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Tide

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

                The Bay of Fundy at high tide
                           Enlarge
                The Bay of Fundy at high tide

                                             The same location at low tide
                                                        Enlarge
                                             The same location at low tide

   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. The reason for this is that the tidal force is related not to
   the strength of a gravitational field, but to its gradient. The field
   gradient decreases with distance from the source more rapidly than does
   the field strength; 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.
   There would also be smaller, superimposed bulges on the sides facing
   toward and away from the Sun. For an explanation see below under Tidal
   physics. 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 difference in height
   between the two is known as the "daily inequality." The daily
   inequality is generally small when the moon is over the equator. 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 12 hours is
   due to the Earth's rotation, and the 24 minutes to the Moon's orbit.
   This is the "principal lunar semi-diurnal" period, abbreviated as the
   M2 tidal component, and it is, on average, half the time separating one
   lunar zenith from the next. The M2 component is usually the biggest
   one, but there are many others as well due to such complications as the
   tilt of the earth's axis and the inclination of the lunar orbit. The
   lunar cycle is what is tracked by tide clocks.

   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. The tides' range is then at its maximum:
   this is called the "spring tide," or just "springs" and is derived not
   from the season of spring but rather from the verb "to jump" or "to
   leap up." 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".
   The Earth and Moon, looking at the North Pole
   Enlarge
   The Earth and Moon, looking at the North Pole

   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. Neaps result in
   less extreme tidal conditions. Normally there is a seven day interval
   between springs and neaps.

   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. Every 7½ lunations, perigee and
   (alternately) either a new or full Moon coincide; 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

   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.
   There would also be smaller, superimposed bulges on the sides facing
   toward and away from the Sun. For an explanation see below under Tidal
   physics. 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 difference in height
   between the two is known as the "daily inequality." The daily
   inequality is generally small when the moon is over the equator. 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 12 hours is
   due to the Earth's rotation, and the 24 minutes to the Moon's orbit.
   This is the "principal lunar semi-diurnal" period, abbreviated as the
   M2 tidal component, and it is, on average, half the time separating one
   lunar zenith from the next. The M2 component is usually the biggest
   one, but there are many others as well due to such complications as the
   tilt of the earth's axis and the inclination of the lunar orbit. The
   lunar cycle is what is tracked by tide clocks.

   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. The tides' range is then at its maximum:
   this is called the "spring tide," or just "springs" and is derived not
   from the season of spring but rather from the verb "to jump" or "to
   leap up." 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".
   The Earth and Moon, looking at the North Pole
   Enlarge
   The Earth and Moon, looking at the North Pole

   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. Neaps result in
   less extreme tidal conditions. Normally there is a seven day interval
   between springs and neaps.

   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. Every 7½ lunations, perigee and
   (alternately) either a new or full Moon coincide; 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.

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.

   Now consider the effect of added external, massive bodies such as the
   Moon and Sun. These massive bodies have strong gravitational fields
   that diminish with distance in space. It is the spatial differences,
   called the gradient in these fields that deform the geoid shape. 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. Gravitational forces follow the inverse-square law (force is
   inversely proportional to the square of the distance), but tidal forces
   are inversely proportional to the cube of the distance. 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). For simplicity, the next few sections use the word
   "Moon" where also "Sun" can be understood.
   The Moon exerts its gravitational pull differently on different parts
   of the earth. The farther the Moon, the weaker its pull. Imagine a
   shell of the outer Earth, this diagram shows the Moon's gravity
   differential over the thickness of the shell.
   Enlarge
   The Moon exerts its gravitational pull differently on different parts
   of the earth. The farther the Moon, the weaker its pull. Imagine a
   shell of the outer Earth, this diagram shows the Moon's gravity
   differential over the thickness of the shell.
   The Moon's gravity differential field at the surface of the earth is
   known as the Tide Generating Force. This is the primary mechanism that
   drives tidal action and explains two bulges, accounting for two high
   tides per day. Other forces, such as the Sun's gravity, also add to
   tidal action.
   Enlarge
   The Moon's gravity differential field at the surface of the earth is
   known as the Tide Generating Force. This is the primary mechanism that
   drives tidal action and explains two bulges, accounting for two high
   tides per day. Other forces, such as the Sun's gravity, also add to
   tidal action.

   Since the Earth's crust is solid, it moves with everything inside as
   one whole, as defined by the average force on it. For a geoid shape
   this average force is equal to the force on its centre. The water at
   the surface is free to move following forces on its particles. It is
   the difference between the forces at the Earth's centre and surface
   which determine the effective tidal force.

   At the point right "under" the Moon (the sub-lunar point), the water is
   closer than the solid Earth; so it is pulled more and rises. 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. the Earth
   moves more toward the Moon than the water does), rising as well. On the
   lateral sides, the water is pulled in a slightly different direction
   than at the centre. The vectorial difference with the force at the
   centre points almost straight inwards to Earth. It can be shown that
   the forces at the sub-lunar and antipodal points are approximately
   equal and that the inward forces at the sides are about half that size.
   Somewhere in between (at 55° from the orbital plane) there is a point
   where the tidal force is parallel to the Earth's surface. Those
   parallel components actually contribute most to the formation of tides,
   since the water particles are free to follow. The actual force on a
   particle is only about a ten millionth of the force caused by the
   Earth's gravity.

   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
       points

   So 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. In reality, the presence of land masses and the depth
   profile of oceans distort this simple pattern significantly.

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
   theoretical amplitude of oceanic tides due to the Moon is about 54 cm
   at the highest point. This is the amplitude that would be reached if
   the ocean were uniform with no landmasses and Earth not rotating.

   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.

   Real amplitudes differ considerably, not only because of global
   topography as explained above, but also because the natural period of
   the oceans is in the same order of magnitude as the rotation period:
   about 30 hours. 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). This means that, if
   the Moon suddenly vanished, and there were no land, the level of the
   oceans would oscillate with a period of 30 hours with a slowly
   decreasing amplitude while dissipating the stored energy. This 30 hour
   value is a simple function of terrestrial gravity, the average depth of
   the oceans, and the circumference of the Earth.

   The distances of Earth from the Moon or the Sun vary, because the
   orbits are not circular, but elliptical. This causes a variation in the
   tidal force and theoretical amplitude of about ±18% for the Moon and
   ±5% for the Sun. So if both are in closest position and aligned, the
   theoretical amplitude would reach 93 cm.

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

   Revolution of the earth around the barycenter
   Revolution of the earth around the barycenter

   Some other explanations in articles on the physics of tides include the
   (apparent) centrifugal force on the Earth in its orbit around the
   common centre of mass (the barycenter) with the Moon. The barycenter is
   located at about ¾ of the radius from the Earth's center. It is
   important to note that the Earth has no "rotation" around this point.
   It just "displaces" around this point in a circular way (see figure).
   Every point on Earth has the same angular velocity and the same radius
   of orbit, but with a displaced center. So the centrifugal force is
   uniform and does not contribute to the tides. However, this uniform
   centrifugal force is just equal (but with opposite sign) to the
   gravitational force acting on the center of mass of Earth. So
   subtracting the gravitational force at the centre of Earth from the
   local gravitational forces at the surface, has the same effect as
   adding the (uniform) centrifugal forces. Although these two
   explanations seem very different, they yield the same results.

   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. However, there is some evidence that Hellenistic Greeks
   were able to explain tides in terms of a mathematical theory of
   gravity. Lucio Russo, an Italian scholar, makes this argument in his
   books Flussi e Riflussi (yet to be published in English) and La
   Rivoluzione Dimenticata (which has been translated into English as The
   Forgotten Revolution). Russo argues that the ancients had a more
   developed theory of gravity than has generally been acknowledged. For
   example, he exhibits excerpts from ancient texts indicating that
   Seleucus of Seleucia (II B.C.) devised a gravitational explanation to
   prove that the Earth revolves around the Sun, rather than vice versa.

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 heights are also very important; for example many rivers
   and harbours have a shallow "bar" at the entrance which will prevent
   boats with significant draught from entering at certain states of the
   tide.

   Tidal flow can be found by looking at a tidal chart or tidal stream
   atlas for the area of interest. 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).

   Nautical charts display the "charted depth" of the water at specific
   locations and on contours. 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.
   "Drying heights" may also be shown on the chart. These are the heights
   of the exposed seabed at the lowest astronomical tide.

   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. The water depth for times other than high
   or low water can be derived from tidal curves published for major
   ports. If an accurate curve is not available, the rule of twelfths can
   be used. 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. Since the outer extremely
   thin layers of the atmosphere are in equilibrium with the layers below,
   the long term effects may not be easily neglected. This means, if the
   extremely thin outer layers are steadily siphoned away, the material is
   re-supplied by lower layers, causing an altogether constant small loss
   of material.

   The Earth's crust, on the other hand, rises and falls imperceptibly in
   response to the Moon's solicitation. 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). For instance, at the CERN or SLAC, the
   very large particle accelerators are designed while taking terrestrial
   tides into account for proper operation. Indeed, despite their
   kilometre-range dimension, centimetric deformations might lead to their
   malfunctioning as a physics experimental apparatus. Among the effects
   that need to be taken into account are circumference deformation for
   circular accelerators and particle beam energy.

   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. The term tidal wave appears
   to be disappearing from popular usage.

   Retrieved from " http://en.wikipedia.org/wiki/Tide"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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