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Thread: Question about Earth tides

  1. #61
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    Quote Originally Posted by grant hutchison View Post
    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.
    I completely agree with all of that. Note how analogous is our use of the term "Big Bang," which also stemmed from a kind of joke, but nevertheless that term has a formal meaning that is in wide usage. Let's hope "lies to children" doesn't similarly catch on, but all the same, it does have a formal meaning, and if it catches on, we are stuck with it. We can fight the good fight to prevent that, because it is indeed a horrible choice of words (so is "Big Bang", so is "dark matter"), but there's only so much we can do!
    Last edited by Ken G; 2017-Jan-06 at 03:32 PM.

  2. #62
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    Quote Originally Posted by Ken G View Post
    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.
    On the other hand, I am certain that they would be very different. The solid earth tide is not insignificant. If it were larger, the ocean tides could disappear.

    We might be able to find some simulations somewhere, but I imagine a good approximation might be the difference between neap and spring tides.
    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.
    That is the essentials of the Laplace equations for the tides, where the potential is the forcing function.
    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!
    As far as this thread is concerned, where the question is whether there are two bulges or not, the solid tides do come into play. Also, as far as the "lies to children" are concerned, the two bulges are mitigated by the two bulges of the sun, in addition to all the other complicating factors involved. But, after all, the two bulges are real, they exist.

    That hints at the reason that there are not significant tides in small (or even large) lakes. The potential surface doesn't vary significantly over the extent of the lake, whether the surface raises or lowers. The surface of the lake tends to parallel the potential surface.

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    Quote Originally Posted by grapes View Post
    That is the essentials of the Laplace equations for the tides, where the potential is the forcing function.
    Yes, and that's why any stance that the tides are caused entirely by changes in potential or entirely by tangential forces is going to be wrong - everything works together.
    And yet you (or maybe it's just me) frequently see someone declaring something to the effect that "there can be no tidal bulges, because the tides are caused by tangential tidal forces, not vertical tidal forces". In effect, they set up straw man, and then attack it with a mistaken argument. You can find an example of that sort of argument in the StackExchange thread I linked to earlier.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    And yet you (or maybe it's just me) frequently see someone declaring something to the effect that "there can be no tidal bulges, because the tides are caused by tangential tidal forces, not vertical tidal forces". In effect, they set up straw man, and then attack it with a mistaken argument. You can find an example of that sort of argument in the StackExchange thread I linked to earlier.
    Can you quote an example? I can't find anyone claiming what you said they have.

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    In fact, I can't find anywhere on the net the phrase "the tides are caused by tangential tidal forces". It will show up eventually, but only to your post, and mine.

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    Quote Originally Posted by Gigabyte View Post
    Quote Originally Posted by grant hutchison
    And yet you (or maybe it's just me) frequently see someone declaring something to the effect that "there can be no tidal bulges, because the tides are caused by tangential tidal forces, not vertical tidal forces". In effect, they set up straw man, and then attack it with a mistaken argument. You can find an example of that sort of argument in the StackExchange thread I linked to earlier.
    Can you quote an example? I can't find anyone claiming what you said they have.
    Here you go:
    There is no tidal bulge.
    [...]
    The question is about the oceanic tides, and there it's the horizontal component that is the driving force.
    And with reference to:
    Quote Originally Posted by Gigabyte View Post
    In fact, I can't find anywhere on the net the phrase "the tides are caused by tangential tidal forces".
    I'm not surprised, since I didn't offer it as a direct quotation. Hence my use of the phrase "something to the effect that". See above, for something to the effect that "the tides are caused by tangential tidal forces", taken from the link I gave.

    Grant Hutchison

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    When I read, in context, that "it's the horizontal component that is the driving force." I see it as the horizontal movement of water is what actually causes the tides, not a lifting (vertical) factor, which is what the dynamic theory does state is the reason for the observed tides. There is no gravity bulge lifting up, it's all about the horizontal movement of the oceans.

    The model animation I posted before, https://www.youtube.com/watch?v=5zi7N06JXD4 shows clearly that it's not some bulge lifted by the moon, it's the horizontal movement of the ocean, caused by gravity, that results in the actual real tides.

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    If you think about the assumed bulge following the moon, it can't actually happen. The speed of the bulge is far too fast for the depth of our oceans. And nowhere do we ever observe a tide moving at supersonic speeds, trying to stay with the moon. It is impossible.

    Now, the real question was, is that an ATM idea? Is the mainstream view that the ocean tides are really two bulges, moving at a 1000 mph across the Pacific ocean twice each lunar month, actually true?

    (that would be when the moon is over the equator of course)
    Last edited by Gigabyte; 2017-Jan-06 at 10:12 PM.

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    Quote Originally Posted by Gigabyte View Post
    When I read, in context, that "it's the horizontal component that is the driving force." I see it as the horizontal movement of water is what actually causes the tides, not a lifting (vertical) factor, which is what the dynamic theory does state is the reason for the observed tides. There is no gravity bulge lifting up, it's all about the horizontal movement of the oceans.

    The model animation I posted before, https://www.youtube.com/watch?v=5zi7N06JXD4 shows clearly that it's not some bulge lifted by the moon, it's the horizontal movement of the ocean, caused by gravity, that results in the actual real tides.
    I seem to see the highs and lows progressing around the equator, though, in that animation. What are the expected characteristics?

    Quote Originally Posted by Gigabyte View Post
    If you think about the assumed bulge following the moon, it can't actually happen. The speed of the bulge is far too fast for the depth of our oceans. And nowhere do we ever observe a tide moving at supersonic speeds, trying to stay with the moon. It is impossible.
    I really thought that's what I see in that animation. What does the animation depict?
    Now, the real question was, is that an ATM idea? Is the mainstream view that the ocean tides are really two bulges, moving at a 1000 mph across the Pacific ocean twice each lunar month, actually true?

    (that would be when the moon is over the equator of course)
    And, the sun lines up with the moon, but, yes?

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    Quote Originally Posted by grapes View Post
    I seem to see the highs and lows progressing around the equator, though, in that animation. What are the expected characteristics?
    https://www.youtube.changescom/watch?v=mJorZMT6W3M

    Yet another animation, showing the M2 Harmonic Constituent, which is the moon only.

    Phil explaining tides

    What is actually happening, for example with the Bay of Fundy tide, is the exact same circular wave (the amphidrome) (see here) comes around twice a day. It's the same wave each time. Tides move in giant circles, the two high tides a day (where this happens) is from the same wave crest each time.

    You can see it in the Dutchs gif animation, any of the animations really.

    https://www.uwgb.edu/dutchs/Graphics...TideNodes0.gif

    So rather than a bulge following the moon, the tide is actually a giant standing rotary wave, pumped by the sun/moon gravity changes, because the earth is spinning.

    If the sun and moon actually caused bulges, like so many people explain, there would be separate bulges for the moon and sun. When the sun and moon are at right angles, there would be four high tides a day, and at full and new moon, just two much larger high tides a day.

    Right? Four bulges. Because while the sun is less, it still causes a tide.

    Well, maybe. But I find this very interesting. The amphidromic points are pretty much for the lunar tides, these being overwhelming in their influence. The sun tide moderates the lunar tide, but at the amphidromic point there is not tide. (those are the red dots in the Dutchs gif animation)

    The center of the amphidrome doesn't have a lunar tide. (by definition, the amphidromic point is the point of no tide) So the solar tide can actually cause a tide at those points, since the lunar influence is zero there. Or so I have read.

    Isn't that something?

    But in any case, we know from actual measurements that there are indeed amphidromes, and that's how the tides work. Not two bulges following the moon.

  11. #71
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    Quote Originally Posted by grapes View Post
    What does the animation depict?
    The incredibly complicated amphidromic systems, which are shown clearly in the giant Flash movie of the entire world, I posted early on.

    Quote Originally Posted by Gigabyte View Post
    Flash movie showing in great detail the amphidromes and the ranges

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

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    Quote Originally Posted by Gigabyte View Post
    the horizontal movement of water is what actually causes the tides, not a lifting (vertical) factor, which is what the dynamic theory does state is the reason for the observed tides. There is no gravity bulge lifting up, it's all about the horizontal movement of the oceans.
    It is incorrect to say there is no gravity bulge affecting tides.

    Each synodic month has spring (big) and neap (small) tides on a weekly cycle. Spring tides are where sun and moon are lined up, around full and new moons, reinforcing each other's gravity to increase the tide. Neap tides are where sun and moon are near first and third quarters, cancelling each other's gravity to reduce the tide.

    These weekly changes in tidal range are only possible as a product of a combined gravity bulge producing the tides.

    Here is an illustrative picture I produced from tidal records, showing this regular weekly change in tidal range in Sydney Australia.
    Attached Images Attached Images

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    Quote Originally Posted by grapes View Post
    On the other hand, I am certain that they would be very different. The solid earth tide is not insignificant. If it were larger, the ocean tides could disappear.
    I just don't believe that. If the solid tides were larger, it would not make the ocean tides disappear, but more to the point, if there were no solid tides at all, that would likely not change the ocean tides much. The reason is simple-- the tides are caused by exactly one thing, a differential fictitious force everywhere that varies with time. What responds to that differential fictitious force is what can move in response to it-- the oceans. The solid Earth does respond faster, due to its fast sound speed, but it doesn't respond much, so it shouldn't have much effect on the water.
    We might be able to find some simulations somewhere, but I imagine a good approximation might be the difference between neap and spring tides.
    I don't see what that would tell us, that difference has nothing to do with solid tides, it has to do with the variation in the size of the equipotential bulges, i.e., in the strength of the differential fictitious forces.
    As far as this thread is concerned, where the question is whether there are two bulges or not, the solid tides do come into play.
    Why? What evidence do we have for that? I don't see it, I think the ocean tides would be virtually identical if the solid Earth was infinitely rigid.

    Also, as far as the "lies to children" are concerned, the two bulges are mitigated by the two bulges of the sun, in addition to all the other complicating factors involved.
    There are not two bulges due to the Sun, the Sun and Moon are nearly in the same plane, so their tidal forces combine to produce a single football-shaped equipotential. All that changes is "how pointy" is the football, versus how flattened it is. There are always two bulges, what is different is the height of the two bulges.
    That hints at the reason that there are not significant tides in small (or even large) lakes.
    It's simply because the lake cannot create a vacuum under it, so it cannot lift up to the height of the constant equipotential. Nor can it be squeezed down below its banks. There can't be sufficient piling up due to lateral pressure forces in the water-- pressure forces that cannot be created if the lake is not large enough. So the equipotential at the surface of a lake varies more like the equipotential varies in the solid crust. It is only in the open ocean where the value of the equipotential that the surface achieves will vary less with time, due to the ability of the ocean to flow laterally (due to the combination of lateral tidal gravity and lateral pressure forces, both driven by the equipotential).
    The potential surface doesn't vary significantly over the extent of the lake, whether the surface raises or lowers.
    Actually, it does, it varies just as much as on the open ocean, but the value of the equipotential that the water surface reaches changes with lunar phase. In other words, the water will tend to lie along the same equipotential surface, but the value of that equipotential can change with time unless the water connects the peaks and the troughs, as in the open ocean. If the water connects the peak and the troughs, and has enough time and freedom to conform to that equipotential, then it will all be at the same equipotential, and so that equipotential value cannot change with time (because it connects the peaks and troughs, so is always the same).
    The surface of the lake tends to parallel the potential surface.
    Yes, very quickly any water surface must conform to an equipotential surface locally, but the global value of the equipotential is what causes tides. The water surface has to reach equilibrium with the global equipotential to get the simple "bulges", and that requires both long distances and long times. The long distances are interrupted by land masses (hopelessly so for lakes), and the long times is curtailed by the changing lunar orientation, though a lot can happen in six hours.
    Last edited by Ken G; 2017-Jan-07 at 08:36 AM.

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    Quote Originally Posted by Gigabyte View Post
    If the sun and moon actually caused bulges, like so many people explain, there would be separate bulges for the moon and sun. When the sun and moon are at right angles, there would be four high tides a day, and at full and new moon, just two much larger high tides a day.
    No, this is not correct. The linearized perturbation to the gravity caused by the Sun and Moon is always essentially a football shape, since the Sun and Moon are close to being in the same plane all the time, and the combination of two football-shaped linearized deformations at any relative angle is still a single football-shaped deformation. There would never be four bulges!
    Last edited by Ken G; 2017-Jan-07 at 08:28 AM.

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    In relation to bulges travelling around the world, there might be an averaged effect but it is clear from the observations that there are strong ( full depth) currents which reverse exactly as you would expect if the moon pulled the water sideways tangentially and then reversed. When you consider the inertia of those masses of water, it is clear the currents will lag the driving force exactly like harmonic motion. For each longitude the moment the moon force reverses is the moment of max flow. But that is for an uninterrupted flow. The continental shelf accelerates the flow considerably and funnels the flows to new latitudes, therefore to new radii which introduces the coriolis forces or conservation of momentum forces. The tidal height is also out of phase with the currents just as a pendulum bob height is out of phase with its speed. If you look at the North Atlantic in the visualisations, you see the high tides simultaneous at east and west with a minimum at the centre. This suggests a second harmonic resonance of that zone illustrating how important ocean resonances are in a regular forcing, which is complicated by the slow variation of the gravity environment of the moon and sun. A simple bulge rotating could never produce simultaneous high tides on both sides of he Atlantic. That,s even if enough averaging can show the 24 hour equipotential cycle. The inertia and momentum of the fluids is so important.
    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

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    Well, we know there is a bulge that moves around the Earth with the moon - in the Southern Ocean. It's visible in the animations, and it's no surprise its there, given that the Southern Ocean has no large landmasses blocking it, and is at a high enough latitude that the speed of the Earth's rotation relative to the moon is a match for the maximum wave velocity. You can see segments of the same thing happening in the Pacific and Atlantic, with high tides sweeping east to west across the north end of these oceans in sequence.
    Elsewhere we have the gyres, which rotate semi-diurnally, but generally repeat the same pattern only diurnally (and, obviously, with long-term variations driven by the moon's orbit and the sun's position). So each time the moon passes over, it "sees" roughly the same patterns of bulges and dips in the oceans below it, because it's driving a forced harmonic motion.
    And if you average the gravitational effect of all those bulges and dips at midlatitudes over time, along with the travelling bulges at high latitudes ... presto! the Newtonian tidal bulge emerges. It has to be there, to drive the observed tidal evolution of the moon's orbit. If the oceans were just sloshing at random, they wouldn't be able to transfer angular momentum from the Earth to the moon.

    There's no doubt an overemphasis on the equilibrium potential model ignores the complexity of the real world, which is a Bad Thing in some circumstances.
    But concentrating exclusively on the oceanic gyres ignores the big picture of how tides are generated and how they work, which is a Bad Thing in other circumstances.

    The trick is to realize that neither viewpoint excludes the truth of the other.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    And if you average the gravitational effect of all those bulges and dips at midlatitudes over time, along with the travelling bulges at high latitudes ... presto! the Newtonian tidal bulge emerges.
    That was the point when I quoted Wikipedia. (citation needed)
    Quote Originally Posted by Gigabyte View Post
    The "bulges" that actually do exist for the Moon to pull on (and which pull on the Moon) are the net result of integrating the actual undulations over all the world's oceans.

    https://en.wikipedia.org/wiki/Tidal_...tum_and_energy

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    Quote Originally Posted by Gigabyte View Post
    That was the point when I quoted Wikipedia.
    I know. And the point I've been making. So saying "there are no bulges" is at best misleading. At any given time the open ocean is covered with bulges, caused by tidal gravity, and they have a summative effect.

    Grant Hutchison

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    Indeed, but there is never a twin bulge, following the moon, like so many mainstream sites show. It's always amphidromes, even in the southern ocean. And large tidal variation is always where an amphidromic system gets piled up by a land mass, or between land masses. Or where two amphidromic systems interact.

    The issue of a tide bulge in the southern ocean is disputed of course. But what is the mainstream view on that? And how can you know?
    Last edited by Gigabyte; 2017-Jan-07 at 06:01 PM.

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    http://www.iupui.edu/~g115/mod12/lecture07.html

    If there was a tide bulge in the southern ocean, you would see a high tide all along the path. But that isn't what is measured. Instead there is a variable semi-diurnal tide, which is due to the other amphidromic systems interacting with the southern ocean bulge. And of course the bulge gets shunted up the Atlantic every time it encounters the narrows between Antarctica and South America. It's fascinating to me.

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    Quote Originally Posted by Ken G View Post
    No, this is not correct. The linearized perturbation to the gravity caused by the Sun and Moon is always essentially a football shape, since the Sun and Moon are close to being in the same plane all the time, and the combination of two football-shaped linearized deformations at any relative angle is still a single football-shaped deformation. There would never be four bulges!
    It's interesting, because the Laplace Dynamic theory of the tides says there are four bulges. As well as amphidromic systems, and it is the interaction of the two different bulges that causes the actual tides. As well as all the other influences.

    The four bulges are the tides, even as they form all the complex amphidromic systems, it's the out of phase four bulges that create the changing tides. But rather than 4 moving bulges following the sun and moon, the circular waves are the actual tides.

    Mainstream? Or ATM crazy?

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    Quote Originally Posted by Gigabyte View Post
    The issue of a tide bulge in the southern ocean is disputed of course. But what is the mainstream view on that? And how can you know?
    You'll need to give me a reference for the "dispute" - it's evident to the naked eye in the tide animations I've provided, and you've provided.

    Quote Originally Posted by Gigabyte View Post
    http://www.iupui.edu/~g115/mod12/lecture07.html

    If there was a tide bulge in the southern ocean, you would see a high tide all along the path. But that isn't what is measured. Instead there is a variable semi-diurnal tide ...
    That's not what the map shows at all. The co-range colours shows that the tidal range is small in some parts of the Southern Ocean, and high in other parts, not that there are no tides. And the tide is, as I've said, diurnal on the Antarctic coast. You can watch the tide make a single diurnal pass along the coast of Antarctica in the gif I already linked to, and see how it is of lower intensity in the regions marked blue in the co-range chart you referenced, but doesn't disappear.

    Grant Hutchison

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    Quote Originally Posted by Gigabyte View Post
    It's interesting, because the Laplace Dynamic theory of the tides says there are four bulges. As well as amphidromic systems, and it is the interaction of the two different bulges that causes the actual tides. As well as all the other influences.

    The four bulges are the tides, even as they form all the complex amphidromic systems, it's the out of phase four bulges that create the changing tides. But rather than 4 moving bulges following the sun and moon, the circular waves are the actual tides.

    Mainstream? Or ATM crazy?
    My bold. I would say the presentation in the first link is just an oversimplified account of a complex phenomenon involving continental coastlines. I don't think Laplace would have found a 4-lobed pattern in an idealized spherical hydrosphere on a spherical lithosphere. At quadrature I would simply expect a reduced ellipticity of the orbital plane cross section, that is, what would be at the equator at equinox. This has been addressed before in this thread.

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    This is a long but interesting review of the scientific analysis
    http://www.springer.com/cda/content/...325-p174603023
    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

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    Quote Originally Posted by Gigabyte View Post
    It's interesting, because the Laplace Dynamic theory of the tides says there are four bulges.
    I recommend you pay no attention to any source that says:
    "The sun, although much larger than the moon, exerts a
    gravitational pull that is less than the moon. In reality the pull is
    much greater, but because the sun is so much farther away from
    the Earth, its gravitational pull lessens. The moon then, is the
    main source of gravitational pull on Earth."
    It obviously has no idea what it is talking about, because if it cannot navigate the distinction between a force and a differential to a force, it has zero hope of understanding tides.

    As well as amphidromic systems, and it is the interaction of the two different bulges that causes the actual tides.
    The amphidromic elements are when you bring in the dynamics. The point here is that a good way to approach tides is by a two-step process: first understand the driver, which is the equipotential, and that has two bulges even when you include the Sun. Then, since this will only give you factor-2 kind of accuracy in general (and won't give you the tides at the beaches any more than it gives you surfable waves there), you can tack on the dynamical modifications to fill in the rest of the story at a considerable ramping up of the complexity.
    The four bulges are the tides, even as they form all the complex amphidromic systems, it's the out of phase four bulges that create the changing tides. But rather than 4 moving bulges following the sun and moon, the circular waves are the actual tides.
    Again; the equipotential never has any bulges that follow the Sun. The difference between neap and spring tides is just the shape of the football (or rugby ball, if you prefer).
    Mainstream? Or ATM crazy?
    Neitheraa-- just incorrect, you are picturing the wrong equipotential.

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    For the oceans (unlike the solid earth tides) there is no football shape. That's the essential fact that modern observations show. The solar tides have their own amphidromic systems, the moon has a different set, and it is the complex interaction of the two tidal systems that creates what we observe.

    ATM or mainstream view?

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    Quote Originally Posted by Gigabyte View Post
    For the oceans (unlike the solid earth tides) there is no football shape.
    The football shape refers to the equipotential surface. That's the driver for everything that happens, so it's silly to say "there is no football shape." However, we have established that the oceans fail to fill the equipotential, for two reasons-- the six hour timescale is a bit fast for the ocean response, and the continents get in the way because they don't at all conform to the equipotential surface, and they inhibit water flow. Hence, the football shape is incompletely matched by the ocean surface, there are currents that represent the incomplete attempt to fill the football, and there are continents that stack up the dynamical effects to a much higher amplitude in limited areas.

    The solar tides have their own amphidromic systems, the moon has a different set, and it is the complex interaction of the two tidal systems that creates what we observe.
    Obviously, the effort of the oceans to conform to the equipotentials of the Moon and Sun system is the cause of all the tides. The equipotential has a roughly football shape at all times and all lunar phases, but the dynamical response to trying to fill it does not competely succeed, leading instead to more complex flows like amphidromic currents and stacking up of tidal effects in resonant zones.

    ATM or mainstream view?
    The mainstream view has been stated many times in this thread, as I just repeated. You only have to say it right.
    Last edited by Ken G; 2017-Jan-07 at 08:01 PM.

  28. #88
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    Quote Originally Posted by grant hutchison View Post
    There's no doubt an overemphasis on the equilibrium potential model ignores the complexity of the real world, which is a Bad Thing in some circumstances.
    But concentrating exclusively on the oceanic gyres ignores the big picture of how tides are generated and how they work, which is a Bad Thing in other circumstances.

    The trick is to realize that neither viewpoint excludes the truth of the other.
    Exactly, there are layers of understanding here. This is routine in science. There's always the clever kid in the back of the high school class that, when the teacher gives the class Newton's universal law of gravity and tells the class that gravity is a force that causes acceleration, pipes up and says that's a myth because gravity is actually a curvature of spacetime that causes no acceleration at all. But this fails to grasp how science works, and how we gain understanding in layers of complexity. There are not complete answers in science, there are tailored answers. We replace what we cannot understand with what we can, and that depends on our capabilities. Still, I agree the bulges have been a bit overemphasized-- they are the drivers, not the response to the drivers.

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    It helps to visualize that the solid earth tide, the deformation of the entire planet due to the moon and solar gravity differentials, is about 30 cm, which is hardly anything at all, considering the size of the planet.

    To imagine how slight a tilt this actually causes, if you had a solid steel bar 10 miles long, so it couldn't deform in any way, and it was on an actual real flat surface, in order to tilt it as much as the earth tide does, you would slip a dime under one end. That would approximate how much the land tilts from low to hide tide, for the solid earth.

    To imagine the difference in weight on your own body the full moon causes, I've heard it is about that of a grain of sand.

  30. #90
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    Quote Originally Posted by Gigabyte View Post
    It helps to visualize that the solid earth tide, the deformation of the entire planet due to the moon and solar gravity differentials, is about 30 cm, which is hardly anything at all, considering the size of the planet.
    Right, that's why I said above that I didn't think the solid tides that do exist play any important role. What matters is the contrast that stacks up tides along the coastlines. The equipotential bulges that matter are over the oceans.

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