The regular, periodic rise and fall of the surface of the sea. The tides are produced by differences in gravitational forces acting on different points on the Earth's surface, and affect all bodies of water to some extent. These so-called tidal forces are produced primarily by the Sun and Moon. The Sun's tidal forces are only about half as strong as those of the Moon, due to the Sun's greater distance from the Earth. The position of the Sun, Moon, and other celestial bodies with respect to the Earth produces variations in the timing and magnitude of the tides.
This article is about tides in Earth's ocean. For other meanings of the word, see Tide (disambiguation).The tide is the cyclic rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the Earth. Tides cause changes in the depth of the sea, and also produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation (see Tides and navigation, below). The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides.
The changing tide produced at a given location on the Earth is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of the rotation of the Earth and the local bathymetry (the underwater equivalent to topography or terrain). Though the gravitational force exerted by the Sun on the Earth is almost 200 times stronger than that exerted by the Moon, the tidal force produced by the Moon is about twice as strong as that produced by the Sun. as the Sun is about 400 times further from the Earth than is the Moon, the gradient of the Sun's field, and thus the tidal force produced by the Sun, is weaker.
Tidal terminology
The maximum water level is called "high tide" or "high water" and the minimum level is "low tide" or "low water." If the ocean were a constant depth, and there were no land, high water would occur as two bulges in the height of the oceans--one bulge facing the Moon and the other on the opposite side of the earth, facing away from the Moon. At any given point in the ocean, there are normally two high tides and two low tides each day just as there would be for an earth with no land; however, rather than two large bulges propagating around the earth, with land masses in the way the result is many smaller bulges propagating around amphidromic points, so there is no simple, general rule for predicting the time of high tide from the position of the Moon in the sky. The common names of the two high tides are the "high high" tide and the "low high" tide; The two low tides are called the "high low" tide and the "low low" tide. On average, high tides occur 12 hours 24 minutes apart.
The time between high tide and low tide, when the water level is falling, is called the "ebb." The time between low tide and high tide when the tide is rising, is called "flow," or "flood." At the times of high tide and low tide, the tide is said to be "turning," also slack tide.
The height of the high and low tides (relative to mean sea level) also varies. Around new and full Moon when the Sun, Moon and Earth form a line (a condition known as syzygy), the tidal forces due to the Sun reinforce those of the Moon. When the Moon is at first quarter or third quarter, the Sun and Moon are at 90° to each other and the forces due to the Sun partially cancel out those of the Moon. At these points in the Lunar cycle, the tide's range is at its minimum: this is called the "neap tide," or "neaps".
Spring tides result in high waters that are higher than average, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents than average.
The relative distance of the Moon from the Earth also affects tide heights: When the Moon is at perigee the range increases, and when it is at apogee the range is reduced. at these times the range of tide heights is greatest of all, and if a storm happens to be moving onshore at this time, the consequences (in the form of property damage, etc.) can be especially severe. (Surfers are aware of this, and will often intentionally go out to sea during these times, as the waves are larger at these times.) The effect is enhanced even further if the line-up of the Sun, Earth and Moon is so exact that a solar or lunar eclipse occurs concomitant with perigee.
Timing
In most places there is a delay between the phases of the Moon and its effect on the tide. The reason for this is that the tide originates in the southern oceans, the only place on the globe where a circumventing wave (as caused by the tidal force of the Moon) can travel unimpeded by land.
The resulting effect on the amplitude, or height, of the tide travels across the oceans.
The exact time and height of the tide at a particular coastal point is also greatly influenced by the local bathymetry. Southampton in the United Kingdom has a double high tide caused by the flow of water around the Isle of Wight, and Weymouth, Dorset has a double low tide because of the Isle of Portland.
There are only very slight tides in the Mediterranean Sea and the Baltic Sea due to their narrow connections with the Atlantic Ocean.
Tidal physics
Ignoring external forces, the ocean's surface defines a geopotential surface or geoid, where the gravitational force is directly towards the centre of the Earth and there is no net lateral force and hence no flow of water. This deformation has a fixed orientation relative to the influencing body and the rotation of the Earth relative to this shape drives the tides around. The Sun's gravitational pull on Earth is on average 179 times bigger than the Moon's, but because of its much greater distance, the Sun's field gradient and thus its tidal effect is smaller than the Moon's (about 46% as strong).
Since the Earth's crust is solid, it moves with everything inside as one whole, as defined by the average force on it.
At the point right "under" the Moon (the sub-lunar point), the water is closer than the solid Earth; On the opposite side of the Earth, facing away from the Moon (the antipodal point), the water is farther from the moon than the solid earth, so it is pulled less and effectively moves away from Earth (i.e. Those parallel components actually contribute most to the formation of tides, since the water particles are free to follow.
These minute forces all work together:
pull up under and away from the Moon pull down at the sides pull towards the sub-lunar and antipodal points at intermediate pointsSo in an ocean of constant depth on an Earth with no land, two bulges would form pointing towards the Moon just under it and away from it on Earth's far side.
Tidal amplitude and cycle time
Since the Earth rotates relative to the Moon in one lunar day (24 hours, 48 minutes), each of the two bulges travels around at that speed, leading to one high tide every 12 hours and 24 minutes.
The Sun similarly causes tides, of which the theoretical amplitude is about 25 cm (46% of that of the Moon) and the cycle time is 12 hours.
At spring tide the two effects add to each other to a theoretical level of 79 cm, while at neap tide the theoretical level is reduced to 29 cm. If there were no land masses and the ocean bottom were flat, it would take about 30 hours for a long wavelength ocean surface wave to propagate halfway around the Earth (by comparison, the natural period of the Earth's crust is about 57 minutes).
The distances of Earth from the Moon or the Sun vary, because the orbits are not circular, but elliptical.
Tidal lag
Because the Moon's tidal forces drive the oceans with a period of about 12.42 hours (half of the Moon's synodic period of rotation), which is considerably less than the natural period of the oceans, complex resonance phenomena take place. The global average tidal lag is 12 minutes, which corresponds to an angle of 3 degrees between the position of the moon and the location of global average high tide. Tidal lag and the transfer of momentum between sea and land causes the Earth's rotation to slow down and the Moon to be moved further away in a process known as tidal acceleration.
Alternative explanation
Some other explanations in articles on the physics of tides include the (apparent) centrifugal force on the Earth in its orbit around the common center of mass (the barycenter) with the Moon. So the centrifugal force is uniform and does not contribute to the tides.
Image: http://www.seafriends.org.nz/oceano/topextide.jpg (52KB)
History of tidal physics
The first well-documented mathematical explanation of tidal forces was given in 1687 by Isaac Newton in the Philosophiae Naturalis Principia Mathematica.
Tides and navigation
Tidal flows are of profound importance in navigation and very significant errors in position will occur if tides are not taken into account. Tidal charts come in sets, each diagram of the set covering a single hour between one high tide and another (they ignore the extra 24 minutes) and give the average tidal flow for that one hour. An arrow on the tidal chart indicates direction and two numbers are given: average flow (usually in knots) for spring tides and neap tides respectively. If a tidal chart is not available, most nautical charts have "tidal diamonds" which relate specific points on the chart to a table of data giving direction and speed of tidal flow.
Standard procedure is to calculate a "dead reckoning" position (or DR) from distance and direction of travel and mark this on the chart (with a vertical cross like a plus sign) and then draw in a line from the DR in the direction of the tide. Measuring the distance the tide will have moved the boat along this line then gives an "estimated position" or EP (traditionally marked with a dot in a triangle). These depths are relative to "chart datum", which is the level of water at the lowest possible astronomical tide (tides may be lower or higher for meteorological reasons) and are therefore the minimum water depth possible during the tidal cycle.
Heights and times of low and high tide on each day are published in "tide tables". The actual depth of water at the given points at high or low water can easily be calculated by adding the charted depth to the published height of the tide. This approximation works on the basis that the increase in depth in the six hours between low and high tide will follow this simple rule: first hour - 1/12, second - 2/12, third - 3/12, fourth - 3/12, fifth - 2/12, sixth - 1/12.
Other tides
In addition to oceanic tides, there are atmospheric tides as well as terrestrial tides (earth tides), affecting the rocky mass of the Earth. Atmospheric tides may be negligible for everyday phenomena, drowned by the much more important effects of weather and the solar thermal tides. However, there is no strict upper limit to the Earth's atmosphere, and the tidal pull increases with the distance from the Earth's centre. Theoretically, the Earth's atmosphere extends beyond the Roche limit of the Earth in the Moon's gravitational field. The amplitude of terrestrial tides can reach about 55 cm at the equator (15 cm of which are due to the Sun), and they are nearly in phase with the Moon (the tidal lag is about two hours only).
While negligible for most human activities, terrestrial tides need to be taken in account in the case of some particle physics experimental equipments (Stanford online).
Since tidal forces generate currents of conducting fluids within the interior of the Earth, they affect in turn the Earth's magnetic field itself.
The loss of rotational energy of the earth, due to friction within the tides, and the torque produced by the gravitational effects of the Sun and Moon on the tidal deformations of the earth's body are responsible for the slowdown of the earth's rotation and the increase of the distance to the Moon, see Tidal force.
Tsunamis, the large waves that occur after earthquakes, are sometimes called tidal waves, but have nothing to do with the tides. Other phenomena unrelated to tides but using the word tide are rip tide, storm tide, hurricane tide, and red tide.
Wikimedia Commons has media related to: Category:Tides
User Comments Add a comment…