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# Why are tides high OPPOSITE the moon?

07/12/2007 3:46 PM

The following is taken from http://science.howstuffworks.com/question72.htm :

The following diagram shows how the moon causes tides on Earth:

In this diagram, you can see that the moon's gravitational force pulls on water in the oceans so that there are "bulges" in the ocean on both sides of the planet. The moon pulls water toward it, and this causes the bulge toward the moon. The bulge on the side of the Earth opposite the moon is caused by the moon "pulling the Earth away" from the water on that side.

Now for my question. Can someone explain the last sentence in the paragraph above? If the gravitational field of the moon causes a bulge in the ocean on the opposite side of the Earth because it pulls harder on the denser rock, how can it pull harder on the less-dense water on the near side?

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#1

### Re: Why are tides high OPPOSITE the moon?

07/12/2007 4:54 PM

The centre of gravity of the earth / moon combination is between the 2 (closer to earth). The earth is then actually in orbit around that point - and falling to that point.

The water bulge at the back is there because of centrifugal force. (Ok you cannot push something with a string but that is about the easiest explanation to understand why!)

The water must be seen as in orbit around the centre point of gravity trying to continue in a straight line. (further from the point) in a wider orbit.

The best way to explain the bulge will ba an experiment with a tennis ball tied to a string. If the ball is soaked in water and swung around in a circle the water will leave the ball at the back. If the ball is swung around slowly or honey is used the fluid it will concentrate at the far side.

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#2

### Re: Why are tides high OPPOSITE the moon?

07/12/2007 5:21 PM

did you 'cut and paste' the graphics?

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#15
In reply to #2

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 11:40 AM

Yes, I did cut and paste. Se the referenced url.

Bill

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#3

### Re: Why are tides high OPPOSITE the moon?

07/12/2007 5:34 PM

GOOD QUESTION!

Maybe another way to look at it is that there isn't a bulge at the back!

The water for the bulge at the front has come from the sides so they are having a low tide...the back isn't so much having a high tide ...as a 'not low' tide..?

I'dunno just a though...maybe I should go to bed!

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#4

### Re: Why are tides high OPPOSITE the moon?

07/12/2007 11:30 PM

Can't pull all the ocean water to one side, so "squishing" occurs in between. I believe that is a scientific term now.... sorry.

Budub budub, that's all folks.

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#5

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 2:58 AM

Imagine there was no moon!
(i.e.. some space side disaster)

If the pull of the moon on the nearside water (tide) balances
the tide created by centrifugal force on the opposite side, a
small deviation in the moon, eg in distance or size, could have
a disasterous water "sloshing about" effect on the earth.

Anyone got a boat? Noah.

jt.

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#8
In reply to #5

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 3:26 AM

"sloshing about"

D'uh...? That's exactly what the tides are!!!???

The far side bulge could be just inertia...it needs a good mathematical model to show what's going on, as it is such a huge system.

Arrrgh no! Del wants to use maths....must have a lie down.

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#6

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 3:02 AM

Liquid is more elastic than solids or if you like solids are more inelastic than liquids.

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#7

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 3:12 AM

Hi bmorrow492.

The picture you have drawn actually shows more or less the tidal bulges on the crust (non-fluid) part of Earth. It is caused by the difference of the moon's gravitational field vectors on the near side, center, far side and perpendicular sides of Earth. This creates so-called tidal gravity forces and Earth is slightly stretched (half a meter or so on each side) by the moon and squeezed by the same amount in the perpendicular direction.[Note 1]

The oceans are so shallow relative to the radius of Earth that no stretching or squeezing are noticeable in them - the waters just lift and sag with the crust; so where do the tides come from? The water is dragged to and fro along the surface by so-called tractive forces, causing oscillating tidal currents, even to the bottom of the deepest oceans.

These tidal currents cause heaping effects as they meet or when they hit shallow waters. Interestingly, the peak current are in-line with the moon-vector (on both sides of Earth), but the peak heaping effects have a 45Â° lead, if viewed with the rotation direction of Earth. See figure.

Hope it clears some of the 'coastal fog' (but probably will cause some other visibility problems).

Jorrie

[1] I've posted a fuller description on this CR4 Blog entry, some time ago.

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#9
In reply to #7

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 3:30 AM

Whew thanks for that Jorrie..I was getting worried

...I sort of though I might have had an inkling that something like that might be going on...

Maybe I get a B- for using the word inertia and spelling my name right?

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#10

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 3:58 AM

Hallo to everybody...

Just ignore the "solid" Earth for a while and let's consider the "water shere" itself...

The gravitatianal force (by the moon) is not equal to the two sides of "water shere"... At the front side (towards the moon) is stronger and at the back side is weaker due to the different distances from the moon... Of course this difference is small but enough to cause a deformation on the shape of the water mass... This phenomeno, of course, doesn't cause such a deformation at the Earth itself as it is a "solid mass" and not a "liquid mass" (as the "water shere" is)...

Though, near the surphase of a black hole such tidal forces are so strong (due to the extremely large gravity) that can dissolve even a solid body... I.e. the tidal forces applied on an astronaut that falls towards a black hole (and near its surphase) can disrupt his bodie (because the gravitational force applied on his head is much smaller than that applied on his feet)...

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#13
In reply to #10

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 8:46 AM

Hi G.K, you wrote: "... Of course this difference is small but enough to cause a deformation on the shape of the water mass... This phenomeno, of course, doesn't cause such a deformation at the Earth itself as it is a "solid mass" and not a "liquid mass" (as the "water shere" is)..."

It may be just your word-choice that was not correct, but it tends to convey the opposite meaning to the real state of affairs. It is the solid crust that is deformed and the water is simply dragged along the surface to form tidal currents.

If this sounds wrong to you, think it through: the 'solid' Earth is 12+ thousand km in diameter and is deformed (stretched and squeezed) by about one meter by the Moon+Sun tidal gravity vector. The waters are at most 4 km deep on each side and the amount of compression and decompression is utterly negligible: ~(4/6000)3 meter!

It's only horizontal components of the tidal force and resultant horizontal flow that cause the coastal tides.

Jorrie

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#11

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 8:19 AM

Here's a corollary to your question.

Why is there no simultaneous tidal flow in our Earth's atmosphere?

James Maxwell Clerk didn't like, "gravity".

Bob

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#12

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 8:33 AM

Hi, Jorrie...

I saw your answer after I had posted mine...

You said: "This creates so-called tidal gravity forces and Earth is slightly stretched (half a meter or so on each side) by the moon and squeezed by the same amount in the perpendicular direction." I have an objection here: How is this possible since the crust is compact solid and not liquid or even elastic???... This "half a meter" expansion of the crust should "travel" around (as the moon goes around the Earth)... Is the crust (under our feet) going "up and down"???... I don't get it...

I, also, read the complete explanation that you give for the phenomeno... It isn't clear how the "water mass" changes its direction of movement (red and blue arrows)...

This explanation (that you gave) is essentially different than the (simpler) one that I gave... I'm very confused... Is your explanation generally accepted from all the scientists (I mean is this the official explanation)???... Enlighten me please...

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#14
In reply to #12

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 9:12 AM

Hi George.

And I saw your last reply only after I posted my 'rebuttal' of you original one.

Yes, the half a meter (or so) of crust movement travels around the Earth with the Moon. Remember, the Earth has the elliptical form it has because of gravity and the centrifugal force at the equator that counters gravity a tiny bit.

Tidal forces also counter gravity a tiny bit (less than 1 micro-g from side to side) on the 'on' and 'away' sides relative to the Moon and enhances gravity the same tiny bit on the perpendicular sides. This is enough to cause the small rise and fall of the crust.

At the risk of frustrating you, I recommend that you download and save the pdf chapter on tidal gravity from my website. It has a much more complete explanation and is more 'engineer-friendly' than 'official' documents by e.g. NOAA.

Jorrie

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#16
In reply to #14

### Re: Why are tides high OPPOSITE the moon?

07/13/2007 12:29 PM

Jorrie-

I guess I am just too dense, but I still do not see the answer to my original question. Following are graphics from your book; http://www.einsteins-theory-of-relativity-4engineers.com/support-files/tidal-gravityy.pdf:

In Figure 9.1 you show a bulge on the side of the Earth toward the Moon. That easy to understand. It is the bulge on the side of the Earth away from the Moon that I still do not understand. In Figure 9.2 you show a stretching force to the right and to the left. What is the "tractive" force that is pulling to the left? Hendrik said it was centrifugal force. That makes some sense, but I would not have expected Earth's orbit around the Moon to be significant enough to produce this effect.

Bill

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#17
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### Re: Why are tides high OPPOSITE the moon?

07/13/2007 2:19 PM

Hi Bill.

Before I give you the easy answer, just a note: Hendrik's right. The whole thing can be described by centrifugal forces around the barycenter, but it is quite tricky, to say the least. And centrifugal forces do not describe the 'tractive force' all that well. So, let's try the easy answer...

The Moon and Earth are falling towards each other - that's what orbits do - they just have enough sideways movement to continually miss each other. Thank God for that!

The rate of 'fall' is governed by the center of mass of each body. The side closest to the other body 'feels' more gravity than the center and wants to fall faster than the center. The side farthest from the other body 'feels' less gravity and wants to fall slower than the center. Hence, there is a stretching force on both bodies.

The 'squeeze' forces shown are simply a matter of the gravitation vector's direction. The 'tractive' forces are a combination of the two: you have a stretch and a squeeze and since the oceans can move 'sideways' on the surface, they respond to this resultant force (the 'solid' Earth can't really respond fast enough to be significant, I suppose).

The 'tractive forces' are the key to the oceanic tides. They cause enormous currents, slow moving, but huge in the volume of water to flow in circular patterns in Earth's oceans.

If this does not ease you mind, shout and I'll describe it from another angle.

Jorrie

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#18

### Re: Why are tides high OPPOSITE the moon?

07/15/2007 3:20 AM

Hmm.... This whole question relies on the fact that although the moon affects the oceans, it is affecting the earth as a whole, and the motion of the rotating earth, moon and orbits of them all contribute to this effect. So when the earth is whipping around, and is being tugged by gravitational forces of the moon, and the moon's direct gravity is stronger on that side of the planet, pulls at the earth and waters, and the only thing really holding the oceans on the other side is the gravity of the earth itself. It will "give" a little bit because of the planetary shift, etc. That's my take on it, but you will find someone to that answer question here.

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#19

### Re: Why are tides high OPPOSITE the moon?

07/17/2007 4:16 AM

Hi Jorrie...

Your last answer is essentially the same as my first answer...

After my simple answer (that I gave in the begining) you show me that I was wrong... I still try to overcome my wrong perception and to become familiar with the continuously "up and down" movement of the solid crust due to tidal forces (it's not easy...although this continuously "movement=stretching" is only half a meter... due to the fact that the crust is solid it's not easy to understand how it can go "up and down" again and again... but I can accept it...) and to understand the water movement as you have described with your answer...

And now we go back again as your last answer is essentially the same as my first answer... Don't confuse me more, please...

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