[chris]

This answer is wrong. The bulge on the opposite side of the earth is caused by a centripetal force created by the earth's rotation, which is in effect trying to throw the oceans off into space.

What?

A centripetal force acts towards the centre of rotation...

[rmolnav]

#59
You are quite right. Centripetal force obliges considered objects to follow a circular path, instead of a straight line.
But those objects (o parts of matter, such as water particles) somehow have to exert an opposite and equal force on the prime mover (the one that causes the centripetal force), following action and reaction principle.
I say "somehow" because there are many ways, depending on each particular case. In the case of the 2nd high tide (opposite to the Moon) is rather tricky.
I would suggest you to read my reply #55, where a very simple case is exposed.

[rmolnav]

#58
"- But what happens when this tidal bulge do when it reaches the western end? Where does all the water go?"
Tides are kind of very soft waves. High tide "excess" of water is not moving (relative to earth) horizontally, water level basically just oscillate vertically. Forgetting local effects, wether included, that is the result of all acting forces, mainly weight, lunar and solar attraction, and centrifugal forces.
When western basin ends, that physical limitation changes the scenario. Tidal effects continue on solid earth, but deformations are logically much, much smaller. And water tends to go back to a spherical shape.
Then, depending of each case, quite interesting phenomena (as what mentioned by you) can occur. But next day, when when Moon reaches the eastern edge of the basin, high tide and the bulge "chase" of the Moon occur again.

[rmolnav]

#58 (continuation)
We should also keep in mind that we say "bulge", but this term is just a massively exaggerated analogy ... Sea surface deformation is relatively very, very tiny. Even strongest tides are just in the order of app. a couple of tens of meters. Locally it could be consider something big, but that hight decreases very, very slowly from high tide place to low tide places, app. 90 degrees eastward and westward the Moon, along 10,000 km ... (at the equator) (!!!)

[rmolnav]

Yesterday I sent some comments to a Scientist I am discussing with, relative to cause of sea tides, and centrifugal forces.
As it could interest other people, I am going to put them here.
Previously I have to remember something additional, necessary to understand my arguments.
Many people don´t realize that "the Moon rotates around the Earth" is not 100% exact … Cosmic rotating objects actually rotate in pairs, around their barycenter, kind of center of gravity of whole masses of the pair.
If one of the objects is much more massive, then they are still orbiting about their barycenter, but the “center of mass” is located inside the larger object, at some distance from its C.G. That is the case of Moon "around" the Earth:
"It is quite clear, for me too, that the addition of all forces of attraction from the Moon, what produces is the required centripetal acceleration for the whole of the Earth.
But, as far as I can understand, that is not the whole picture.
I see you yourself have been very close to the "edges" of my stand when saying:
"If you're on the side of the Earth facing away from the Moon, you are being pulled slightly less than the Earth as a whole, so your centripetal acceleration due to the Moon is a bit LOWER".
Please kindly note those places further from the Moon are also further from the axis of rotation. And required centripetal acceleration, for same angular speed as the whole Earth, must be actually HIGHER, proportionally to the radius.
The bulk of our planet kind of obliges both closer and further parts to rotate at same angular speed.
Any portion of Earth, either liquid or solid, if only with the rotation movement, has to satisfy the law that the addition of all force vectors acting ON it, divided by its mass, has to be the required centripetal acceleration.
That only can be reached through additional forces, resulting from internal stresses.
Let us imagine a narrow cylinder connecting closest to and furthest from the Moon Earth surface points (I suppose isotropic material and perfect spherical Earth, for the sake of simplicity), and let us divided it into e.g. 1 m pieces.
Forces acting laterally on those pieces would be equilibrated.
And longitudinally acting forces on each of them would be own weight (attraction from the whole Earth), attraction from the Moon, and tensile or compressive stresses/forces exerted by contiguous pieces. Shear stresses would also be present, but for simplification I won´t take them into consideration.
Let us consider four contiguous pieces A, B, C and D at furthest half of the Earth, A the closest to Moon and axis of rotation.
Between contiguous pieces stresses/forces are opposite but equal (3rd Newton´s Principle).
Required centripetal acceleration of B is slighty bigger than A´s, and slightly smaller than C´s. Considering only changes in comparison with when no rotation, one can deduce that the addition of forces on B (exerted by A and C) has to be centripetal, but smaller than what exerted on C by B and D, because the further from rotation axis, the bigger Moon´s attraction "deficit".
That means that every pair of contiguos pieces are pulling each other, half of those forces inwards (centripetal), and the other half outwards, for me CENTRIFUGAL forces (because they are in the sense of fleeing from a center).
I know that word is considered "politically incorrect" among many Physicists … But, whatever we call them, they are REAL forces, which apparently have been obliterated by many.
And I can´t really understand why! Those forces are present across the universe: planet/satellites, star/planets, twin stars … For me, Inner bulges are due to the higher attraction from the other massive object, but at outer bulges those centrifugal forces are paramount ...

[Janus]

In pdf linked in reply #23, I can see that Schutz also mentions only the different gravitatory pulls from the Moon …
I repeat the question on my post #22:
"If those three zones of the Earth experience only those gravitatory accelerations from the Moon, why ALL of them don´t cause a movement of the Earth towards the Moon? “
Tomorrow I´ll try to explain my “theory” of the centrifugal force used in my first post, which I consider necessary to have a correct explanation of the issue. I´m slow when typing in English, and have no time now. But just an analogy that I consider useful:
Imaging an athlete of hammer trow speciality. He or she can´t keep verticality when throwing the hammer. It is necessary to lean a little backwards. Otherwise the hammer could not be rotated. Both the athlete and the hammer will rotate around an axis situated near the forward part of the athlete´s body.
If the hair of the athlete is long and not  fixed by some device, instead of keeping its normal downward direction due to its weight, it will move back and upwards …
That cannot be due to anything similar to what stated by Schutz relative to tides: CENTRIFUGAL force is the cause.

Both the Earth and Moon are in free fall.  So would be an Earth-Moon pair falling directly towards each other.  IOW, if you were to stop both the Earth and Moon in their orbital motion, so that they suddenly started to fall directly towards each, the tidal bulges would not change in shape at that moment.  If the Moon had been falling in towards the Earth from some far distance, at the moment it reached the present Earth-Moon distance, the tides would be the same as they would be for the Moon orbiting at that distance. The relative motion of the Earth and Moon with respect to each other doesn't matter.

[evan_au]

If the Moon had been falling in towards the Earth from some far distance, at the moment it reached the present Earth-Moon distance, the tides would be the same as they would be for the Moon orbiting at that distance.

Resonance is one factor in the height of tides - the most extreme tides in the world (eg near Anchorage in Alaska) occur because the resonant frequency of the ocean basin is close to the driving frequency of the Earth's rotation (slightly over 12 hours between high tides).

If the Earth & Moon were falling directly towards each other (without the Earth rotating), these resonances would not occur, and these instances of extreme tide heights would not occur.

Similarly, there are regions in the middle of Earth's oceans which have almost zero tidal range (amphidromic points), because the oscillations cancel out at that point.  If the Earth & Moon were falling directly towards each other, these oscillations would not occur, and I expect that the distribution of amphidromic points would be quite different.

[rmolnav]

#65 evan_au
On previous posts I´d already said that local circumstances, resonance mentioned by you included, does affect the hight of tides.
But that doesn´t mean what I said on #63 to be main causes of tides is erroneous ...
I saw both #64 and 65 just some minutes ago. Regarding what said by Janus on #64, I´ll reply at another moment. I need more time, firstly to "ruminate" about it, and after that to write the reply.

[rmolnav]

#66 (Continuation)
Just after posting #66 I´ve realized what you said about resonances has a basic error. Janus, I presume, was "stoping" the rotation of the pair Earth/Moon. But that rotation (around their barycenter, 28/29 day cycle) is not which allows the resonances mentioned by you. It is the DAYLY Earth´s rotation about its own axis !!

[evan_au]

The relative motion of the Earth and Moon with respect to each other doesn't matter.

the resonances mentioned by you. It is the DAYLY Earth´s rotation about its own axis !!

I agree with you, rmolnav. The Earth's rotation relative to the Moon is the major driver for tides (followed by the Earth's rotation relative to the Sun).
- If the Moon were "far away", it's tidal pull would be small in amplitude, but would occur every 12 hours, as the Earth rotates.
- At its present distance (300,000 km), the Moon's tidal pull is larger than that from the Sun, but it occurs about every 12 hours and 20 minutes, as the Moon has rotated in its orbit in that 12 hours, and it takes a bit longer than 12 hours for the Earth to rotate to it's original position relative to the Moon. This changes the resonances a bit.
- If we imagine the Moon only 100,000 km away, the tidal range would be huge (27x higher = inverse cube law), but the high tides will be a lot longer than 12 hours apart, and will have very different resonances.

[rmolnav]

#64 Janus and #68 evan_au
After "ruminating" Janus´s post last night, a few minutes ago I saw evan_au´s ...
On last one I´ve already seen some details I can´t agree with, but some more time is necessary to prepare and post a well thought reply.
Janus: For a couple of year I´ve been posting tens and tens of replies and/or comments, trying to convey my stand step by step, giving well recognized physical principles I support my statements on, pinning down others´concrete statements I can´t agree with, drawing parallels with several practical cases ... Both here and on  "http://www.thenakedscientists.com/forum/index.php?topic=68025.50", "What is centrifugal force?".
Just as an example, as an introduction to a physical analysis of pendulum´s movement, I posted on last one (#60):
"It seems simple, but there are small but important details that, if not being careful, one can misinterpret facts, or at least confuse others …
That´s why I am now going to consider only the scenario statically (and step by step), without any movement: fixed hanging point, string and weight (I´ll call it W), all in a straight vertical line.
- a) Primary acting force: Earth pulls downwards W.
- b) W does not move. According to 2nd Newton´s principle, the sum of all forces acting on W has to be null.
- c) The unique object that can exert another force on W is the string: it must somehow pull W, with equal but opposite force (watch out: those two opposite forces are not action/reaction forces (3rd principle); they are acting on a unique object, and 3rd principle is about two objects exerting a force on each other).
- d) If the string pulls upwards W, applying now 3rd principle to that pair of objects, we can deduce that W must be pulling downwards the string lower extreme.
- e) That force seems to be a centrifugal one (and centripetal the one mentioned on - c)), but we should keep in mind that if there is no rotatory movement at all, a proper “center” does not actually exist.
- f) And when with movement, we have also to be careful with the term “centripetal force”, because it usually refers to the the radial component of adding up all forces acting on W,  which divided by the mass would give us the “centripetal acceleration” that makes W not to follow a rectilinear trajectory. And in many cases some of those added forces may be in the sense of the “center”, but compensated by others and not producing any acceleration by themselves".
AND YOU, not quoting anything from my last post here but from another one and a half year old, without specifying any concrete steps of my arguments you don´t agree with, and without giving any physical principle to support your stand, JUST SAY:
"Both the Earth and Moon are in free fall.  So would be an Earth-Moon pair falling directly towards each other.  IOW, if you were to stop both the Earth and Moon in their orbital motion, so that they suddenly started to fall directly towards each, the tidal bulges would not change in shape at that moment.  If the Moon had been falling in towards the Earth from some far distance, at the moment it reached the present Earth-Moon distance, the tides would be the same as they would be for the Moon orbiting at that distance. The relative motion of the Earth and Moon with respect to each other doesn't matter"
PLEASE KINDLY note that if I´d just said something is white, you could say I´m wrong, that it is black ...
But It would be OF NO USE, to you, me and any other reader !!   
   

[Bill S]

Fascinating link,  Bill. Having lived by the sea - Cornwall,  then East Anglia (UK) - all my life,  I've long been aware that tides were not as simple as the two bulge model suggests, but I've never tried answering my own questions.  There's some food for thought in your link.

[rmolnav]

#68 evan–au
You say:
" If the Moon were "far away", it's tidal pull would be small in amplitude, but would occur every 12 hours, as the Earth rotates".
Sorry but I´m afraid you are mixing things ...
Firstly, there are neither two nor one Moon tidal pull ... Its pull is continuous, no matter how far away the Moon were.
Two so called bulges always happen. And they continuously change their longitudinal position with Earth 24 h. own rotation.
It is clear that the one app. below the Moon is due to higher attraction there, due to less distance to the Moon.
Many people consider the opposite bulge to be also due to similar reason: that part is further from the Moon, pull there is smaller than mean pull to the whole Earth, and water there kind of falls behind ...
I feel pretty sure that those differences in gravitational pulls are not sufficient to explain further high tides. Here and on "http://www.thenakedscientists.com/forum/index.php?topic=68025.50" I have explained many, many times my reasons.
I neither can repeat now all that staff, nor ask you to read it.
Nevertheless, I´ll put just a few condensed lines.
The whole Earth has to rotate around Moon/Earth barycenter at 2PI/app.28 d. angular speed. Moon´s pull on the whole Earth produces the centripetal force for that rotation. That means centripetal force matches distance from C.G of Moon and Earth.
But further Earth parts, on the one hand they are being pulled less by the Moon. And on the other, being required centripetal acceleration proportional to distance to the barycenter, to rotate at same angular speed centripetal force should be higher ...
That "deficit" of centripetal force can only be compensated by internal stresses/forces. The result is that "rows" of material between bulges are being stretched by internal tensile stresses, two opposite between each pair of contiguous particles (according to 3rd Newton´s Principle).
Half of them are inward and compensate centripetal force deficit, and the other half are outward (I consider them to be centrifugal, in the sense of fleeing from a center), and produce both earth deformation and further high sea tides ...
Rest of #68, as far as I can understand, has other errors, but I´ll refer to them tomorrow on another post.

[rmolnav]

#72 (Continuation)
evan_au said on #68:
"At its present distance (300,000 km), the Moon's tidal pull is larger than that from the Sun"
If with "Moon´s tidal pull" you mean its gravitational attraction, that is not right. Even being the Moon much closer, the Sun is so much massive that its pull is higher.
What is actually larger is the centripetal force "deficit" (see #71) at Earth´s parts further from the barycenter, that happens to lie within Earth, app. 2/3 of its radius away from its C.G. That is due to the fact that in Earth´s rotation around the Sun, required centripetal acceleration at furthest part (midnight) is relatively much more similar to centripetal acceleration of the whole Earth. And gravitational pulls are more similar too.
We could say that four "bulges" actually happen: two visible related to the Moon, and two smaller related to the Sun, but not visible because we see sea surface deformation due to both effects added up.
App. twice a month, when the three celestial objects form a line (full or new Moon), "non visible" Sun related bulges are at same longitudinal position as Moon related ones, and we have so called spring tides (in Spanish, literately translated, "live tides"), with tide´s range at its maximum (visible and invisible bulges add up). When instead of in line they are 90º apart (Moon at one of its quarters), Sun related "invisible" bulges are situated where Moon related low tides, and we have "neap tides" ("dead tides in Spain), with tide´s range at its minimum.

[rmolnav]

Last couple of months I´ve been discussing our "issue", and also the sheer concept of centrifugal force, on "live science". On some of their last web pages they had shown the idea I´m refuting that centrifugal f. is a kind of taboo, something not real …
Some minutes ago I sent them:
It´s funny. Just a few minutes ago I saw that Lauren Cox, more than six years ago, on a page ALSO FROM LIVESCIENCE,
http://www.livescience.com/29621-what-causes-the-tides.html
showed a stand very, very similar to mine (linking cause of tides to REAL centrifugal forces):
"The moon's gravitational pull on the earth is strong enough to tug the oceans into bulge. If no other forces were at play, shores would experience one high tide a day as the earth rotated on its axis and coasts ran into the oceans' bulge facing the moon.
… As the moon circles the earth, the earth moves in a very slight circle too, and this movement is enough to cause a centrifugal force on the oceans.
… This inertia, or centrifugal force, causes the oceans to bulge on the opposite side facing the moon ..."
But I wouldn´t say "This inertia, or centrifugal force ...". It is a real, "inertial" force, in the sense that it is due to 3rd Newton´s Principle, based (as the other two) on inertia physical REALITY."
http://www.livescience.com/52488-centrifugal-centripetal-forces.html

[rmolnav]

#70 Bill S
Perhaps I should have commented your post before ...
In my mother tongue (Spanish), "fascinante" sounds perhaps too much ...
Anyhow, thank you for what could affect my own interventions. If any doubt or disagreement, please send them. I´ll do my best to try and clear our ideas.
Recently I´ve sent a couple of comments to another related topic. I think it could be of your interest:

"Will we eventually lose the moon?"
https://www.thenakedscientists.com/forum/index.php?topic=36739.msg461127#msg461127

[rmolnav]

Looking for something else in Youtube, a couple of days ago I found a VERY interesting video, which thoroughly analyzes both tides basic causes (globally considered: Moon, Sun ... the two "standard" bulges), and local tides. Don´t miss it!
Said that, its authors are among the certainly big group of scientists that don´t want even mention centrifugal forces ... Short time ago I sent a comment explaining why I consider they are wrong, as far as that concrete issue is concerned:

[rmolnav]

Said that, its authors are among the certainly big group of scientists that don´t want even mention centrifugal forces ... Short time ago I sent a comment explaining why I consider they are wrong, as far as that concrete issue is concerned

In my last post I intended to include just the link to the youtube page, but the video "play" engine (?) turns up …
But what don´t appear are sent comments …
What follows is what I sent, not literally (I had a lapsus and had to correct some details)
Both Moon and Sun, causes of tides, are similarly considered in the video, without even mentioning centrifugal forces.
As we know, Moon is main cause (particularly in timing), and Sun tides affect total tides only in their cycling intensity.
But Moon/Earth dynamics is rather trickier than Sun/Earth´s. as far as distribution of forces is concerned. That is why I´ll refer to last one before.
Diagram timed in the video 2:23 is about Moon related bulges. Let us imagine a similar diagram, but with the Sun instead of the Moon. Shown "medium"  gravitational pull (average as if acting on Earth´s C.G.) is rested both at nearer and farther sides, trying to explain the bulges ...
The authors don´t consider those vectors any centrifugal force, they´re just comparing to find kind of "relative bulges" caused by "relative forces". In other words, the usual concept of inertial frame of reference, which I consider applies to distances, velocities and accelerations, but that applying it to forces is an erroneous "trick". But I will not discuss that now any further. 
What´s causing "medium" black vector at C.G.? : the centripetal acceleration required for the rotation. But that REQUIRED (to orbit at each distance) acceleration is NOT uniform across the Earth, horizontally in our diagram. Farther parts of the Earth, to rotate at same angular speed (an average of 2π radians a year), NEED higher centripetal acceleration than closer ones: just the OPPOSITE to what happens with gravitational forces.
ONLY internal stresses can compensate that imbalance ... Thanks to them, the excess of pull on closer parts is transmitted, from right to left in the diagram, to farther parts ...
But according to Newton´s 3rd Motion Law, if at any considered Earth´s section, right part is pulling (towards the Sun) contiguous left part, this one is also pulling the former towards the left, same force but opposite direction: OUTWARDS.
All those internal forces are centrifugal REAL forces, which, together with their "mirror" centripetal ones (the fraction not necessary for the rotation), stretch the Earth, solid parts included.
Logically, that deformation is much bigger where liquid parts, our oceans. And the TWO Sun related bulges ARE DUE TO THAT.
In another post, better tomorrow, I´ll put what Moon related …This way could be better for some of possibly interested folks: t could facilitate a posterior understanding of the trickier case of Moon related bulges.

[rmolnav]

ONLY internal stresses can compensate that imbalance ... Thanks to them, the excess of pull on closer parts is transmitted, from right to left in the diagram, to farther parts ...

AS A CONTINUATION of my post of yesterday, yet another analogy, to help understanding.
Yesterday I happened to have to buy a new tyre for my byke. When I had it hanging from my right hand, I felt how heavy it was … and saw some deformation: its circular shape got slightly oval.
What physically was happening is clear: gravity was acting across the tyre, but my still hand was preventing it from getting a g downwards acceleration. My hand was pulling upwards, and total force acting on the tyre was null (forgetting consequences of Earth´s movements …)
But my hand upward force was neither acting on the tyre´s C.G., nor uniformly distributed across the tyre (as gravity was). Similarly to what I said yesterday, through internal stresses my hand´s pull was being transmitted and distributed along the tyre, acting both upwards and downwards (Newton´s 3rd Law), each part of the tyre was still, but the tyre was being stretched vertically.
Then I tried and spinned myself with my arm almost horizontal, the tyre still gripped by my hand, as if I were going to throw it like a disc by an athlete.
I was feeling I had to pull the tyre inwards, the higher the angular speed, the higher the necessary pull. My hand was making rotate actually only what it was touching, but required force was much bigger than if my hand were gripping only some ten cm piece of tyre ...  Rest of the tyre rotated because, almost identical reason as what quoted, "the excess of pull on closer parts was being transmitted, outwards in this case, to farther parts …"
In Earth rotation around the Sun I was talking about yesterday, centripetal force is not initially exerted through physical contact at a spot as in the tyre case…
Gravitational pull is distributed over Earth sphere. Not having a pull "concentration" at any spot, "excesses of pull" are much smaller, but, as explained yesterday, they also happen. And also centrifugal REAL forces appear, due to mentioned Newton´s 3rd Law.
Because of that, deformations due to all those forces (particularly tidal bulges) have to be relatively much smaller than in the tyre case. And that is what really happens. Sun related bulges (by the way, very much exaggerated term …) are only a few meters from high and low tides … 10,000 km apart !! (at the equator).
 

[rmolnav]

In another post, better tomorrow, I´ll put what Moon related …This way could be better for some of possibly interested folks: t could facilitate a posterior understanding of the trickier case of Moon related bulges.

Here it is ... though a day later due to additional analogy of yesterday (hope useful).
Mentioned video diagram timed 2:23 is misleading: it seems like a static situation … Authors not only ignore any possible centrifugal force: they don´t even mention any centripetal force. If only what shown were the physical reality there, why Earth doesn´t moves toward the Moon?. Because of same reason as Moon doesn´t move towards the Earth: both are actually rotating around their barycenter, and gravitational pulls are causing their centripetal accelerations. 
As that barycenter is situated app. 2/3 Earth´s radius from its center (logically, at Moon side), Earth´s rotation in this "couple dance" is a kind of wobbling, similar to when a child plays with a hula-hoop.
Though Earth daily spinning generates much, much stronger internal stresses and centrifugal forces (some 28/29 times higher angular speed …), it acts always same way: trying and increasing Earth´s diameter at the equator and lower latitude parallels.
That Earth´s wobbling generates internal stresses and deformations which we can perceive added to those mentioned stronger, permanent ones.
Let us imagine video diagram timed 2:23 with a vertical line some 2/3 Earth´s radius at right side of its center: the axis of this rotation.
Earth parts at the left of that axis require a total force per unit of mass proportional to its distance to that axis, in order to get required centripetal acceleration at ach considered spot. But gravitational pull from Moon is the farther the smaller (to the square of the distance to the Moon). That imbalance can be compensated only by internal stresses: parts closer to the axis have to add an inward pull on contiguous farther parts … Then we have Newton´s 3rd Law, and each part pulls inner contiguous one with same but opposite (and REAL) force, that is, outwards, "fllying from a center" (axis of rotation), CENTRIFUGAL …
At Earth parts at right side of that axis, required centripetal forces are smaller, also the farther from the axis, the bigger. BUT now gravitational pulls from the Moon are opposite to required centripetal forces. Only own gravitational Earth´s pull can supply required centripetal forces …That´s similar to what happens due to daily Earth´s rotation: it tends to increase sea level at that spherical segment. And that effect adds to what derived from the obvious fact that there Moon´s gravitational pull is bigger, and a bulge also occurs at that side, due to both reasons ...
And due to Earth´s daily spinnig, those two opposite bulges seem to rotate around Earth, trying to keep in line with Moon. Result: the main part (Sun caused tides have to be added) of our "popular" (but not so well known) tides …