i know the mainstream view "gravity tides" but that would sugest that the high tide points at the moon, and i don't think it does. the only other thing it could be is the earth moving with the barycentre wobble. as someone put it "the wobble sloshes the sea around"

There is a website associated with this board!

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There is also a lengthy thread here on the tides.

In short, tides are caused by differential gravitational forces. The moon pulls on the closer side more because, well, it's closer.

thanks for that. in the java applet, the moon was directly above the high tide. i don't think that is right. the moon is nearer above low tide.
this is what i mean, mainstream " gravity causes tides". i too, have heard this but i'm just saying "maybe not". and a lot of people seem offended because the've learnt differently. it's only supposed to be thought provocking

What do you mean the moon is "nearer" above low tide. It's not like the moon comes swooping down near earth and then jumps back up into the sky. High tides are "above" the moon because that is where the greatest graviational attraction is.

http://csep10.phys.utk.edu/astr161/lect/time/tides.html

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in the java applet, the moon was directly above the high tide. i don't think that is right. the moon is nearer above low tide.

That's because the Moon is going round the earth every twenty-eight days, but the Earth is spinning every twenty-four hours. The tidal bulge created by the pull of the Moon's gravity is pulled ahead of the Moon by the Earth's rotation, hence the Moon does not lie exactly over the high tide but rather lags behind.

This is slowing the Earth's rotation (the bulge is being dragged by the Moon), and causing the Moon to recede (the bulge is also dragging the Moon, pulling it forward and into a slightly higher orbit). Eventually this will result in the entire system becoming locked so that one side of Earth always faces the Moon as well as one side of the Moon always facing Earth. This situation has already occurred in the Pluto/Charon system.

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"The very powerful and the very stupid have one thing in common: They don't alter their views to fit the facts, they alter the facts to fit their views." The Doctor, Doctor Who: The Face of Evil.

[quote="Jason Thompson
The tidal bulge created by the pull of the Moon's gravity is pulled ahead of the Moon by the Earth's rotation, hence the Moon does not lie exactly over the high tide but rather lags behind.[/quote]

so the common theory sujests the moon being over the high tide but allows room for this not being the case. or maybe the theory is wrong.

Maybe, but that's a really huge maybe.

Imagine you're holding a garden hose, that has a steady stream of water coming out of it with enough pressure to push it out a few feet if the hose is horizontal. You move like a top spinning, and the stream bends. Now, all that water is still going straight out from you, but because you moved the stream is curved. The "end" of the stream needs time to catch up to the "beginning" (the hose end). That's a bit like what Jason Thompson meant when he said:

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h00!

so the common theory sujests the moon being over the high tide but allows room for this not being the case.

No, the common theory says that the Moon pulls on the Earth creating the tidal bulge and this bulge is carried ahead of the Moon by the Earth's faster rotation. It makes perfect sense, and fits with observation.

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"The very powerful and the very stupid have one thing in common: They don't alter their views to fit the facts, they alter the facts to fit their views." The Doctor, Doctor Who: The Face of Evil.

The common theory works just fine, its just simplified in that java animation. The actual motion of the tides at any location on earth is highly complex and includes things like water depth and shore geometry.

You are making something out of nothing.

(shameless plug)

... or you could go out and buy a copy of "Bad Astronomy" since it's now in its final printing (if you haven't already done so), and read chapter 7.

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Good point. Surprisingly, I haven't bought the book. I read a lot, but most books are from the library. Sorry BA. :P

You have to factor in the mass and density of the earth, verses then mass of the ocean - since the the earth is also tugged toward the moon, the ocean on the other side is left behind - hence the higher tide near the opposite side...of course, the rotations of the earth and moon come into play too, and the position of the sun, but when the dust is all settled, the mechanics work out almost perfectly with newtonian predictions...

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jwj

It's a big universe out there...is it really unwinding, really burning out?

Anyone wanta talk wave resonance for tidal height variations?

Also, what would happen to the Moon's orbit if it and Earth were perfect isotropic spheroids and inelastic? Would energy be exchanged? [assume current rotation and orbital speeds]

I suppose "frame dragging" would count. #-o

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you said.........the mechanics work out almost perfectly with newtonian predictions..... (how do you do all that bordering????) but newton also said "two bodies don't fell gravity from each other if they are falling at each other" which is roughly whats happening with the earth and moon. the moon moves all the way round the earth and the earth moves around the barycentre.

Wait a sec. The Earth-moon barrycenter is within the earth. [Link, additionally if you've ever wanted to know more than pi=3.14 you'll see it here]

So, you're saying the earth moving around the barrycenter causes the oceans to 'slosh' and thus the tides. However, if the point of rotation is within the earth, then over then years friction with the ocean beds would have dampened the 'sloshing' to a stop. Then following though the logic you seem to be presenting, all the water would have been pulled by centrifugal force to the fartherest point from the center of rotation. Thus the side closest to the barrycenter would be at a perpetual low tide (if not completely dry) and the point fartherest away would be at perpetual high tide. Considering the age of the earth, I'd say enough time has passed that this would have either happened by now or it'd be very obivous it was headed that way.

The earth and moon aren't point particles and therefore, they feel tidal forces. Newton's laws do predict that. The fact that a cc of water on the near-side of the earth is closer to the moon than one on the far side means it feels more gravitational force. The difference is the tidal force.

From that website:
Another miracle of pi?

From its tides page:
It's actually about three times that.
Many magnitudes less than that.
The only tide page I've seen with as bad an explanation was NOAA's.
Not every day. Every century.
The moon's "most massive side" does not face the Earth directly.

I didn't check it out too much, just wanted someplace that mentioned where the barrycenter was. Shoulda looked a bit harder.

Sorry I looked into it! I've done the same thing.

I should have read this first as it seems to answer my question regarding wether or not the Moon's orbit would be affected by Earth if both bodies were inelastic, sperhical and homogenous.

If they were, there would be no "bulge" to slow the Earth and speed-up the Moon along it's orbit.

I am not sure what frame dragging would do. Let's see what Probe B says.

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It is one thing to state tidal energy is transferred to the moon, it is more complex if we try to use this transfer to increasing orbital speed of the moon, which appears to be happening. The textbook mechanism for this is the 'swing pushing' of the orbit of the moon relative to the rotational velocity of the earth. This causes the bulk of the tide to slightly lead the path of the moon, encouraging it to orbit faster.

HOWEVER: The energy transfer pushes the moon to a higher orbit and it should not take long for an equalibruim to be reached between the drag components of the earth/moon orbit and this tidal transfer rate, as this rate should decrease as the distance increases...UNLESS the net tidal force is actually increasing, which due to global warming, appears to be the case: Ice melting at the poles sloshes, flows and bulges at the equator. This belt-widening has also been observed.

There are other alternatives - I read one article proposing magna tides, but the acceleration of the moon is certainly fertile ground for alternative cosmologists (that would be me) and woo woos (that might be me, too!) to plough.

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jwj

It's a big universe out there...is it really unwinding, really burning out?

Why does that big pendulum thing behave erratically during an eclipse? Is it even relevant? Feel free to tell me to shut up and go away!

And no oceans either, eh?
Frame dragging is not going to be significant, as far as the moon is concerned, if the theory we have is correct.

If you consider oceans, the bodies are no longer inelastic and homogenous, so I think it's covered.

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Right. You can have oceans, but they can not be moved at all. I suppose it is a bit more hypothetical than usual.

However, if no energy can transfer in this scenario, then I may understand better what is happening. I'm guessing no energy is exchanged for either the Moon's orbit or the Earth's spin rate.

They would still have a barrycenter and would both rotate around it.

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I'm not sure what you're getting at. Even if you allow oceans (even moveable ones), and elasticity, there is no energy exchange necessarily. In fact, it's not clear exactly where the energy exchange does take place, in the actual situation.

The idea is to eliminate the drag due to a "bulge". If Earth is completely (and impossibly) rigid, there would be no fast rotating bulge to pull on the moon and increase it's orbital speed.

If I learn that I am wrong, then I will see that I don't actually understand this aspect of the tidal interaction.

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If there is no tidal bulge, there will be no tidal drag.

But the drag is ultimately just "friction" somewhere. Where, we're not exactly sure.

If the Earth deformed perfectly, and had tides, the bulge would line up with the moon and would not be carried ahead of the earth/moon line.