moon and tide?

Question

someone mentioned, that the moon and sun are alined at new moon and full moon, i am being tol;d there is highest tides at full moon, that is when the sun is looking over the sholder of earth and the moon is face on to earth therefor if the 2 heavenly bodies are pulling, there wouldnt be a tide, no you smart guys are not answering my question, your dribbling the ball round the goal posts but not scoring goals. come on tell me how the moon can infulance such a body of water as our great oceans?

Answer 1

1. The moon pulls on the earth because of its gravity.

2. The moon's gravity is greater on the side of earth closest to the Moon, and weakest on the side of Earth opposite to the moon.

3. Because of #2, the oceans on the side of Earth closest to the moon are pulled harder by the moon's gravity than the earth as a whole. And since oceans are liquid, they flow towards the pulling force, making a little bulge on the side of Earth facing the moon.

4. {!! THIS IS IMPORTANT !!} Also because of #2, the force of the moon's gravity on the earth as a whole is greater than the force of the moon's gravity on the side of earth opposite the moon. That causes the earth as a whole (the rocky part) to move a little closer to the moon than the oceans on the side opposite the moon. And since the oceans are liquid, they don't move along with the rocky part, but are "left behind" and (in effect) form another little bulge on the side of Earth opposite the moon.

5. Because of #3 and #4, there are TWO high tides every 24 hours, not just one: once when the moon is roughly overhead, and once when the moon is roughly underfoot.

6. The Sun pulls tides too, but much less than the Moon does: about 1/3 of the moon's strength.

7. The tides pulled by the Sun have two bulges too, just like the tides pulled by the moon, and for the same reason.

8. When the moon is either full OR new, the moon's tidal bulge is ADDED TO the Sun's tidal bulge, resulting in tides that are about 1 1/3 larger than average (1 + 1/3).

9. When the moon is at first quarter or last quarter, the Sun's tidal bulge is SUBTRACTED FROM the moon's bulge, resulting in tides that are about 2/3 as large as average (1 - 1/3 = 2/3).

Answer 2

The moon's tidal effect on the earth stretches it out slightly. Imagine it bulging like a bubble. It bulges out on the near and far side at the same time. This is because it is not just pulling on the near side but it's gravity is pulling on the whole earth, differentially, with distance. That is, pulling on the near side more than it is on the far side. So there is a high tide on opposite sides of the earth. And it isn't just the oceans that are afftected. The whole earth bulges. It's a very slight amount, though. Now when you add the sun to the picture, it does the same thing, only not quite to the extent that the moon does.



When the sun and moon and earth are all lined up, like they are at new moon or at full moon, the tidal effects of the moon and sun help each other. Each causes the earth to stretch along the same line as the other. Water, being fluid and forming an unbroken layer around the earth, is more free to move in the direction of those tidal bulges.



Note, the earth's rotation drags the bulges eastward by several degrees, so they are not really on a line with the moon or sun.



Chait... and Sparkle... would you please stop cutting and pasting long articles. That is not really what Y!A is all about. And that's why we have links. Again, to both of you, I say it's rude.

Answer 3

Just ask the bloody question. Reading that is like having my head pushed through cold mashed potato.



The answer, whether you like it or not, is gravity. And it has two effects which give us tides.



The first is because the mass of the Moon attracts the Earth towards it, and vice versa. The easiest bit of the Earth that can be moved is the water. Therefore, the side of the Earth facing the moon is at high tide.



This tide also occurs on the opposite side of the Earth away from the moon because the moon is also pulling the Earth itself. The surface on the opposite side of the Earth is also pulled towards the moon, which is why the tide is high on that side as well. This is assisted by the gravity of the sun.



That also answers your question about how there can be a high tide with the sun and moon on opposite sides. The sun has a similar effect on the water to the moon, but nowhere near as great. They don't actually balance each other, they complement each other, and it's mostly the moon.



When sun and moon are on the same side, it's very similar to having them on the opposite sides, but with an even higher tide.

Answer 4

The moon's gravity tension acts on on rocks, bones, flesh, as properly as water. The Moon does not "pull" the tide up in direction of it. The result's greater like water in a young infants exterior swimming pool, in case you gently push & pull on it with a rhythm. In a rapid time, you will get some stable length waves going decrease to and fro. interior a similar way, the moon pushes & pulls on the earth in a on a daily basis cycle (somewhat approximately 24 hours, 50 minutes). This motives tides to surge around the sea basins. At any particular place, the tide cycle would desire to be as quickly as an afternoon, or two times an afternoon. The severe tide does not could be while the moon is rapidly overhead. The gravity result on YOU is decrease than a million/10th of a million%. in the experience that your weight is 200 pounds, the end result's decrease than 3 oz.

Answer 5

Wow, Chaitnay and Sparkle sure do know how to use copy-paste, dont they? I need a nap after all that!

Answer 6

HA! cold mashed potatoes. thats a good one too Tom...

Answer 7

Tides are the rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tidal phenomena can occur in any object that is subjected to a gravitational field that varies in time and space, such as the Earth's land masses. (see Other tides).



Tides noticeably affect the depth of marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides very important for coastal navigation (see Tides and navigation). 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 (see Intertidal ecology).



The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the local shape of the sea floor.[1] Sea level measured by coastal t



Introduction and tidal terminology



Types of tides.A tide is a repeated cycle of sea level changes in the following stages:



Over several hours the water rises or advances up a beach in the flood tide.

The water reaches its highest level and stops at high tide. Because tidal currents cease this is also called slack water or slack tide. The tide reverses direction and is said to be turning.

The sea level recedes or falls over several hours during the ebb tide.

The level stops falling at low tide. This point is also described as slack or turning.

Tides may be semidiurnal (two high tides and two low tides each day), or diurnal (one tidal cycle per day). In most locations, tides are semidiurnal. Because of the diurnal contribution, there is a difference in height (the daily inequality) between the two high tides on a given day; these are differentiated as the higher high water and the lower high water in tide tables. Similarly, the two low tides each day are referred to as the higher low water and the lower low water. The daily inequality changes with time and is generally small when the Moon is over the equator.[2]



The various frequencies of astronomical forcing which contribute to tidal variations are called constituents. In most locations, the largest is the "principal lunar semidiurnal" constituent, also known as the M2 (or M2) tidal constituent. Its period is about 12 hours and 24 minutes, exactly half a tidal lunar day, the average time separating one lunar zenith from the next, and thus the time required for the Earth to rotate once relative to the Moon. This is the constituent tracked by simple tide clocks.[3]



Tides vary on timescales ranging from hours to years, so to make accurate records tide gauges measure the water level over time at fixed stations which are screened from variations caused by waves shorter than minutes in period. These data are compared to the reference (or datum) level usually called mean sea level.[4]



Constituents other than M2 arise from factors such as the gravitational influence of the Sun, the tilt of the Earth's rotation axis, the inclination of the lunar orbit and the ellipticity of the orbits of the Moon about the Earth and the Earth about the Sun. Variations with periods of less than half a day are called harmonic constituents. Long period constituents have periods of days, months, or years.





[edit] Tidal range variation: springs and neaps



An artist's conception of spring tide

An artist's conception of neap tideThe semidiurnal tidal range (the difference in height between high and low tides over about a half day) varies in a two-week or fortnightly cycle. 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 tide's range is then maximum: this is called the spring tide, or just springs and is derived not from the season of spring but rather from the verb meaning "to jump" or "to leap up". When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the earth, and the forces due to the Sun partially cancel those of the Moon. At these points in the lunar cycle, the tide's range is 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. Neaps result in less extreme tidal conditions. There is about a seven day interval between springs and neaps.



The changing distance of the Moon from the Earth also affects tide heights. When the Moon is at perigee the range is increased and when it is at apogee the range is reduced. Every 7½ lunations, perigee coincides with either a new or full moon causing perigean tides with the largest tidal range. If a storm happens to be moving onshore at this time, the consequences (in the form of property damage, etc.) can be especially severe.



Tidal forces



A schematic of the Earth-Moon system (not to scale), showing the entire Earth following the motion of its center of gravity.The tidal force produced by a massive object (Moon, hereafter) on a small particle located on or in an extensive body (Earth, hereafter) is the vector difference between the gravitational force exerted by the Moon on the particle, and the gravitational force that would be exerted on the particle if it were located at the center of mass of the Earth. Thus, the tidal force depends not on the strength of the gravitational field of the Moon, but on its gradient. The gravitational force exerted on the Earth by the Sun is on average 179 times stronger than that exerted on the Earth by the Moon, but because the Sun is on average 389 times farther from the Earth, the gradient of its field is weaker. The tidal force produced by the Sun is therefore only 46% as large as that produced by the Moon.



Tidal forces can also be analyzed from the point of view of a reference frame that translates with the center of mass of the Earth. Consider the tide due to the Moon (the Sun is similar). First observe that the Earth and Moon rotate around a common orbital center of mass, as determined by their relative masses. The orbital center of mass is 3/4 of the way from the Earth's center to its surface. The second observation is that the Earth's centripetal motion is the averaged response of the entire Earth to the Moon's gravity and is exactly the correct motion to balance the Moon's gravity only at the center of the Earth; but every part of the Earth moves along with the center of mass and all parts have the same centripetal motion, since the Earth is rigid.[10] On the other hand each point of the Earth experiences the Moon's radially decreasing gravity differently; the near parts of the Earth are more strongly attracted than is compensated by inertia and experience a net tidal force toward the Moon; the far parts have more inertia than is necessary for the reduced attraction, and thus feel a net force away from the Moon. Finally only the horizontal components of the tidal forces actually contribute tidal acceleration to the water particles since there is small resistance. The actual tidal force on a particle is only about a ten millionth of the force caused by the Earth's gravity.





The Moon's (or Sun'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 tidal equipotential bulges, accounting for two high tides per day.The ocean's surface is closely approximated by an equipotential surface, (ignoring ocean currents) which is commonly referred to as the geoid. Since the gravitational force is equal to the gradient of the potential, there are no tangential forces on such a surface, and the ocean surface is thus in gravitational equilibrium. Now consider the effect of external, massive bodies such as the Moon and Sun. These bodies have strong gravitational fields that diminish with distance in space and which act to alter the shape of an equipotential surface on the Earth. Gravitational forces follow an 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 ocean surface moves to adjust to changing tidal equipotential, tending to rise when the tidal potential is high, the part of the Earth nearest the Moon, and the farthest part. When the tidal equipotential changes, the ocean surface is no longer aligned with it, so that the apparent direction of the vertical shifts. The surface then experiences a down slope, in the direction that the equipotential has risen



Tidal observation and prediction

From ancient times, tides have been observed and discussed with increasing sophistication, first noting the daily recurrence, then its relationship to the Sun and Moon. Pytheas travelled to the British Isles and seems to be the first to have related spring tides to the phase of the moon. The Naturalis Historia of Pliny the Elder collates many observations of detail: the spring tides being a few days after (or before) new and full moon, and that the spring tides around the time of the equinoxes were the highest, though there were also many relationships now regarded as fanciful. In his Geography, Strabo described tides in the Persian Gulf having their greatest range when the moon was furthest from the plane of the equator. All this despite the relatively feeble tides in the Mediterranean basin, though there are strong currents through the Strait of Messina and between Greece and the island of Euboea through the Euripus that puzzled Aristotle. In Europe the Venerable Bede around 730 A.D. described how the rise of tide on one coast of the British Isles coincided with the fall on the other and described the progression in times of the same high tide along the Northumbrian coast.



Eventually the first tide table in China was recorded in 1056 A.D. primarily for the benefit of visitors wishing to see the famous tidal bore in the Qiantang River.The first known tide-table is thought to be that of John, Abbott of Wallingford (d. 1213), based on high water occurring 48 minutes later each day, and three hours later upriver at London than at the mouth of the Thames. William Thomson led the first systematic harmonic analysis to tidal records starting in 1867. The main result was the building of a tide-predicting machine (TPM) on using a system of pulleys to add together six harmonic functions of time. It was "programmed" by resetting gears and chains to adjust phasing and amplitudes. Similar machines were used until the 1960s.



The first known sea-level record of an entire spring–neap cycle was made in 1831 on the Navy Dock in the Thames Estuary, and many large ports had automatic tide gages stations by 1850.



William Whewell first mapped co-tidal lines ending with a nearly global chart in 1836. In order to make these maps consistent, he hypothesized the existence of amphidromes where co-tidal lines meet in the mid-ocean. These points of no tide were confirmed by measurement in 1840 by Captain Hewett, RN, from careful soundings in the North Sea.[9]





[edit] Timing



The same tidal forcing has different results depending on many factors, including coast orientation, continental shelf margin, water body dimensions.In most places there is a delay between the phases of the Moon and the effect on the tide. Springs and neaps in the North Sea, for example, are two days behind the new/full Moon and first/third quarter. This is called the age of the tide.[13]



The exact time and height of the tide at a particular coastal point is also greatly influenced by the local bathymetry. There are some extreme cases: the Bay of Fundy, on the east coast of Canada, features the largest well-documented tidal ranges in the world, 16 metres (53 ft), because of the shape of the bay.[14] Southampton in the United Kingdom has a double high tide caused by the interaction between the different tidal harmonics within the region. This is contrary to the popular belief that the flow of water around the Isle of Wight creates two high waters. The Isle of Wight is important, however, as it is responsible for the 'Young Flood Stand', which describes the pause of the incoming tide about three hours after low water. Ungava Bay in Northern Quebec, north eastern Canada, is believed by some experts to have higher tidal ranges than the Bay of Fundy (about 17 metres or 56 ft)[citation needed], but it is free of pack ice for only about four months every year, whereas the Bay of Fundy rarely freezes.



Because the oscillation modes of the Mediterranean Sea and the Baltic Sea do not coincide with any significant astronomical forcing period the largest tides are close to their narrow connections with the Atlantic Ocean. Extremely small tides also occur for the same reason in the Gulf of Mexico and Sea of Japan. On the southern coast of Australia, because the coast is extremely straight (partly due to the tiny quantities of runoff flowing from rivers), tidal ranges are equally small.



Tidal Current

The flow pattern due to tidal influence is much more difficult to analyse, and also, data is much more difficult to collect. A tidal height is a simple number, and applies to a wide region simultaneously (often as far as the eye can see), but a flow has both a magnitude and a direction, and can vary substantially over just a short distance due to local bathymetry, and also vary with depth. Although the centre of a channel is the most useful measuring site, mariners will not accept a current measuring installation obstructing navigation! A flexible attitude is required. A flow proceeding up a curved channel is the same flow, even though its direction varies continuously along the channel. Even the obvious expectation that the flood and ebb flows will be in reciprocal directions is not met, as the direction of a flow is determined by the shape of the channel it is coming from, not the shape where it will shortly be. Likewise, eddies can form in one direction but not the other.



Nevertheless, analysis proceeds on the same basis. At a given location in the simple case, the great majority of the flood flow will be in one direction, and the ebb flow in another (not necessarily reciprocal) direction. Take the velocities along the flood direction as positive, and along the ebb direction as negative, and proceed as if these were tide height figures. In more complex situations, the flow will not be dominated by the main ebb and flow directions, with the flow direction and magnitude tracing out an ellipse over a tidal cycle (on a polar plot) instead of along the two lines of ebb and flow direction. In this case, analysis might proceed along two pairs of directions, the primary flow directions and the secondary directions at right angles. Alternatively, the tidal flows can be treated as complex numbers, as each value has both a magnitude and a direction.



As with tide height predictions, tide flow predictions based only on astronomical factors do not take account of weather conditions, which can completely change the situation. The tidal flow through Cook Strait between the two main islands of New Zealand is particularly interesting, as on each side of the strait the tide is almost exactly out of phase so that high tide on one side meets low tide on the other. Strong currents result, with almost zero tidal height change in the centre of the strait. Yet, although the tidal surge should flow in one direction for six hours and then the reverse direction for six hours, etc. a particular surge might last eight or ten hours with the reverse surge enfeebled. In especially boisterous weather conditions, the reverse surge might be entirely overcome so that the flow remains in the same direction through three surge periods and longer.



Tides and navigation

Tidal flows are of profound importance in navigation and very significant errors in position will occur if they 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 draft from entering at certain states of the tide.



The timings and velocities of 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, with 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 the direction and the average flow speed (usually in knots) for spring and neap tides. 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 to counteract the effects of tides on navigation is to (1) calculate a "dead reckoning" position (or DR) from distance and direction of travel, (2) mark this on the chart (with a vertical cross like a plus sign) and (3) draw a line from the DR in the direction of the tide. The distance the tide will have moved 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).





Civil and maritime uses of tidal dataNautical charts display the "charted depth" of the water at specific locations with "soundings" and the use of bathymetric contour lines to depict the shape of the submerged surface. These depths are relative to a "chart datum", which is typically 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, which 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 earth tides. All of these are continuum mechanical phenomena, the first two being fluids and the third solid (with various modifications).



Atmospheric tides are negligible from ground level and aviation altitudes, drowned by the much more important effects of weather. Atmospheric tides are both gravitational and thermal in origin, and are the dominant dynamics from about 80 km to 120 km where the molecular density becomes too small to behave as a fluid.



Earth tides or terrestrial tides affect the entire rocky mass of the Earth. The Earth's crust shifts (up/down, east/west, north/south) in response to the Moon's and Sun's gravitation, ocean tides, and atmospheric loading. While negligible for most human activities, the semidiurnal amplitude of terrestrial tides can reach about 55 cm at the equator (15 cm is due to the Sun) which is important in GPS calibration and VLBI measurements. Also to make precise astronomical angular measurements requires knowledge of the earth's rate of rotation and nutation, both of which are influenced by earth tides. The semi-diurnal M2 Earth tides are nearly in phase with the Moon with tidal lag of about two hours.[18] Terrestrial tides also need to be taken in account in the case of some particle physics experiments.[19] For instance, at the CERN or SLAC, the very large particle accelerators were designed while taking terrestrial tides into account for proper operation. Among the effects that need to be taken into account are circumference deformation for circular accelerators and particle beam energy.[20] Since tidal forces generate currents of conducting fluids within the interior of the Earth, they affect in turn the Earth's magnetic field itself.



When oscillating tidal currents in the stratified ocean flow over uneven bottom topography, they generate internal waves with tidal frequencies. Such waves are called internal tides.



The galactic tide is the tidal force exerted by galaxies on stars within them and satellite galaxies orbiting them. The effects of the galactic tide on the Solar System's Oort cloud are believed to be the cause of 90 percent of all observed long-period comets.[21]

Answer 8

The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. To a much smaller extent, tides also occur in large lakes, the atmosphere, and within the solid crust of the earth, acted upon by these same gravitational forces of the moon and sun.

What are Lunar Tides

Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it. Each day, there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide. There is about 12 hours and 25 minutes between the two high tides.



Tides are the periodic rise and falling of large bodies of water. Winds and currents move the surface water causing waves. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and earth interact. As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. Since the earth is rotating while this is happening, two tides occur each day.

What are the different types of Tides

When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.

Spring Tides

When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.



Neap Tides

During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.

The Proxigean Spring Tide is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years.



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High Tide / Low Tide Examples



A view of the tides at Halls Harbour on Nova Scotia's Bay of Fundy. This is a time lapse of the tidal rise and fall over a period of six and a half hours. During the next six hours of ebb the fishermen unload their boats on the dock. That's a high tide every 12 and 1/2 hours! There are two high tides every 25 hours.



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A Few Facts About Lunar Tides





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The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth's centrifugal force created by its spin, you get tides.



The sun's gravitational force on the earth is only 46 percent that of the moon. Making the moon the single most important factor for the creation of tides.



The sun's gravity also produces tides. But since the forces are smaller, as compared to the moon, the effects are greatly decreased.



Tides are not caused by the direct pull of the moon's gravity. The moon is pulling upwards on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides.



Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides.



Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon).



When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides.



Spring tides and neap tide levels are about 20% higher or lower than average.



Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet



The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified. (In the Bay of Fundy, tides have a range of 44.6 ft.)



The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada.



Because the earth rotates on its axis the moon completes one orbit in our sky every 25 hours (Not to be confused with moon's 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours.



Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes.



The type of gravitational force that causes tides is know as "Tractive" force.





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FAQs About Lunar Tides From - "The Astronomy Cafe"





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Why are there no ocean tides at the equator?

"Tides are a very complex phenomenon. For any particular location, their height and fluctuation in time depends to varying degrees on the location of the Sun and the Moon, and to the details of the shape of the beach, coastline, coastline depth and prevailing ocean currents. The tidal bulge of the Moon follows along the path on the earth's surface which intersects with the orbital plane of the Moon. This plane is tilted about 23 degrees with respect to the equatorial plane of the earth. The result is that near the equator, the difference between high tide and low tide is actually rather small, compared to other latitudes. To see this, draw a circle inscribed in an ellipse, with the major axis of the ellipse rotated by 23 degrees with respect to the circle's horizontal diameter. Now measure the height of the elliptical contour just above the 'equator' of the circle. You will see that it is quite small compared to other positions on earth, particularly at latitudes of 23 degrees or so. Even larger differences can occur depending on the shape of a bay or inlet or continental shelf." - Dr. Odenwald's ASK THE ASTRONOMER



Why are ocean tides so different everywhere?

"Because they depend on many factors including the geometry of your local coastline, and exactly where the Sun and Moon are located. Also, like the surface of a vibrating drum, the world oceans have vibratory modes that get stimulated in changing ways from minute to minute. Finally, there are storms at sea and elsewhere which move large quantities of water. Detailed forecasts are available for high and low tides in all sea ports." - Dr. Odenwald's ASK THE ASTRONOMER



Why aren't the Atlantic and Pacific coast tides the same?

"The nature of tides on the Earth's oceans is very complex. The oceans are, of course, being periodically 'forced' by a number of tidal sources including the Moon and the Sun, but this forcing has a number of different periods and harmonics. The two dominant periods are sue to the Sun and Moon, these are referred to as the S1 and M2 'modes' which have roughly 12 hour periods because they raise TWO water tides on the ocean diametrically opposite each other. But, for a variety of reasons, any given port will not have two high and two low tides each day; also called 'semi-diurnal tides'. A careful monitoring of the tides at any port for several years will show that in addition to the major modes, there are as many as 300 minor or 'harmonic' modes as well.

The World Ocean is a complex dynamical system. The natural velocity of a water disturbance depends on the depth and salinity of the water at each point it passes. When bodies of land circumscribe bodies of water, they produce a collection of resonating systems that favor water oscillations with certain frequencies over others. From among the 300+ harmonics that can be measured, every port and coastal location has its own unique signature depending on its latitude, longitude, water depth and salinity. The result is that the 'two high two low' tide rule can be strongly modified so that the time between successive high tides can be greater than or less that 12 hours in many cases. The result is that for some locations, there can be days when only one high tide occurs. Looking at the Atlantic and Pacific Coast tide tables for 1995, the data for the various 'Standard Ports' showed that virtually all days had two high tides and two low tides in San Diego, San Francisco, New York and Charleston. There were, however a few days every few months when only a single high tide occurred." - Dr. Odenwald's ASK THE ASTRONOMER



What is a Proxigean Spring Tide?

"The Moon follows an elliptical path around the Earth which has a perigee distance of 356,400 kilometers, which is about 92.7 percent of its mean distance. Because tidal forces vary as the third power of distance, this little 8 percent change translates into 25 percent increase in the tide- producing ability of the Moon upon the Earth. If the lunar perigee occurs when the Moon is between the Sun and the Earth, it produces unusually high Spring high tides. When it occurs on the opposite side from the Earth that where the Sun is located ( during full moon) it produces unusually low, Neap Tides. The High, High Tide is called the Proxigean Spring Tide and it occurs not more than once every 1.5 years. Some occurrences are more favorable that others.



A very interesting book "Tidal Dynamics" by Fergus J. Wood, published in 1986 by Reidel Publishing Company, talks at great length about these tides, and their environmental consequences.



Because of the gravitational nature of the interaction between the Earth, the Moon, and the water on the Earth, there is a curious amplification event called 'evection' that occurs when the Moon is at its closest 'perigee' distance called its 'proxigee'. The Moon draws even closer to the Earth than its ordinary perigee distance. Because of the complex dynamics of the Earth's oceans, their inertia, friction with the ocean floor, internal viscosity and the distribution of the continents, the maximum tides do not always coincide with the optimal times of proxigee. Still, these tides can produce enormous damage when all factors come together optimally. There are many recorded instances of unusually high storm or coastal flooding during the proxigean times. On January 9, 1974 the Los Angeles Times reported 'Giant Waves Pound Southland Coast".



During the last 400 years, there have been 39 instances or 'Extreme Proxigean Spring Tides' where the tide-producing severity has been near the theoretical maximum. The last one of these was on March 7 1995 at 22:00 hours Greenwich Civil Time during a lunar Full Moon. There were, in fact cases of extreme tidal flooding recorded during these particular spring tides which occur once every 31 years." - Dr. Odenwald's ASK THE ASTRONOMER



If the Moon were to escape, what would happen to the Earth's oceans?

"What happens is that the lunar water tides on the Earth go away, but the solar water tides still occur, but with about 1/3 or so the amplitude. There are still daily high and low tides, but they would be noticeably smaller. There would be no 'Spring' or 'Neap' tides, however."- Dr. Odenwald's ASK THE ASTRONOMER



Why does the Moon produce TWO water tides on the Earth and not just one?

"It is intuitively easy to understand why the gravitational pull of the Moon should produce a water tide on the Earth in the part of the ocean closest to the moon along the line connecting the center of the Moon with the center of the Earth. But in fact not one but TWO water tides are produced under which the Earth rotates every day to produce about two high tides and two low tides every day. How come?



It is not the gravitational force that is doing it, but the change in the gravitational force across the body of the Earth. If you were to plot the pattern of the Moon's 'tidal' gravitational force added to the Earth's own gravitational force, at the Earth's surface, you would be able to resolve the force vectors at different latitudes and longitudes into a radial component directed towards the Earth's center, and a component tangential to the Earth's surface. On the side nearest the moon, the 'differential' gravitational force is directed toward the Moon showing that for particles on the Earth's surface, they are being tugged slightly towards the Moon because the force of the Moon is slightly stronger at the Earth's surface than at the Earth's center which is an additional 6300 kilometers from the Moon. On the far side of the Earth, the Moon is tugging on the center of the Earth slightly stronger than it is on the far surface, so the resultant force vector is directed away from the Earth's center.



The net result of this is that the Earth gets deformed into a slightly squashed, ellipsoidal shape due to these tidal forces. This happens because if we resolve the tidal forces at each point on the Earth into a local vertical and horizontal component, the horizontal components are not zero, and are directed towards the two points along the line connecting the Earth and the Moon's centers. These horizontal forces cause rock and water to feel a gravitational force which results in the flow of rock and water into the 'tidal bulges'. There will be exactly two of these bulges. At exactly the positions of the tidal bulges where the Moon is at the zenith and at the nadir positions, there are no horizontal tidal forces and the flow stops. The water gets piled up, and the only effect is to slightly lower the weight of the water along the vertical direction.



Another way of thinking about this is that the gravitational force of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth."- Dr. Odenwald's ASK THE ASTRONOMER



What Causes Tides?

"There are several kinds of tides. The ones that break upon a beach every 10 seconds to a minute are caused by sea level disturbances out in the ocean produced by such things as storms. Also, the various circulation currents of sea water can have velocity components directed towards the land which will bring water up onto the beach. As this water travels towards the beach from deep water to shallow water, its amplitude will increase until it finally 'breaks' as a full-fledged breaker, suitable for surfing etc.



Now, underlying this minute to minute activity is a slower water wave which causes an alternating pattern of high-tide, low-tide, high-tide, low-tide in most places on the Earth that are directly on the ocean. This roughly 6 hour cycle is caused by the gravitational tugging of the Moon upon the Earth. This 'tidal' pull causes the shape of the solid Earth to be not perfectly round by something like a few dozen yards over its entire 27,000 mile circumference. The Earth gets distorted a small bit, but because it is solid rock its a small effect. The water in the oceans, however, gets distorted into a roughly ellipsoidal ( football-like) shape with a much larger amplitude. The orientation of this shape changes from minute to minute as the Moon orbits the Earth, which is why the high and low tide times change all the time. The Moon causes these tides by deforming the oceans, and as the Earth rotates under this ocean bulge, it causes a high tide to propagate onto beaches. Because there are two bulges, we get two high tides, and also two low tides each day.



The Sun also causes tides on the Earth because even though it is so far away, it is very massive. These solar tides are about half as strong as the ones produced by the Moon, and they cause the so-called Spring tides and the Neap Tides. When the bulge of ocean water raised by the Moon is added the a similar tidal bulge raised by the Sun, you get a higher, high tide called the Spring Tide. When the solar low tide is added to the lunar low tide, you get the Neap Tide.



There may be even weaker tides caused by the gravitational influences of the planets Mars and Venus, but they are probably lost in the daily noise of individual tides."- Dr. Odenwald's ASK THE ASTRONOMER



When the Earth, Moon and Sun are aligned for Spring Tides, are they highest at Full or New Moon?

"Spring tides are about the same height whether at New or Full Moon, because the tidal bulge occurs on both sides of the Earth...the side toward the Moon ( or sun) and the side away from the Moon (or Sun). They will not be equally high because the distance between the Earth and Sun, and the Earth and Moon both vary and so will their tide producing effectiveness. The highest Spring tides occur when the Moon is at its closest to the Earth...the so-called Perigee Tide."- Dr. Odenwald's ASK THE ASTRONOMER