moon is at first or third quarter the Sun and Moon are acting in different directions. The high tides are then at their lowest and the low tides are at their highest; these are the so-called neap tides. The Moon and Sun also raise measurable tides in the solid Earth and its atmosphere. The Earth raises tides in the Moon, its force being a maximum when the Moon is at perigee.

tide / tīd/ • n. the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun: the changing patterns of the tides | they were driven on by wind and tide. ∎  the water as affected by this: the rising tide covered the wharf. ∎ fig. a powerful surge of feeling or trend of events: he drifted into sleep on a tide of euphoria we must reverse the growing tide of racism sweeping the country. • v. [intr.] archaic drift with or as if with the tide. ∎  (of a ship) float or drift in or out of a harbor by taking advantage of favoring tides. PHRASES: turn the tide reverse the trend of events: the air power that helped to turn the tide of battle.PHRASAL VERBS: tide someone over help someone through a difficult period, esp. with financial assistance: she needed a small loan to tide her over.DERIVATIVES: tide·less adj.

tide1
A. †portion of time, season, age; †hour; (arch.) point of time, due time; definite time of day or of the year (surviving in eventide, noontide, springtide); church anniversary or festival (arch. except as in Eastertide, Shrovetide, Whitsuntide) OE.;

B. swelling of the sea or its alternate rising and falling XIV. OE. tīd = OS. tīd (Du. tijd), OHG. zīt (G. zeit), ON. tíō :- Gmc. *tīdiz, f. *tī- :- IE. *dī- *dā(i)- divide, cut up, repr. by Gr. daiesthai divide, distribute, Skr. dâti, dyâti cuts, harvests, shares. In B prob. after MLG. (ge)tīde, tīe, MDu. ghetīde (Du. (ge)tij), a special development of the sense ‘fixed time’.
So tide vb. (arch.) happen, befall. OE. tīdan, earlier ġetīdan, f. the sb.

tide
1. The periodic rise and fall of the Earth's oceans, caused by the relative gravitational attraction of the Sun, Moon, and Earth. The effect of the Moon is about twice that of the Sun, giving rise to the spring-neap cycle of tides. Variation in tides is caused by: (a) changes in the relative positions of the Sun, Moon, and Earth; (b) uneven distribution of water on the Earth's surface; and (c) variation in the sea-bed topography. Semi-diurnal tides are those with two high and two low waters (period 12 hours and 25 minutes) during a tidal day (24 hours and 50 minutes). Diurnal tides have one high and one low water during a tidal day.

2. See EARTH TIDES.

tide The periodic rise and fall of the Earth's oceans, caused by the relative gravitational attraction of the Sun, Moon, and Earth. The effect of the Moon is about twice that of the Sun, giving rise to the spring–neap cycle of tides. Variation in tides is caused by: (a)changes in the relative positions of the Sun, Moon, and Earth;(b)uneven distribution of water on the Earth's surface; and(c)variation in the seabed topography. Semidiurnal tides are those with two high and two low waters (period 12 hours and 25 minutes) during a tidal day (24 hours and 50 minutes). Diurnal tides have one high and one low water during a tidal day.

tide a particular time, season, or festival of the Christian Church; tide meaning ‘time, period, era’ is recorded from Old English (in form tīd) and is of German origin, ultimately related to German Zeit.

From late Middle English, the word has also meant (now the current meaning) the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun.

See also happy as a clam at high tide, a rising tide lifts all boats, save the tide, time and tide wait for no man.

tide Periodic rise and fall of the surface level of the oceans caused by the gravitational attraction of the Moon and Sun. Tides follow the Moon's cycle of 28 days, so they arrive at a given spot 50 minutes later each day. When the Sun and Moon are in conjunction or opposition, the greatest tidal range occurs, called spring tides. When they are in quadrature, when the Moon is half-full, tidal ranges are lowest and are called neap tides.

tide n.
1. the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun: the changing patterns of the tides | they were driven on by wind and tide.

2. the water as affected by this: the rising tide covered the wharf.
v. (of a ship) float or drift in or out of a harbor by taking advantage of favoring tides.

Tide

a stream; a current of things or emotions.

Examples : tide of blood; of emigration, 1830; of emotions; of events; of feelings; of upright freedom, 1519; of popular prejudice, 1777; of sorrows, 1738.

tide2 flow or carry along like the tide XVI; get over, surmount XVII. f. TIDE1 B.

tide •abide, applied, aside, astride, backslide, beside, bestride, betide, bide, bride, chide, Clyde, cockeyed, coincide, collide, confide, cried, decide, divide, dried, elide, five-a-side, glide, guide, hide, hollow-eyed, I'd, implied, lied, misguide, nationwide, nide, offside, onside, outride, outside, pan-fried, pied, pie-eyed, popeyed, pride, provide, ride, Said, shied, side, slide, sloe-eyed, snide, square-eyed, starry-eyed, statewide, Strathclyde, stride, subdivide, subside, tide, tried, undyed, wall-eyed, wide, worldwide •carbide • unmodified •overqualified, unqualified •dignified, signified •unverified • countrified •unpurified • unclassified •unspecified • sissified • unsanctified •self-satisfied, unsatisfied •unidentified • unquantified •unfortified • unjustified • uncertified •formaldehyde • oxhide • rawhide •cowhide • allied • landslide • bolide •paraglide • polyamide • bromide •thalidomide • selenide • cyanide •unoccupied