# effect of lunar/solar gravity on shape of earth

by daniel6874
Tags: earth, effect, gravity, lunar or solar, shape
P: 64
 Quote by olivermsun Perhaps, but from what I could see from the OP, there was nothing wrong with Gamow's explanation.
The problem would be that if what Russ said about centrifugal force is true, the explanation cannot be correct. I am suspending judgment until I understand the argument better.
 Sci Advisor P: 2,426 The computation I showed gives you a differential in forces acting on the front and far sides of the planet or other object. All it says is that the object will be stretched out. Id est, there will be some sort of bulging. Whether that bulging is symmetric or not is a matter of higher order effects which we have not yet discussed. If you want to use Mathematica to graph what the Earth's surface would look like if it was a perfect fluid, taking into account Earth's gravity, Earth's rotation, and Moon's gravity, I can help you out with this. But maybe you should send me a PM, so that we don't clutter this topic. Edit: Unless there are other people who want to see it. I suppose, I can just set it up in Mathematica and upload the notebook.
P: 421
 Quote by daniel6874 The problem would be that if what Russ said about centrifugal force is true, the explanation cannot be correct. I am suspending judgment until I understand the argument better.
I'm not sure the posts in the thread have much if anything to do with Gamow's explanation (which appears to be equivalent to Feynman's description, quoted in the other, similar discussion), so you won't understand the argument any better by reading what Russ said.
P: 3,181
 Quote by xts "Tensorial"? "Simanek's gradient"? "Schwarzschild's metric"? Guys, stick to common sense! Tide is a phenomenon that on a full/new moon you may go to the beach in the afternoon to pick crabs, moules, and other tasty living beings, while on the quartermoon you could drown in the place you used to pick crabs a week before, rather than being able to pick something for a dinner. No tensorial analysis is able to explain why on Bretonian beach tides are 2 metres high, while on Lofoten they may be 8 meters high. And even Simanek won't help you hunting crabs.
Or in the words of Bill O'Reilly: You can't explain that!
P: 64
 Quote by olivermsun I'm not sure the posts in the thread have much if anything to do with Gamow's explanation (which appears to be equivalent to Feynman's description, quoted in the other, similar discussion), so you won't understand the argument any better by reading what Russ said.
I think that Russ and K^2 did address the OP. A quick summary of the positions so far might include:

1. Gamow as per the OP.

2. Feynman, and [extrinsic to this post] [ crackpot link deleted ] daniel6874, the link you cited is not a good link for this site.. And that F. himself saw problems with his idea.

3. A quaint older text suggesting that the pull of the moon is stronger on the earth than the water, so the earth shifts like an egg-yolk toward the moon, leaving a bulge on the far side.

4. A reasonable note by xts that water for high tides is shifted from low-tide areas, and glosses from others [outside this post] on the effect of resonant land formations and separation of oceans to explain tidal height discrepancies.

5. An series of notes by K^2 that suggest the explanation of forces depends on choice of coordinate system.

6. Cited by Russ, http://www.lhup.edu/~dsimanek/scenario/tides.htm, an author who calculates the "tide-generating force" as the gradient of the moon's gravitational potential [I mistakenly said "force"], with a picture that seems to explain tidal bulges in terms of gravitation alone. The author then proceeds to outline what appears to be K^2's approach--the use of a rotating coordinate system with barycenter at the origin, noting that the mathematical origin of the "fictitious" forces is often omitted in this approach.

7. A gloss by harrylin on essentially the egg-yolk theory of 3 above, but updated I think to account for discussion in the post.

8. A.T. posts http://www.vialattea.net/maree/eng/index.htm, which seems to agree with K^2 but like Russ emphasizes the non-use of fictitious forces.

I am tempted to conclude that Russ, K^2, Gamow, and Feynman are all correct, taking into account simplifications and assumptions. After reading the author's home-page in 2 above I no longer think it bears pursuing. Thanks to all who helped with this apparently not-so-easy question.
Mentor
P: 13,651
I'll start off by noting that I am not a fan of the centrifugal force explanation of the tides.

That said, one cannot say that a centrifugal force explanation is incorrect. K^2 was spot-on correct way back on page 2:
 Quote by K^2 It. Is. Arbitrary. You can chose ANY coordinate system. In any coordinate system, the results will be the same. In some systems, part of the tidal bulging is due to centrifugal force. In other systems, there is no centrifugal force. You can choose ANY system you like and do the computations. You'll get the SAME answer.
General relativity says that all frames of reference are equally valid. Do the math right and you will come up with the same correct answer regardless of perspective. How can you say that one explanation is right and another is wrong when both explanations yield the same end results? You can't. What you can say is that one explanation is clean and simple in comparison to another that is twisted and ugly. The extra terms that arise in a rotating frame explanation vanish from the perspective of a non-rotating frame. While those extra terms are not essential to explaining the tides, they are not wrong.

The explanations that result from a non-rotating perspective and a barycentric frame rotating with the Moon's orbital rate are valid with respect to the tidal forcing functions. Both are correct with regard to the shape of the Earth itself in response to these forcing functions. The Earth itself undergoes tidal deformations. See http://en.wikipedia.org/wiki/Earth_tide, for example.

Neither explanation is correct with regard to oceanic tides. There is no tidal bulge. There are two huge land masses, the Americas and Eurasia+Africa, that prevent this from arising. To get a correct picture of the oceans' responses to the tidal forces you need to go to yet another perspective, one based on the rotating Earth. There you'll get a picture of amphidromic points, points with virtually no tides, about which the tides appear to rotate.

Picture: http://svs.gsfc.nasa.gov/stories/top...erns_hires.tif
Movie: http://svs.gsfc.nasa.gov/vis/a000000...01332/full.mov

Since all frames are equally valid, I suppose one could come up with an explanation of the ocean tides from the perspective of a non-rotating frame or a frame that rotates with the orbit of the Earth and Moon about one another. Nobody does that because such an explanation would be hideously complex.

An example of a hideously bad explanation based on centrifugal force, courtesy of our government: http://co-ops.nos.noaa.gov/restles3.html. This is so convoluted that I can't see if they've done the math right. There's just too much hand waving and there is no math to check. If they did do the math right they would of course have come up with the same answer as the much, much simpler gravity gradient based explanation.
 Mentor P: 21,669 No matter how many times I say I understand that the math works out, people keep harping on it, so I must not be making myself clear. Let me try a completely different approach: I dropped my pen and it fell to the ground. The centrifugal force due to the earth's rotation caused the pen to fall to the ground. True or false? And why or why not?
P: 421
 Quote by D H Addendum An example of a hideously bad explanation based on centrifugal force, courtesy of our government: http://co-ops.nos.noaa.gov/restles3.html. This is so convoluted that I can't see if they've done the math right. There's just too much hand waving and there is no math to check. If they did do the math right they would of course have come up with the same answer as the much, much simpler gravity gradient based explanation.
What's wrong with it? It's pretty much what Gamow and Feynman were talking about.

If you think about it carefully, rotating coordinates aren't so counter-intuitive for explaining tide-producing forces. The tidal "bulges" (in all the simplified examples here) are stationary relative to the earth-moon line, not to relative to fixed points on a non rotating earth in an inertial frame.
P: 421
 Quote by russ_watters No matter how many times I say I understand that the math works out, people keep harping on it, so I must not be making myself clear. Let me try a completely different approach: I dropped my pen and it fell to the ground. The centrifugal force due to the earth's rotation caused the pen to fall to the ground. True or false? And why or why not?
Well, causes are a philosophical thing.

But physics-wise, I'd like to hear why anybody would explain a pen falling toward the ground as being due to the centrifugal force due to the earth's rotation.
Mentor
P: 21,669
 Quote by olivermsun Well, causes are a philosophical thing.
I disagree, but perhaps that is the whole problem here...
 But physics-wise, I'd like to hear why anybody would explain a pen falling toward the ground as being due to the centrifugal force due to the earth's rotation.
Since as people harped on me previously, we can construct rotating or non-rotating frames from which to analyze the question and all will give the same result, right? So then any effect captured by the calculations can said to be the cause.
P: 421
 Quote by russ_watters I disagree, but perhaps that is the whole problem here...
Well, not to point at you specifically, but there does seem to be much ado about nothing in some recent threads. There are a few obviously wrong explanations being tossed about, and then there are the persistent disagreements which are based on people's opinions about what is the "right" frame of reference for physics.

 Since as people harped on me previously, we can construct rotating or non-rotating frames from which to analyze the question and all will give the same result, right? So then any effect captured by the calculations can said to be the cause.
As I said, if you think it makes any sense to explain a falling pen that way, then I'd like to hear that construction.
 Mentor P: 21,669 You're missing my point: I don't think it works that way, but that is what I think I am seeing from others. What I'm seeing from others in this thread is this: 1. Since we can construct a mathematical model of the tides that incorporates a centrifugal force component, we can say that the centrifugal force causes the tides. So why can't we then say this: 2. Since we can construct a mathematical model of the motion of a falling pen that incorporates a centrifugal force component, we can say that the centrifugal force causes the tides. Why is one correct and the other not?
P: 3,179
 Quote by D H [..] Addendum An example of a hideously bad explanation based on centrifugal force, courtesy of our government: http://co-ops.nos.noaa.gov/restles3.html. This is so convoluted that I can't see if they've done the math right. There's just too much hand waving and there is no math to check. If they did do the math right they would of course have come up with the same answer as the much, much simpler gravity gradient based explanation.
At first sight they seem to give exactly the same explanation:

'the effect of the external gravitational force produced by another astronomical body may be different at different positions on the earth because the magnitude of the gravitational force exerted varies with the distance of the attracting body.' (but why "may be"?!)

Also (and happily!) they do point out that 'It is important to note that the centrifugal force produced by the daily rotation of the earth on it axis must be completely disregarded in tidal theory.'

PS: note that I fully agree with your criticism on the way they phrase things.
Mentor
P: 13,651
 Quote by russ_watters I dropped my pen and it fell to the ground. The centrifugal force due to the earth's rotation caused the pen to fall to the ground. True or false? And why or why not?
False, of course. Centrifugal force is directed outwards. It explains why your pen takes longer to fall to the ground than it would on a non-rotating planet with the exact same acceleration due to gravity.

Let g' be the local gravitational acceleration vector due to the Earth's mass only. From the perspective of a frame rotating with the Earth, your pen accelerates Earthward at g' due to the Earth's mass and accelerates outward at r×Ω2 due to the Earth's rotation. Here r is the vector from the Earth's rotation axis to the pen and Ω is the Earth's rotation vector, 1 revolution per sidereal day. The pen's net acceleration is g=g'-r×Ω2. Assuming that gravity and centrifugal acceleration change very little over the short fall, the time the pen takes to fall is given by $\sqrt{2h/g}$. This calculation becomes even easier if you just use g in the first place rather than g'. For example, the standard value for Earth gravity, g0=9.80665 m/s2 includes both the gravitational and centrifugal forces at sea level and at a latitude of 45.5 degrees.

From the perspective of an inertial observer, the only force acting on the pen is gravity. The pen has an initial horizontal velocity due to the Earth's rotation that puts it on a curved trajectory. That initial horizontal velocity coupled with the Earth's curvature means the pen will fall for a distance slightly greater than the initial height h above the surface of the Earth. Account for that increase in the vertical distance traveled by the pen and make a simple second order assumption (the first order correction is zero) and you will get the exact same answer as above, $\sqrt{2h/g}$. Just not as easily.

This is one of those occasions where working in a rotating frame is easier than an inertial one.
Mentor
P: 13,651
 Quote by russ_watters Since we can construct a mathematical model of the tides that incorporates a centrifugal force component, we can say that the centrifugal force causes the tides.
Better: We can say that centrifugal forces can be used to explain the tides. Saying that they cause the tides is going a bit too far IMO.

Consider a moon the same mass as ours that is in an orthogonal orbit about some planet the same size as the Earth. (Orthogonal orbit: An orbit with a zero angular momentum. When the moon hits the planet, the impact angle will be orthogonal to the ground.) At the instant that this hypothetical moon is one lunar distance from the planet, the tidal forces exerted by this moon on that planet will be exactly the same as the tidal forces exerted by our orbiting Moon on the Earth. Explaining that this is the case is a piece of cake if you use a gravity gradient based explanation. Now try from explaining using a centrifugal force based explanation.

 So why can't we then say this: 2. Since we can construct a mathematical model of the motion of a falling pen that incorporates a centrifugal force component, we can say that the centrifugal force causes the tides. Why is one correct and the other not?
You most certainly could explain the tides from the perspective of an Earth-fixed frame. Explanation follows my response to harrylin's post.

 Quote by harrylin Also (and happily!) they do point out that 'It is important to note that the centrifugal force produced by the daily rotation of the earth on it axis must be completely disregarded in tidal theory.'
That's because oceanographers are elided. (Elided because of the old mantra, If you can't say something nice don't say anything at all.)

The reason that centrifugal forces due to the Earth's rotation must be disregarded is because those forces have already been taken into account. The starting point for an Earth-fixed based explanation of the tides is the geoid. The geoid is an equipotential surface of the gravitational plus centrifugal forces. Accounting for those centrifugal force again would be a double booking.
P: 421
 Quote by D H That's because oceanographers are elided. (Elided because of the old mantra, If you can't say something nice don't say anything at all.)
Hmm...what does this have to do with anything?
Mentor
P: 13,651
 Quote by olivermsun What's wrong with it? It's pretty much what Gamow and Feynman were talking about. If you think about it carefully, rotating coordinates aren't so counter-intuitive for explaining tide-producing forces. The tidal "bulges" (in all the simplified examples here) are stationary relative to the earth-moon line, not to relative to fixed points on a non rotating earth in an inertial frame.
What's wrong with it is that there is no math. Where people do use math in a centrifugal force-based explanation they inevitably do it wrong, and then hand-wave the mistakes away. I know that a correct explanation is possible, but I have yet to see one.

Compare that to a gravity gradient based approach. The desired outcome is the gravitational acceleration due to the Moon (or Sun) relative to the Earth as a whole. The Earth as a whole is accelerating toward the Moon by

$${\boldsymbol a}_e = \frac {GM_m}{||{\boldsymbol R}||^3}\,{\boldsymbol R}$$

where R is the vector from the center of the Earth to the center of the Moon. The gravitational acceleration toward the Moon at some point r away from the center of the Earth is

$${\boldsymbol a}_p = \frac {GM_m}{||{\boldsymbol R}-{\boldsymbol r}||^3}\,({\boldsymbol R}-{\boldsymbol r})$$

The relative acceleration is the difference between these two vectors:

$${\boldsymbol a}_{\mbox{tidal}} = {\boldsymbol a}_p - {\boldsymbol a}_e$$

The vector R can always be expressed in terms of a unit vector $\hat{\boldsymbol r}$ directed along $\boldsymbol r$ and some other unit vector $\hat{\boldsymbol{\theta}}$ normal to $\hat{\boldsymbol r}$:

$${\boldsymbol R} = R(\cos\theta \,\hat{\boldsymbol r} + \sin\theta\,\hat{\boldsymbol{\theta}})$$

Using this notation and making a first order approximation for r<<R yields

$${\boldsymbol a}_{\mbox{tidal}} \approx \frac {GM_m r}{R^3} \left((3\cos^2\theta-1)\,\hat{\boldsymbol r} + 3\cos\theta\sin\theta\,\hat{\boldsymbol {\theta}}\right)$$

Note that this covers the entire globe, not just the sublunar point. Try to do the same with a centrifugal-based approach. This is one of those cases where the inertial explanation is much easier, and much more general, than a rotating frame explanation.
Mentor
P: 13,651
 Quote by olivermsun Hmm...what does this have to do with anything?
The places you are most likely to find very bad and/or mathematically incorrect explanations of the tides are textbooks and web sites written by oceanographers.

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