Tide

The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally (see equilibrium tide).

Tidal motion generally lessens moving away from continental coasts, so that crossing the cotidal lines are contours of constant amplitude (half the distance between high and low water) which decrease to zero at the amphidromic point.

Investigation into tidal physics was important in the early development of celestial mechanics, with the existence of two daily tides being explained by the Moon's gravity.

[c] In De temporum ratione (The Reckoning of Time) of 725 Bede linked semidurnal tides and the phenomenon of varying tidal heights to the Moon and its phases.

His explanation of the tides (and many other phenomena) was published in the Principia (1687)[27][28] and used his theory of universal gravitation to explain the lunar and solar attractions as the origin of the tide-generating forces.

[e] Newton and others before Pierre-Simon Laplace worked the problem from the perspective of a static system (equilibrium theory), that provided an approximation that described the tides that would occur in a non-inertial ocean evenly covering the whole Earth.

[31] Pierre-Simon Laplace formulated a system of partial differential equations relating the ocean's horizontal flow to its surface height, the first major dynamic theory for water tides.

[37] From ancient times, tidal observation and discussion has increased in sophistication, first marking the daily recurrence, then tides' relationship to the Sun and moon.

In the 2nd century BC, the Hellenistic astronomer Seleucus of Seleucia correctly described the phenomenon of tides in order to support his heliocentric theory.

The first tide table in China was recorded in 1056 AD primarily for visitors wishing to see the famous tidal bore in the Qiantang River.

The first known British tide table is thought to be that of John Wallingford, who died Abbot of St. Albans in 1213, based on high water occurring 48 minutes later each day, and three hours earlier at the Thames mouth than upriver at London.

[44] In order to make these maps consistent, he hypothesized the existence of a region with no tidal rise or fall where co-tidal lines meet in the mid-ocean.

[45][46][32] Much later, in the late 20th century, geologists noticed tidal rhythmites, which document the occurrence of ancient tides in the geological record, notably in the Carboniferous.

The shape of the shoreline and the ocean floor changes the way that tides propagate, so there is no simple, general rule that predicts the time of high water from the Moon's position in the sky.

For example, the high tide at Norfolk, Virginia, U.S., predictably occurs approximately two and a half hours before the Moon passes directly overhead.

[62] The tidal forces due to the Moon and Sun generate very long waves which travel all around the ocean following the paths shown in co-tidal charts.

There are some extreme cases; the Bay of Fundy, on the east coast of Canada, is often stated to have the world's highest tides because of its shape, bathymetry, and its distance from the continental shelf edge.

Isaac Newton's theory of gravitation first enabled an explanation of why there were generally two tides a day, not one, and offered hope for a detailed understanding of tidal forces and behavior.

At each place of interest, the tide heights are therefore measured for a period of time sufficiently long (usually more than a year in the case of a new port not previously studied) to enable the response at each significant tide-generating frequency to be distinguished by analysis, and to extract the tidal constants for a sufficient number of the strongest known components of the astronomical tidal forces to enable practical tide prediction.

The tidal patterns are decomposed into many sinusoids having many fundamental frequencies, corresponding (as in the lunar theory) to many different combinations of the motions of the Earth, the Moon, and the angles that define the shape and location of their orbits.

Consider further that the tidal force on a location depends also on whether the Moon (or the Sun) is above or below the plane of the Equator, and that these attributes have their own periods also incommensurable with a day and a month, and it is clear that many combinations result.

Organisations quoting a "highest astronomical tide" for some location may exaggerate the figure as a safety factor against analytical uncertainties, distance from the nearest measurement point, changes since the last observation time, ground subsidence, etc., to avert liability should an engineering work be overtopped.

The lines rotate around the amphidromic points counterclockwise in the northern hemisphere so that from Baja California Peninsula to Alaska and from France to Ireland the M2 tide propagates northward.

The alternation in high tide heights becomes smaller, until they are the same (at the lunar equinox, the Moon is above the Equator), then redevelop but with the other polarity, waxing to a maximum difference and then waning again.

A flow is a vector quantity, with magnitude and direction, both of which can vary substantially with depth and over short distances due to local bathymetry.

So far, there are few installed systems for tidal power generation (most famously, La Rance at Saint Malo, France) which face many difficulties.

Tidal heights are also important; for example many rivers and harbours have a shallow "bar" at the entrance which prevents boats with significant draft from entering at low tide.

The distance the tide moves the boat along this line is computed by the tidal speed, and this gives an "estimated position" or EP (traditionally marked with a dot in a triangle).

One easily visible feature is vertical zonation, in which the community divides into distinct horizontal bands of specific species at each elevation above low water.

Atmospheric tides are both gravitational and thermal in origin and are the dominant dynamics from about 80 to 120 kilometres (50 to 75 mi), above which the molecular density becomes too low to support fluid behavior.

Simplified schematic of only the lunar portion of Earth's tides, showing (exaggerated) high tides at the sublunar point and its antipode for the hypothetical case of an ocean of constant depth without land, and on the assumption that Earth is not rotating; otherwise there is a lag angle. Solar tides not shown.
Earth's rotation drags the position of the tidal bulge ahead of the position directly under the Moon showing the lag angle.
In Maine (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift .
Tide coming in, video stops about 1 + 1 ⁄ 2 hours before high tide
Tide coming in
Illustration by the course of half a month
Spring tide: the Sun, moon, and earth form a straight line. Neap tide: the Sun, moon, and earth form a right angle.
The types of tides
Global surface elevation of M2 ocean tide (NASA) [ 10 ]
Three graphs. The first shows the twice-daily rising and falling tide pattern with nearly regular high and low elevations. The second shows the much more variable high and low tides that form a "mixed tide". The third shows the day-long period of a diurnal tide.
Types of tides (See Timing (below) for coastal map)
Low tide at Bangchuidao scenic area, Dalian , Liaoning Province , China
Low tide at Ocean Beach in San Francisco , California , U.S.
Low tide at Bar Harbor , Maine , U.S. (2014)
Map showing relative tidal magnitudes of different ocean areas
M 2 tidal constituent. Red is most extreme (highest highs, lowest lows), with blues being least extreme. White cotidal lines converge in blue areas indicating little or no tide. Around these convergences, called amphidromic points , curved arrows show the direction of the tides, each indicating a synchronized 6-hour period. Tidal ranges generally increase with increasing distance from amphidromic points. Tide waves move around these points, generally counterclockwise in the N. Hemisphere and clockwise in the S. Hemisphere [ 14 ] [ 15 ]
Brouscon's Almanach of 1546: Compass bearings of high waters in the Bay of Biscay (left) and the coast from Brittany to Dover (right).
Brouscon's Almanach of 1546: Tidal diagrams "according to the age of the moon".
Diagram showing a circle with closely spaced arrows pointing away from the reader on the left and right sides, while pointing towards the user on the top and bottom.
The lunar gravity residual field at the Earth's surface is known as the tide-generating force . This is the primary mechanism that drives tidal action and explains two simultaneous tidal bulges; Earth's rotation further accounts for two daily high waters at any location. The figure shows both the tidal field (thick red arrows) and the gravity field (thin blue arrows) exerted on Earth's surface and center (label O) by the Moon (label S).
The harbour of Gorey, Jersey falls dry at low tide.
World map showing the location of diurnal, semi-diurnal, and mixed semi-diurnal tides. The European and African west coasts are exclusively semi-diurnal, and North America's West coast is mixed semi-diurnal, but elsewhere the different patterns are highly intermixed, although a given pattern may cover 200–2,000 kilometres (120–1,240 mi).
The same tidal forcing has different results depending on many factors, including coast orientation, continental shelf margin, water body dimensions.
A regular water level chart
Graph showing one line each for M 2, S 2, N 2, K 1, O 1, P 1, and one for their summation, with the X axis spanning slightly more than a single day
Tidal prediction summing constituent parts. The tidal coefficients are defined on the page theory of tides .
Graph with a single line rising and falling between 4 peaks around 3 and four valleys around −3
Tides at Bridgeport, Connecticut , U.S. during a 50-hour period.
Graph with a single line showing tidal peaks and valleys gradually cycling between higher highs and lower highs over a 14-day period
Tides at Bridgeport, Connecticut, U.S. during a 30-day period.
Graph showing with a single line showing only a minimal annual tidal fluctuation
Tides at Bridgeport, Connecticut, U.S. during a 400-day period.
Graph showing 6 lines with two lines for each of three cities. Nelson has two monthly spring tides, while Napier and Wellington each have one.
Tidal patterns in Cook Strait. The south part (Nelson) has two spring tides per month, versus only one on the north side (Wellington and Napier).
Chart illustrating that tidal heights enter in calculations of legally significant data such as boundary lines between the high seas and territorial waters. Chart shows an exemplar coastline, identifying bottom features such as longshore bar and berms, tidal heights such as mean higher high water, and distances from shore such as the 12 mile limit.
US civil and maritime uses of tidal data
Tidal Indicator, Delaware River, Delaware c. 1897. At the time shown in the figure, the tide is 1 + 1 ⁄ 4 feet above mean low water and is still falling, as indicated by pointing of the arrow. Indicator is powered by system of pulleys, cables and a float. (Report Of The Superintendent Of The Coast & Geodetic Survey Showing The Progress Of The Work During The Fiscal Year Ending With June 1897 (p. 483))
Photo of partially submerged rock showing horizontal bands of different color and texture, where each band represents a different fraction of time spent submerged.
A rock, seen at low water, exhibiting typical intertidal zonation.