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Thread: Question about Earth tides

  1. #31
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    This animation improves on the TOPEX model.

    https://www.youtube.com/watch?v=5zi7N06JXD4

  2. #32
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    Certainly the cause of the tides is known to be the gravity from the sun and moon, along with the rotation of the planet, since the changes in their positions and distance match exactly with the changes in the tides. In fact, the main factors for calculating tidal energy is based on those factors.

  3. #33
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    Just for fun, if you use the simple force equation f = G.m.m/D^2 for 1 kg and the moon, you get a force of about 40 micro newton.
    So compared with the weight of about 10 N there is not much direct lift.
    Now if you work out the sideways velocity v = ft (f is the ac cn.)
    I chose for simplicity a six hour period ie three hours each side of the max sideways acceleration,
    You get a velocity of about 0.9 m/s.
    That applies to actually all the water in an ocean, so the resultant sideways momentum is a big number.
    However the water particle moves about 100 m in that six hours. Not enough to accumulate in a bulge.
    The sideways or tangential forces dominate completely. The land masses and coriolis accelerations cause the bulges on our planet. The local natural frequencies of the water masses must also be significant.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  4. #34
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    Quote Originally Posted by Gigabyte View Post
    Really? Did anyone notice the clock? The "tidal bulge" in the southern ocean made it all the way around the pole in 12 hours.

    Think about it.
    Couple of things going on there. One is that it's impossible to show a diurnal tide on an animation depicting one semi-diurnal cycle. If you look at the tide magnitude animation, you can see a single high tide sweeping around Antarctica in 24 hours - which is exactly what you'd expect at high latitude, as I've mentioned before. So although amphidromes rotate once in ~12 hours, successive rotations produce completely different tidal effects at high latitude - you get alternating high and low tides with each rotation.
    Second thing is that the isochrone animation doesn't show all the amphidromes, because its resolution is too small - the complicated double gyre in the North Sea isn't shown, for instance. There's an amphidromal point tucked hard against the Antarctic coast off the Amery Ice Shelf. So if you follow the isochrone animation through a single cycle, a high tide sweeps west to the Amery Shelf and then circulates in that small bay, while the amphidromal sectors you see continuing on westwards represent a low tide. On the next animation cycle, a high tide leaves the Amery and heads over to the amphidrome in the Drake Passage, while a low tide sweeps into the Amery and circulates. You can see how the Amery bay sucks in and spits out tidal cycles quite well on the tide magnitude animation.
    So each animation cycle shows half the story for Antarctica, and half the amphidromal sectors passing along the Antarctic coast represent the opposite tide from the other half. It takes two cycles of the animation (two amphidromal rotations) to complete one Antarctic diurnal tide cycle.

    Grant Hutchison

  5. #35
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    Quote Originally Posted by 01101001 View Post
    I thought this was a spherical-cow matter, a lie we tell to children (Wikipedia), to help people to understand tides.

    I'm surprised anyone would categorize this simplification as mainstream physics instead of simple mainstream pedagogy.
    Read the Wiki more carefully, "lies to children" are not restricted to pedagogy, they also include scientific theory. Every law of physics falls under the definition of a "lie to children," we are simply the children using the lies. Of course, it requires a rather literal use of the term "lie", which can be debated, nevertheless that's what the term means.
    It's a germ of truth in a kernel of complexity.
    Exactly, or more generally, it is "understanding." Understanding always comes in degrees, and all scientific knowledge involves a kind of balance between confidence and skepticism. So all explanations are pedagogy, as soon as someone asks "why are there tides" you have entered pedagogy, and you about to issue a lie to children. The issue is what lies are good ones that benefit understanding, and what lies are bad ones that feed misconceptions and are a barrier to understanding. It's not always easy to tell the difference!
    Last edited by Ken G; 2017-Jan-05 at 06:41 PM.

  6. #36
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    Quote Originally Posted by profloater View Post
    Just for fun, if you use the simple force equation f = G.m.m/D^2 for 1 kg and the moon, you get a force of about 40 micro newton.
    So compared with the weight of about 10 N there is not much direct lift.
    Now if you work out the sideways velocity v = ft (f is the ac cn.)
    I chose for simplicity a six hour period ie three hours each side of the max sideways acceleration,
    You get a velocity of about 0.9 m/s.
    That applies to actually all the water in an ocean, so the resultant sideways momentum is a big number.
    However the water particle moves about 100 m in that six hours. Not enough to accumulate in a bulge.
    Six hours is about 20,000 seconds, so at about 1 m/s, that's about 20,000 meters, not 100 meters.

  7. #37
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    Quote Originally Posted by Ken G View Post
    Six hours is about 20,000 seconds, so at about 1 m/s, that's about 20,000 meters, not 100 meters.
    So we're talking about a wall of water three kilometres deep (on average) moving twenty kilometres towards the sublunar point from all directions. Seems like that small sublunar surface bulge might be quite feasible, after all. Especially since the bulge would represent a move towards hydrostatic equilibrium, rather than a deviation from it.

    Grant Hutchison

  8. #38
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    Quote Originally Posted by Ken G View Post
    Six hours is about 20,000 seconds, so at about 1 m/s, that's about 20,000 meters, not 100 meters.
    Yes oops I dropped a hundred, so the distance is about 10,000 m 10 km, sorry, it makes more sense at 10 km, the v is at the end of the time. But my point is that sideways momentum dwarfs any direct lift as evidenced by smaller seas and lakes.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  9. #39
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    Actually, direct lift is quite important, as the bulge is only about 1 foot.

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    [Still trying not to get lost, conceptually]So, if there were no Moon (I mean, beyond the obvious implications of how its formation is tied up in Earth's geologic history), would the tides as we experience them be largely unchanged? The "sloshing about" of the oceans would happen regardless, right?

    CJSF
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    Electrons are free
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  11. #41
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    Yes, but then the solar components would be the driving factor, and tides would be smaller.

  12. #42
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    Quote Originally Posted by Ken G View Post
    Actually, direct lift is quite important, as the bulge is only about 1 foot.
    There is no bulge. And the height difference for the amphidromic systems varies from zero to several meters. (the really high tides are from the amphidromic wave piling up against a shore, or bay, or some land form).

    https://en.wikipedia.org/wiki/Theory...ory_of_Laplace

    This graphic shows where that happens.

    h_tpxo6.jpg

    http://volkov.oce.orst.edu/tides/global.html
    Egbert, Bennett, and Foreman,1994 and Egbert and Erofeeva, 2002
    Last edited by Gigabyte; 2017-Jan-05 at 09:58 PM.

  13. #43
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    Flash movie showing in great detail the amphidromes and the ranges

    https://web.archive.org/web/20091107...obal_tide.html

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    Physics stack page on why there are no twin bulges

    http://physics.stackexchange.com/que.../121858#121858

  15. #45
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    Quote Originally Posted by Gigabyte View Post
    Yes, but then the solar components would be the driving factor, and tides would be smaller.
    So, the tidal forces that *would* ideally make bulges, drive the system, but the other dynamics basically erase them or redistribute the energy?

    CJSF
    "The sun is a quagmire
    It's not made of fire
    Forget what you've been told in the past
    Electrons are free
    (Plasma!) Fourth state of matter
    Not gas, not liquid, not solid"

    -They Might Be Giants, "Why Does The Sun Really Shine?"


    lonelybirder.org

  16. #46
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    Quote Originally Posted by CJSF View Post
    So, the tidal forces that *would* ideally make bulges, drive the system, but the other dynamics basically erase them or redistribute the energy?

    CJSF
    It's a tricky matter. From what I've read, even on an all ocean world, other factors would cause the amphidromic systems to form (like in the Pacific, where there is nothing stopping a bulge from racing across the ocean)

    The depth of the ocean on our world prevents a tide from travelling at a 1000 mph around the equator, but even on a world with a deep ocean, some calculate there would still be no twin bulges, because of the way water acts, which is where it gets complicated, and there seems to be a dispute over that fact.

  17. #47
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    Ocean tides, believe it or not, are very very complicated things. While the changing influence of gravity of the moon and sun are no doubt the drivers, where the car actually goes, how fast and how high, that is all far different than two bulges following the moon. We know that with out any doubt, because of measurements of the world and the oceans.

  18. #48
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    Quote Originally Posted by profloater View Post
    Yes oops I dropped a hundred, so the distance is about 10,000 m 10 km, sorry, it makes more sense at 10 km, the v is at the end of the time. But my point is that sideways momentum dwarfs any direct lift as evidenced by smaller seas and lakes.
    It seems you're imagining that large amounts of water have to relocate from a long distance away in order to build the bulge. But, in the equilibrium potential model, the bulge is already there. It moves around the Earth by subsiding hydrostatically in the east and rising hydrostatically in the west, to conform to the shifting equipotential surface - it doesn't have to be formed anew at every high tide.
    For a small lake, or a sea with a narrow inlet, like the Mediterranean, all that happens is that the water of the lake/sea drops to a lower potential as a sort of virtual tidal bulge passes over, because it can't change its volume by filling from a nearby body of water at a higher gravitational potential.

    Grant Hutchison

  19. #49
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    I see no merit in calling a simplified explanation a "lie to children." To me that connotes deliberately making a false statement while pretending to make a true one with no caveats. Let's just give our novice readers credit for ordinary intelligence and explain that the coastlines and irregular bottom of the ocean scramble the distribution of the water and make it difficult to tease out the mean distribution relative to the Moon's gravitational gradient. That provides the necessary caveat.

  20. #50
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    Quote Originally Posted by Hornblower View Post
    I see no merit in calling a simplified explanation a "lie to children."
    Hear him, hear him. That started out as a joke by Ian Stewart and Jack Cohen, which sounded to me like it would have worked as a throw-away line in Third Rock From The Sun, but instead it was assimilated by the academic equivalent of the Borg, endlessly and solemnly repeated, and stopped being funny somewhere around 1999.
    And the equilibrium equipotential model is just a terrible example of a "lie to children" because a) it's fundamental to understanding the dynamic theory of tides and b) it's hideously complicated to explain clearly.

    Grant Hutchison

  21. #51
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    Quote Originally Posted by Gigabyte View Post
    Ocean tides, believe it or not, are very very complicated things. While the changing influence of gravity of the moon and sun are no doubt the drivers, where the car actually goes, how fast and how high, that is all far different than two bulges following the moon. We know that with out any doubt, because of measurements of the world and the oceans.
    Actually the measurements do show the two bulges. What we're looking at can be obscured by the solar tidal effect, higher frequency components, and intervening land masses, as we've already discussed. Also, the (significant) deformation of the solid earth is not visible in those tide animations--the land doesn't flow, it simply deforms. If it were perfectly elastic, the sold earth would deform to match the bulges, and there would be *no* tidal changes in ocean depths.

    But the most important effect has not been discussed here (and I haven't found it on those threads in the physics forums). The shape of the earth, as far as gravity is concerned, is represented by the geoid, the equipotential surface that corresponds to the time-averaged ocean surface, and represents sea-level in the continental interiors. The two bulges that the moon's gravity produce are bulges in that equipotential surface. Their geographic manifestations are rises in the solid earth (but not enough to match the equipotential surface) and movement of the ocean--the bulges are the hydrodynamic forcing function for what we call the tides.

    So, the bulges are definitely there, it's just not easy to see them in the animation (if there were no water at all, or if the earth only had large lakes, there would be nothing to see of the bulges)

  22. #52
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    Quote Originally Posted by CJSF View Post
    So, the tidal forces that *would* ideally make bulges, drive the system, but the other dynamics basically erase them or redistribute the energy?
    The other dynamics don't erase the tidal bulges, they just make them harder to see. All that sloshing still contains a systematic east-west drift that shows up most prominently to the naked eye in the Southern Ocean. If you average all the sloshing bulges in the animations over multiple lunar orbits, you can pull a gravitational signal out of all the other noise, which represents that lunar tidal equipotential surface, moving around the Earth at the same angular velocity as the moon. It actually must be there, otherwise the moon wouldn't be undergoing the tide-mediated recession from the Earth that we observe.

    Grant Hutchison

  23. #53
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    Actually, the phrase "lie to children" has come to obtain a formal meaning described in that Wiki, it is not just the intersection of the word "lie" and "children," unfortunate as those word choices may be. I am fine with replacing them with something much more aptly chosen, like "layers of idealization," but that's simply what the term is taken to mean. There are a lot of phrases I am no fan of, including "dark matter", "blackbody radiation", "Big Bang", and "survival of the fittest." Yet these terms all have formal meanings we simply have to live with, or take up the good fight of replacing them with more correctly descriptive terms. Certainly, people are forever taking the terms and interpreting them literally, thereby introducing misconception, but that just happens all the time. We even had a thread here where someone was trying to argue that a completely wrong explanation for the mass-luminosity relation was a "lie to children," apparently because if you take that phrase literally, you can apply to any old garbage. But most of us agree that the tidal bumps introduced by the equipotential are not garbage, they are simply the first layer of idealization that leads to understanding.
    Last edited by Ken G; 2017-Jan-06 at 02:39 AM.

  24. #54
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    So may I summarise that the solid tidal forces create bulges in the geoid shape of the solid earth which is of course far from solid all through, using the equipotential explanation with its d^3 term? The water would follow that distortion if we had a water world. Then the fluids all over the surface are pulled using the Newtonian gravity terms with D^2 tangentially, creating massive momentum of moving fluids which is pushed around by the land masses as the earth rotates.
    The confusion often found is that the moon literally lifts the waters and it does not. So we need to understand that there are solid tides and fluid tides, all driven by the moon and sun gravity forces.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  25. #55
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    Quote Originally Posted by profloater View Post
    So may I summarise that the solid tidal forces create bulges in the geoid shape of the solid earth which is of course far from solid all through, using the equipotential explanation with its d^3 term? The water would follow that distortion if we had a water world. Then the fluids all over the surface are pulled using the Newtonian gravity terms with D^2 tangentially, creating massive momentum of moving fluids which is pushed around by the land masses as the earth rotates.
    The confusion often found is that the moon literally lifts the waters and it does not. So we need to understand that there are solid tides and fluid tides, all driven by the moon and sun gravity forces.
    I see a similarity there, but what do you mean by "solid tidal forces"?

    ETA: The geoid shape includes the surfaces of the oceans. It's often said to be "sea level" in fact.

  26. #56
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    Quote Originally Posted by grapes View Post
    I see a similarity there, but what do you mean by "solid tidal forces"?

    ETA: The geoid shape includes the surfaces of the oceans. It's often said to be "sea level" in fact.
    Yes sea level must be , in a standard atmosphere, a liquid level controlled by the local gravity and that variable, varies. I mean by solid the viscoelastic structure of the earth which must be modified by the centrifugal effect of spin and the tidal effects of spin mainly In relation to moon and sun.
    Then there are the surface fluids, the free water, the ground water, the atmosphere, which have no elastic constraint but do get pushed by the spinning land protrusions. They are continually subject to tangential acceleration toward the moon/sun as the earth spins. The forces and accelerations are small, 40 microN per kg is small, but are sustained for each geographical zone for hours. Obviously it is cyclic reversing acceleration, while the solid earth will stretch in a viscoelastic way, the fluids pick up momentum and then strike the land masses. So the tidal bulge is a feature of the whole planet including surface water but the tides sailors and coastal observers see is due to the tangential accelerations, as are the currents which move in those complex ways in order to create the lifts we observe. It may be counterintuitive that the tides which we see as a surface effect are caused by the whole bulk of the ocean being pulled sideways by the moon. Ground water must also move around invisibly with tides but there the porosity of the rocks is a major variable.
    Last edited by profloater; 2017-Jan-06 at 01:58 PM. Reason: Units
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  27. #57
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    Quote Originally Posted by Ken G View Post
    Actually, the phrase "lie to children" has come to obtain a formal meaning described in that Wiki ...
    Well, I don't want to get into a sidebar discussion on a thread that's already sloshing around like an oceanic gyre. But the problem with "lie to children" (in my view) is that it is applied to different phenomena by different people - and a label that means different things to different people will always hinder rather than help discourse.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Well, I don't want to get into a sidebar discussion on a thread that's already sloshing around like an oceanic gyre. But the problem with "lie to children" (in my view) is that it is applied to different phenomena by different people - and a label that means different things to different people will always hinder rather than help discourse.
    I agree that any time that phrase is invoked, it needs to come with explanatory remarks (like the way it was linked to Wiki above), or else it will spiral into miscommunication stemming from what an incredibly poor choice of words those are!

  29. #59
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    Quote Originally Posted by Ken G View Post
    I agree that any time that phrase is invoked, it needs to come with explanatory remarks (like the way it was linked to Wiki above), or else it will spiral into miscommunication stemming from what an incredibly poor choice of words those are!
    Well, they were a great choice of words when used as a joke in the short comedic science-fiction story that Cohen and Stewart used in their original book. The poor choice of course came only when people decided to adopt the phrase as a blanket label for a not-quite-clearly-defined set of related phenomena that they could then argue about confusedly.

    Grant Hutchison

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    Quote Originally Posted by profloater View Post
    So may I summarise that the solid tidal forces create bulges in the geoid shape of the solid earth which is of course far from solid all through, using the equipotential explanation with its d^3 term? The water would follow that distortion if we had a water world. Then the fluids all over the surface are pulled using the Newtonian gravity terms with D^2 tangentially, creating massive momentum of moving fluids which is pushed around by the land masses as the earth rotates.
    The confusion often found is that the moon literally lifts the waters and it does not. So we need to understand that there are solid tides and fluid tides, all driven by the moon and sun gravity forces.
    Actually, I don't think the solid tides are worth mentioning at all. We certainly should not picture the situation as being that the continents are causing the oceans to slosh around, instead, the fact that the continents do not participate in the sloshing is what strongly alters said sloshing. If the solid Earth responded not one iota to tidal forces, I doubt the ocean tides would be any different. Finally, the idea that the gravity of the Moon lifts the water directly suffers the problem that lifting water would create a vacuum at the bottom of the ocean, which won't happen-- but if the equipotential is raised, the water level will try to rise all the same, it will simply rise via lateral currents. So the gravity does "lift" the water, but it does so by piling it up via sideways motion. This is also what happens in a water wave at the beach-- the water doesn't "lift" as if it was leaving a vacuum under it, but it sure does rise up over you as you stand there. It does so via lateral currents that pile it up, and in the case of tides, the need to do that comes from the lifting of the equipotential surface. Remember, the amazing thing about gravity is that it does not care the mass of the objects it is moving-- regardless of that mass, if there is not force balance, motion will result.

    To clarify this, let's imagine an absurd but telling scenario. Let's say you have a huge lake, and somehow you create an extra downward gravitational force on one half of the lake, and a reduced gravitational force on the other half of the lake. All the gravity is purely vertical, none of it is horizontal. What will happen? You might think that since upward forces can't directly lift water without leaving a vacuum underneath, and since downward forces can't squeeze water any significant amount, essentially nothing would happen to that lake. But that would be wrong, because the new forces create a new equipotential surface that is lower on one half of the lake and higher on the other half, and so that's what the surface of the lake is going to try to follow. It will take time to reach that configuration, so if the gravity is periodically flipping which side is raised, and there is not time to reach the new equilibrium, there will be perpetual sloshing going on. Also, if there is a big island in the middle of the lake, the sloshing pattern will be affected. But at the end of the day, the driving of the sloshing is still going to be the raising and lowering of that equipotential surface, which is going to look a whole lot like "lifting the water", even though the water will mostly be moving sideways (due to internal pressure forces in the water) in its efforts to reach that equipotential shape.

    So the example I'm giving is one where there are no lateral gravitational forces at all, so all lateral forces are pressure forces in response to gravity, yet you'll still get tidal bulges. In the actual situation, there are lateral tidal forces, which are about equal to the vertical ones, so this shows that in the real situation, the water receives roughly equal direct forces from gravity as it does pressure forces from its effort to find the equipotential. So yes, lateral forces are key, but only some part of them are due to lateral tidal forces, a roughly equal part is due to pressure forces in response to the vertical tidal forces. We could attribute that latter component to "lifting" of the water by tidal forces, though the lifting itself is replaced by pressure effects that produce lateral currents as in my lake analogy. Above all, note that we did not have to do anything to the solid matter at the bottom of the lake, nor anything to the solid island, to get the sloshing of the water!
    Last edited by Ken G; 2017-Jan-06 at 03:37 PM.

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