[Halc]

Your basic error comes from same misconception as if somebody said moon (or sun) rotates around earth once every 24 hours ... That is only an apparent movement !

It is apparent movement that I'm talking about.  The tides go up and down twice a day, and that requires movement relative to Earth's surface of thousands of cubic km of water each 6 hours.  In most places, this is not a problem, since the necessary movement is only about 1km or less, and 6 hours is plenty of time for gravity to shove water that far.  In fact, the variance in the Atlantic comes more from resonance than it does from the gravity boost, so a longer day might actually make the tides lower.

So my example of a place that would get much higher tides were the day to be longer is the Mediteranean.  No resonance there, and all the water needed to raise the tides need to fit through Gibraltar, and the gap simply isn't large enough to allow the tide to come in before it starts running out a few hours later.

"Sublunar" bulge is always almost in line with the moon, and its actual cycle is, as moon´s, 28/29 days ... A little more than 7 (rather than 6) days  ahead of next low tide !!

I'm well aware of that, but no water is stationary relative to this bulge.  If it was, it would kill us: New York City and Shanghai wiped out twice daily by a wave of water moving west at over 1000 km/hr.

[Le Repteux]

This explanation predicts that tides would be negative if the Earth spun the other way since the inner part has the greatest tangential speed and wants to move outward, and the outer part has the least tangential speed and wants to move inward.  Venus is such a case (it rotates backwards), at it has normal solar tides towards and away from the sun just like Earth.

This argument looks solid, but I didn't find any data on the Venus tides to check it out. Do you have a link to them? A similar argument can also be raised about how tidal locking could happen to the moon's surface since it is still rotating a bit, thus still going at a different orbital speed around the earth than its own CG. The answer may reside in the way the tidal bulges affect the proper speed. In my explanation, it is the orbital speed that produces the tides, and the bulges are not only moving radially, but they also move tangentially a bit in a direction that opposes the direction of rotation. In other words, it is the bulges that produce the deceleration of the proper rotation, and it happens where the orbital speed counts, not in between, so in reality, the rotation might stop decelerating just because the bulges are already going at the right speed on their orbital trajectory while the whole planet is still rotating faster than that a bit, and we can apply the same reasoning to Venus since its proper rotation is also very slow.

In fact though, the bulges suffer a deceleration followed by an acceleration, they raise and fall in the same day, so with my explanation, they should appear and disappear without decelerating the proper rotation of the planet when it's slow, while still decelerating it when it's fast, which looks difficult to explain. After all, maybe the bulges don't need orbital rotation to raise, but still need it to decelerate the proper rotation. I don't like the friction explanation, I can't understand it, so I'm looking for another one, and the orbital speed one seems to apply so well to orbital receding.

 

[Le Repteux]

That rotation could be considered as the addition of a revolving without rotation (as earth moves), and a spinning around its own axis over same some 28 days of the orbiting.
But the effect of that "spinning" shouldn´t be considered like an "equatorial bulge" as what happens on earth ...

All the rotations depend on inertial motion, whether they are held by gravitation or by a rope, and inertial motion isn't a radial motion, it is tangential, so the outward motion when the force stops pulling is only incidental. It certainly takes a force to keep the inertial motion circular, but it is the same force whether it is a rope or gravitation. It can't be both though, either it is a rope or it is gravitation. When the earth spins, it is inertia that produces its outward motion (not force), but it is gravitation that produces the inward force (not motion), so the reason why the equatorial bulge doesn't spread away from the earth is  gravitation, and the reason why a body on orbital motion doesn't fly away too. It is the same kind of inertial motion, and the same kind of force, so to me, the two kinds of rotation are equivalent.

[rmolnav]

So my example of a place that would get much higher tides were the day to be longer is the Mediteranean.  No resonance there, and all the water needed to raise the tides need to fit through Gibraltar, and the gap simply isn't large enough to allow the tide to come in before it starts running out a few hours later.

On the one hand, at some areas of the Mediterranean resonance actually causes some tides.
On the other, the reason why global moon and sun related tidal effects are only very slightly perceived there is not just a "lack of time" for that Gibraltar "bottle neck" to allow sufficient water in. Open ocean bulges build with not only water from other meridian areas (not too far from the equator), but also from other N and S higher latitude areas ... Otherwise tides almost wouldn´t happen at such high latitudes, where they can actually be very strong.
 

Quote
"Sublunar" bulge is always almost in line with the moon, and its actual cycle is, as moon´s, 28/29 days ... A little more than 7 (rather than 6) days  ahead of next low tide !!
I'm well aware of that, but no water is stationary relative to this bulge.  If it was, it would kill us: New York City and Shanghai wiped out twice daily by a wave of water moving west at over 1000 km/hr.

I can´t understand what you mean! Precisely because of daily earth spinning, that moon-related bulge (per se moving much slowly, 40,000 km/app. 28 days) relatively to earth solid parts it does move at 40,000 km/app. 24h = 1,667 km/h ... But no serious problems because those tidal waves are relatively very, very small (not more than a few meters, if any, of difference between high and low tide, some 10,000 km apart (at the equator) !!)

[Colin2B]

My bold.  I disagree.  This explanation predicts that tides would be negative if the Earth spun the other way since the inner part has the greatest tangential speed and wants to move outward, and the outer part has the least tangential speed and wants to move inward.  Venus is such a case (it rotates backwards), at it has normal solar tides towards and away from the sun just like Earth.

My fault. I was in a hurry and didn’t give a full explanation, thinking @Le Repteux  would work out what I was saying.
Let’s start with the moon which was the example I suggested. It orbits around the barycentre with it’s same face to the earth, so it is not spinning relative to the earth/moon line. The distance from the barycentre to the moon’s centre of mass can be determined by the balance between the centripetal force (due to earth’s gravitational pull) and the centrifugal force (due to speed of orbit of the centre of mass). If we consider points on the earth/moon line on the near and far sides of the moon, it is reasonable to suggest that although these points have the same angular velocity as the CG they are moving at a faster or slower orbital speed than the CG, hence they could be considered to be seeking a different orbit. The nearside moving towards earth, the farside away.
The next question is an interesting one. You introduce the spin of a planet eg Venus that changes the planet’s surface speed. However, I would argue, as I suggested to @Le Repteux, that the spin has no effect on the orvbit. If you consider the locus of points on the surface the spin does not cause any net increase or decrease in the orbital speed and it is orbital speed which is the prime cause of any centrifugal forces. At any instant in time the forces on any mass on the earth/moon line are the gravitational pull and the centrifugal force and we can treat the spin separately because, it has no effect on the net orbital speed of any part of the mass. Also, as I mentioned to @Le Repteux  we can treat the spin as a separate component of the overall system and it's only effect is to create centrifugal force causing an equal bulge on the equator of the spin, it does not cause a net motion towards or away from the earth.

[Halc]

The next question is an interesting one. You introduce the spin of a planet eg Venus that changes the planet’s surface speed. However, I would argue, as I suggested to Le Repteux, that the spin has no effect on the orbit.

I didn’t suggest that the spin has any (short term) effect on the orbit.  Yes, Le Repteux has a thread open about tidal forces slowly pushing orbiting things away, and that is true.  The spin of the sun has a higher angular velocity than any of the planets, so that tide slowly pushes each of them away.  Most planets also spin faster than their orbits, putting thrust on the sun that also contributes to higher orbits.  Venus is an exception there, and its negative spin actually degrades its orbit, but not as much as the tide on the sun expands that orbit.  All this is relevant to the other thread, but your response above seems to concern this point.

The discussion was about tides, with a suggestion that the tides might be partially caused by higher linear speeds at points furthest out, but I pointed out that points on Venus furthest from the sun have the lowest linear speed and should be ‘seeking lower orbit’, and the points closest to the sun have the greatest linear speed and thus should be ‘seeking higher orbit’.  If that were so, the tides on Venus would be to the sides, not towards and away from the sun as all solar tides are.

If you consider the locus of points on the surface the spin does not cause any net increase or decrease in the orbital speed and it is orbital speed which is the prime cause of any centrifugal forces. At any instant in time the forces on any mass on the earth/moon line are the gravitational pull and the centrifugal force and we can treat the spin separately because, it has no effect on the net orbital speed of any part of the mass. Also, as I mentioned to Le Repteux we can treat the spin as a separate component of the overall system and it's only effect is to create centrifugal force causing an equal bulge on the equator of the spin, it does not cause a net motion towards or away from the earth.

All this seems to be about orbital speed, something on which I was not commenting in this topic, until what I wrote above in this post now.

The comment of mine that you quoted in the immediately preceding post concerned centrifugal explanations for the tides.  Instead, I agree with your last line there that we can treat the spin as a separate component of the overall system and yes, all it does is that standing bulge everywhere at the equator.
Tides are partially an effect of spin.  The spin rate coupled with the resonant frequency of oceans and shorelines causes higher tides in some places than others.  Move the continents or alter the spin and the tides will be higher in different places.  But high tide is always towards and away from the gravity gradient and the bulge will form regardless of the axis or intensity of spin.

[rmolnav]

The spin of the sun has a higher angular velocity than any of the planets, so that tide slowly pushes each of them away.  Most planets also spin faster than their orbits, putting thrust on the sun that also contributes to higher orbits

Sorry, but I can´t understand what you mean ...

The discussion was about tides, with a suggestion that the tides might be partially caused by higher linear speeds at points furthest out

Colin2B and me have said several times that idea (about actual linear speeds and orbits), considering as a whole the spinning added to the revolving, is rather a "bad" approach ... Especially when, as in our case of earth tides, linear speeds caused by those different movements are so different ...
Keep in mind that, on the one hand, v (linear speed) = ω r (r: radius of curvature, app. the distance from center of each path to the earth considered point).
The angular speed ω of the spinning is some 28 times bigger than the one of the revolving.
And the radius some 3/2 times bigger too.
And on the other hand, related centrifugal forces are proportional to ω²r ... Enormously higher in the spinning !!
"Normal" bulges are actually quite insignificant compared to the so called equatorial bulge !!
And to tackle earth tides as in what quoted is utterly absurd !!
And regarding Venus special case, I´m afraid you say wrong things, but prefer not to refute them without having quite clear what on my first phrase in bold ... 

[Le Repteux]

I just had a flash. If the earth would stop spinning, the equatorial bulge would get down and the tidal bulges would follow it, but for my proposition to be true, they should also disappear, and eventually reappear if ever the orbital speed was also stopped because then, the two sides of the earth would not be accelerating at the same rate towards the moon. If only the spinning is stopped, then the only fleeing motion left is due to orbital speed, and all the different parts of the earth are fleeing almost the same distance away from the moon in the same time, but there is still a difference in the force from the moon on those different parts since they are not necessarily at the same distance from it: if the cg of the earth is pulled a certain distance towards the moon to absorb the fleeing motion, then the outer part is pulled less and the inner one more. This way, contrary to what I was proposing, the spinning would have effectively no effect on the tidal bulges, and it seems to match David's explanation, but I'm not sure rmolnav's will agree with it since he seems to need a centrifugal force where I only need a tangential motion.

[Halc]

The spin of the sun has a higher angular velocity than any of the planets, so that tide slowly pushes each of them away.  Most planets also spin faster than their orbits, putting thrust on the sun that also contributes to higher orbits

Sorry, but I can´t understand what you mean ...

The sun spins every 25 days, which is faster than the orbit of any planet (88 days of Mercury being the fastest), so the tides on the sun push all of the planets outward, and correspondingly slow the sun’s spin.

And regarding Venus special case, I´m afraid you say wrong things, but prefer not to refute them without having quite clear what on my first phrase in bold ...

In my quote above your bolded phrase, I was speaking of the tides on the sun caused by any planet (including Venus), not of the tides on any particular planet.  Venus is not a special case in this regard. That part you quoted was related to how tides might have an effect on orbital radius, which is a discussion going on in another thread.  This thread is about two tidal bulges instead of just one.

[Colin2B]

All this seems to be about orbital speed, something on which I was not commenting in this topic

In that case we are talking at cross purposes and misunderstanding each other; my reply to @Le Repteux  was only to do with orbital speed.

I just had a flash. If the earth would stop spinning, the equatorial bulge would get down and the tidal bulges would follow it,   ............. contrary to what I was proposing, the spinning would have effectively no effect on the tidal bulges,

The spinning has no effect on tides whatsoever. The centrifugal force causing the equatorial bulge is balanced by earth’s gravity so the ‘new’ level is the geoid ie sea level and any tide raising forces produce an increase relative to this. Not sure if that is what you were trying to say, if so my comments are only for clarification.

[rmolnav]

The sun spins every 25 days, which is faster than the orbit of any planet (88 days of Mercury being the fastest), so the tides on the sun push all of the planets outward, and correspondingly slow the sun’s spin.
Quote
And regarding Venus special case, I´m afraid you say wrong things, but prefer not to refute them without having quite clear what on my first phrase in bold ...
In my quote above your bolded phrase, I was speaking of the tides on the sun caused by any planet (including Venus), not of the tides on any particular planet.  Venus is not a special case in this regard.

But those "tides" on the sun ...
1) ... are almost negligible, even the ones caused by biggest planets ...
2) ... given the distances involved, tangential component of each "bulge" would also be quite negligible ...
2) ... those "individual" bulges would be scattered around sun´s surface kind of at random (and varying with time) ...
3) ... and therefore it is not like the earth-moon scenario whatsoever, where we continuously have a gap between a bulge and actual sublunar meridian (the bulge ALWAYS eastwards from the moon), which is what pulls forward the moon, not just earth eastward spinning ...
So, to extrapolate earth-moon scenario to sun-planets case is quite erroneous ...   

[Le Repteux]

The centrifugal force causing the equatorial bulge is balanced by earth’s gravity so the ‘new’ level is the geoid ie sea level and any tide raising forces produce an increase relative to this. Not sure if that is what you were trying to say, if so my comments are only for clarification.

Yes, that's what I was saying about the spinning, but as I said, I prefer to use the concept of centrifugal motion instead of centrifugal force, just in case @rmolnav would understand what @David Cooper was telling him. If we draw a tangential speed vector on a circle, the tip of the arrow is where the body would be if no force was applied on it, and the force produces the acceleration needed to bring it back on the circle in the same time it took to get away. That centripetal force can be called inertial if the body is held by a rope, or gravitational if it held by gravitation, but to me, we shouldn't oppose it to a centrifugal one because it is a bit misleading. We could if the body's speed was affected by it, but it is not the case since it is perpendicular to the tangential motion. The only opposition the force faces is from the body's mass, as if it was at rest at a certain distance away from the circle. Of course it is different if the trajectory is an ellipse, because then, the force is not often perpendicular to the tangential vector and part of it serves to accelerate or decelerate the body tangentially to its trajectory.

[rmolnav]

If we draw a tangential speed vector on a circle, the tip of the arrow is where the body would be if no force was applied on it, and the force produces the acceleration needed to bring it back on the circle in the same time it took to get away. That centripetal force can be called inertial if the body is held by a rope, or gravitational if it held by gravitation, but to me, we shouldn't oppose it to a centrifugal one because it is a bit misleading. We could if the body's speed was affected by it, but it is not the case since it is perpendicular to the tangential motion. The only opposition the force faces is from the body's mass, as if it was at rest at a certain distance away from the circle.

1) "If we draw a tangential speed vector on a circle, the tip of the arrow is where the body would be if no force was applied on it ...": "Almost" quite right: you should have added "after a second" (or any other used unit of time).
2) "... and the force produces the acceleration needed to bring it back on the circle in the same time it took to get away. That centripetal force can be called inertial if the body is held by a rope, or gravitational if it held by gravitation ...": quite right the first phrase, but not the second.
 The centripetal force that makes an object rotate (instead of following in the direction of a previously acquired "tangential" speed), is always an "active" force (not to be called "inertial"), both if gravitational or the tension of a rope ...
3) "...  but to me, we shouldn't oppose it to a centrifugal one because it is a bit misleading. We could if the body's speed was affected by it, but it is not the case since it is perpendicular to the tangential motion. The only opposition the force faces is from the body's mass..."
It´s not "we" who "oppose it (the centripetal f.) to a centrifugal one" ... Centrifugal f. is the way "inertia" (certainly due to the body´s mass, and its speed) can manifest itself, not always in the same way ...
Even being "perpendicular to the tangential motion", it could for instance cause deformations ...
Rather than "a bit misleading" I would say it is something difficult to grasp, let alone to successfully convey to others ... I´ll keep trying. As I´ve already said, I have some "fresh" ideas for it, but prefer not to post them until having them better elaborated, precisely not to "mislead" anybody (or the fewer the better ...)

[Le Repteux]

Centrifugal f. is the way "inertia" (certainly due to the body´s mass, and its speed) can manifest itself, not always in the same way ...
Even being "perpendicular to the tangential motion", it could for instance cause deformations ...

A force causes an acceleration in the direction it is applied, whether the body is moving or not. We can even consider that the body is at rest to measure the force. What opposes the force is the mass of the body, and that body has to move for the force to be measured. When the force pulls on the body, the body moves towards the force, not away, and this motion is not clear when studying rotation. We may still imagine that a rotation produces an outward force, thus centrifugal, but it is an outward motion that it produces, and that motion is then pulled back in the direction of the force. If that outward motion is due to the spinning earth, then it produces equatorial bulges, and if it is due to its orbital motion, then it produces the tides, but it is the same kind of motion and the same kind of force, so there is no need to separate them, and it is what you seem to be doing.

[rmolnav]

We may still imagine that a rotation produces an outward force, thus centrifugal, but it is an outward motion that it produces, and that motion is then pulled back in the direction of the force. If that outward motion is due to the spinning earth, then it produces equatorial bulges, and if it is due to its orbital motion, then it produces the tides, but it is the same kind of motion and the same kind of force, so there is no need to separate them, and it is what you seem to be doing.

Well, first of all, an "outward motion" is not a proper physical variable, let alone a force ...
But within layman language, I would tell you those water motions (when equatorial bulge and tides) happen in opposition to own water weight (the most important force there), and after the imagined tangential movement and centripetal force pulling back, water remains further from earth CG, actually weighing a little bit less.
I, and eminent physicist, call that diminishing of weight centrifugal force ... If you prefer, I also could call it an "outward" force, certainly one of the ways inertia can manifest itself ...
 

[rmolnav]

Just some additional details about what said on my last post:

...those water motions (when equatorial bulge and tides) happen in opposition to own water weight (the most important force there), and after the imagined tangential movement and centripetal force pulling back, water remains further from earth CG, actually weighing a little bit less

What in bold refers only to antipodal bulge ...
As I´ve said many times, considering only the earth revolving around moon-earth barycenter, all earth particles follow identical circles, and their locations (within own circular path) are ALWAYS the farthest from the moon. That implies centrifugal force is always in the sense opposite to the moon.
Therefore, where sublunar bulge water (and solid elements) weights actually increase a tiny little bit ...(due to that inertial effect, the centrifugal force, in same sense as earth´s own pull).
But as moon´s pull there is bigger than that inertial force, sublunar bulge builds (towards the moon).
But at the antipodes moon´s pull is smaller than centrifugal force, the later prevails, and also a bulge (in the sense opposite to the moon) builds there.
In the case of the so called equatorial bulge all centrifugal forces are radially in the "fugal" sense from earth c.g., and the only existing gravitational pull is own weight of earth particles, uniform around the equator ... That´s why there the weights of all particles diminish, proportionally to ω²r (r the distance to earth axis of spinning, the closer to the equator the bigger ...).
Solid earth get deformed (angular speed ω is some 28 times bigger than in the moon-earth dance), and water from both hemispheres piles up towards the equator.

 
 

[Le Repteux]

Well, first of all, an "outward motion" is not a proper physical variable, let alone a force ...

During rotation, the motion is not directly outwards, it is tangential, and the distance being increasing if we cut the force is thus only incidental. That motion is due to inertia, reason why we call it inertial. While saying "an outward motion is not a proper physical variable", what do you mean exactly? Do you mean that inertial motion is not physical?

[rmolnav]

Well, first of all, an "outward motion" is not a proper physical variable, let alone a force ...

During rotation, the motion is not directly outwards, it is tangential, and the distance being increasing if we cut the force is thus only incidental. That motion is due to inertia, reason why we call it inertial. While saying "an outward motion is not a proper physical variable", what do you mean exactly? Do you mean that inertial motion is not physical?

An adjective such as "inertial" can be used scientifically, or in a broader layman sense ...
Therefore, in that last sense, you can use it for any noun with almost any relation to inertia ...
But in physics science, the "variables" can be speed, mass, acceleration, force, momentum ... all with their own units. Never just a "motion" ...
And that "motion" is not actually due to "inertia", it is due to an initial speed, whatever its cause.
Due to "inertia", objects "tend" to keep constant their velocity vector. If no force opposes that inertial "tendency", objects keep moving without any "problem". But if objects are "forced" to change their velocity vector, they somehow react (they are kind of stubborn: "no, no, I don´t want to change my velocity vector"...). And inertia manifests itself, in different ways ...
In our case, inertia manifests itself as a force, equal but opposite to the centripetal force, the "guilty" for the velocity vector change ... An outward force ... centrifugal (in its broad sense) because it is kind of "fugitive" (< > fugal) from the moon (the "center" of gravitational pull, source of acting centripetal forces ...)

[Le Repteux]

In our case, inertia manifests itself as a force, equal but opposite to the centripetal force, the "guilty" for the velocity vector change ... An outward force ... centrifugal (in its broad sense) because it is kind of "fugitive" (< > fugal) from the moon (the "center" of gravitational pull, source of acting centripetal forces ...)

To me, the definition of inertia is a bit weird: a body goes straight ahead because it has inertia, and it resists to change direction or speed also because it has inertia. A force is not a motion, and inertia is about both, so it means that it is probably still misunderstood. When nothing forces a body to change direction or speed, it goes straight ahead, as if something inside it would know how to move this way, and whenever something is on the way, it resists to change direction or speed, but it still changes them after a while if it is free to move, and it is probably that time that we call inertia. If it would change instantly, there would be no resistance and no inertia, and there would be no motion either. We can't measure it at our scale, but if I'm right about inertia, the atoms are probably going straight ahead for a while during rotation for us to be able to measure a force, and they probably feel no force during that time otherwise they would not be free to move. That's what makes me think that the equatorial bulge and the tidal ones are driven by the same phenomenon, and I think that you could admit it on that basis, but you probably have another viewpoint on inertia that makes you think your way. We behave like atoms as far as inertia is concerned, we all resist to change, and we all finally change since we are free to move, but it takes time. The problem with us is that even if we cannot know if the direction we took is the right one, we still think it is if nothing forces us to change it, but why would an atom change its direction or speed when it is not forced to? :0)

[rmolnav]

... a body goes straight ahead because it has inertia, and it resists to change direction or speed also because it has inertia

To me those two phrases of you are kind of the two sides of a coin: the body, either "feels" a null total force exerted on it, or it feels a not null force ... Inertia phenomenon makes it maintain its velocity vector (including the case of a null vector) in the first case (without any other "inertial" effect), or, in the second case, it experiences an acceleration vector (a=f/m) ... It is in this last case when other inertial effects may occur, not always the same way, depending on the individual forces acting on the object (the breakdown of mentioned total force vector).
 

When nothing forces a body to change direction or speed, it goes straight ahead, as if something inside it would know how to move this way, and whenever something is on the way, it resists to change direction or speed, but it still changes them after a while if it is free to move, and it is probably that time that we call inertia. If it would change instantly, there would be no resistance and no inertia, and there would be no motion either .

Sorry, but that´s utterly erroneous !!
Since the very first instant (an infinitesimal fraction of time), mentioned acceleration vector changes the object speed, though logically only an infinitesimal amount of change ... (dv=a*dt: infinitesimal speed vector change equals to acceleration vector multiplied by the infinitesimal fraction of time the total force has been acting).
And please, try not to make more difficult for people to understand the already "tricky" concept of "inertia" saying things such as "it is probably that time that we call inertia" ...!!