Talk:Tidal locking/Archive 1

Earth/Sun
Am I correct in assuming that at some point (however far off) the Earth will become tidally locked with the Sun and we'll have a cold and likely uninhabitable side of the planet?

You are probably correct. It will most certainly happen three years from now, just after I am graduated from college. (Sorry, but I had to.) If we assume that the moon used to not be locked to us, then we can assume that the earth will become locked to the sun, thereby making the entire planet (save maybe the twilight zones) uninhabitable, not just the dark side. -- D. F. Schmidt (talk) 16:43, 23 August 2005 (UTC)


 * I think the Earth would be locked to the Moon and thus it couldn't be locked to the Sun at the same time.  Grue  20:03, 25 September 2005 (UTC)


 * The articles says that tidal locking applies to bodies that closely orbit each other, and it's stated above that if there's a huge difference in mass, only the one with less mass will be tidally locked. So, the earth's rotation won't slow down due to tidal locking in any significant way at any time in the forseeable future. MarXidad 20:59, 27 February 2006 (UTC)


 * "Closely orbit each other"? If I have it correct, the earth has slowed down in its daily rotation (I'm not sure where I got this from, except I know that it was somewhere on Wikipedia). Why would the earth slow down, but its proximity to the sun (which has significantly more mass)? I don't think it would lock or be so affected by Venus. D. F. Schmidt 16:58, 25 April 2006 (UTC)
 * The Earth has slowed down due to interaction with the Moon, especially due to friction caused by the tides. The Venus thing is almost certainly a coincidence. Deuar 18:36, 25 April 2006 (UTC)


 * If the Earth becomes tidally locked with the Sun will the Earth be able to support an atmosphere?
 * No reason why not, but any atmosphere will probably be blasted away much sooner (in only 5 billion years or so) when the Sun goes into red giant phase. Deuar 10:53, 31 March 2006 (UTC)
 * Thank you kind sir :)


 * If the Earth had no moon, the timescale for the Earth to be tidally locked to the Sun would be in the order of tens of billions of years.
 * The presence of the moon complicates things. The Earth would tidally lock to the moon first. After that, the Sun continues to act on both bodies, slowing the rotation of both. The interaction is complex but I think it works like this. Both bodies rotate slower, cauing the tidal bulges of both bodies to lag behind the centre line connecting the centre of gravity of both bodies. This causes the orbit of the moon to shrink. Eventually the moon will be orbiting the Earth so closely that it breaks up and accretes onto the Earth. Then the Sun drags what is left to a tidal lock many billions of years from now.
 * Of course, the Sun will expand into a red giant long before this happens so the point is moot anyway. --B.d.mills 10:31, 3 July 2006 (UTC)

Please Clarify: Self-gravitation

 * Larger astronomical bodies which are near-spherical due to self-gravitation, become slightly prolate (ovoid).

Could someone elaborate on this sentence? Why only large bodies? Does tidal locking not apply to small bodies? Define self-gravitation. (I assume it means the forces pulling it into its own center, but why would this only apply to large bodies?) Does the material of the body (gaseous vs. rock) make a significant difference? It took me a couple read-throughs to realize the the orientation of the major axis of the prolate spheroid is towards the primary. (I saw the diagram at spheroid and assumed that the major axis was also the axis of rotation. Took me a while to break out of that mindset.)  Could someone who knows what they're talking about (i.e. not me) spell out this part of the article a bit more? Thanks! --Nmagedman 22:21, 20 May 2006 (UTC)

[note: in response to this, Deuar did significant work rewriting the page. --Nmagedman]


 * Since I've restored the old one, I guess this now needs an answer. Which is, in small bodies the internal forces (structrural integrity) can be large enough to oppose gravity. Larger bodies (earth, moon) are essentially fluid. William M. Connolley 13:39, 24 May 2006 (UTC)
 * The tidal bulges are there in small bodies as well, but are tiny in comparison to their structural irregular shape. Example: all the small and irregularly shaped inner moons of Jupiter are tidally locked. Deuar 18:17, 26 May 2006 (UTC)

I thought the old explanation was clearer, myself William M. Connolley 18:45, 21 May 2006 (UTC)

Overall, I agree, but they both have their strengths and weaknesses. The new explanation clarified many points that I didn't get from the first one. Thank you, Deuar. I greatly appreciate your effort! I need to review both versions and then can try merging the best of both of them. --Nmagedman 17:58, 22 May 2006 (UTC)


 * Hmm, well, maybe. How about:


 * Rock (ice, gas, water, whatever) is not elastic, and when it is continually being reshaped by two bulges passing by during every rotation, friction is created. The planet is always pulling the bulge somewhat backwards, so the effect is that the moon is being gradually slowed down by this friction. Over time the rotational speed slows down. The rotational energy of the moon is being lost into space as heat.


 * I don't believe this (that the planet is slowed by friction). The planet is slowed because there is a gravitational torque on the bulge (as the previous version said). Friction is just a manifestation of this. William M. Connolley 19:13, 22 May 2006 (UTC)


 * Hi again. Yeah, well, re-reading what I wrote (this time in the light of day), I can see it could use improvement. The list I made is too choppy, for one. I'm a bit pressed for time for a couple of days, so i'll just be lurking for a while.
 * Regarding friction/torque, you need more than just a constant torque or a constant force to slow things down. What is also needed is a way to lose energy as time goes by (the friction). Suppose you had a body making a rotation in 10 hours. Now you start applying a small constant torque on it. It will indeed change its rotation rate so that now it takes, say, 10 hours plus one minute. But this rotation rate will not change any further if you have no friction. Deuar 08:57, 23 May 2006 (UTC)


 * Not at all sure I understand you. From 2 viewpoints: (1) postulate, if you will, a frictionless fluid body (either a real superfluid helium one, or an abstract mathematical one). Are you asserting that tidal forces will not cause it to slow and lock? (2) as you've just said, the torque on the bulge slows the thing down a bit; but that doesn't get rid of the bulge, or the torque, so it will slow further. Further, what you need is a way to lose angular momentum. Friction doesn't do that; the torque does. William M. Connolley 15:20, 23 May 2006 (UTC)


 * Hi William, yeah well you're posing pretty good questions. It seemed simple at first but it looks more involved now. I'm still convinced that you need friction because energy must be permanently lost somehow but there's also angular momentum to lose. Thinking... Deuar 21:11, 23 May 2006 (UTC)
 * Hmm - I think i get it now. Consider this analogy: we've got a pendulum made from a weight hanging on a spring (so the spring is the arm of the pendulum), and the pendulum can go all the way around the central attachment. It goes around the attachment in a vertical plane. Furthermore there's no friction in the attachment, or with the air. The weight is like a rock on the moon's surface, the spring is like the moon's gravity, and the downward earth gravity is like the planet's. By the way, the weight is a rock on the surface, not the bulge! It becomes a part of the bulge only for a time when the spring is stretched more than average.(I was confused about this for a while) Anyway, we give the pendulum a big push, so that the weight easily flies all the way around the attachment - this is the moon's rotation. So as our "rock" approaches the bottom of a swing, it is like approaching the planet-facing point. Because the earth's gravity is acting against the spring here, it streches a bit, and the rock is further down than if the spring was stiff, and it's like being part of the "bulge". It continues to go around, and as it begins to rise up from the bottom of its rotation, it experiences a torque from the earth's gravity, and begins to slow down. As you rightly pointed out! No friction. Now, it's genrally going pretty fast, so soon it passes the height of the attachment, and later gets up to its maximum height, exactly above the attacment point. It's slower now then whan it was at the bottom. However, now, once we're over the top, the torque from the earth's gravity starts applying torque in the opposite way and the weight begins to speed up as it travels around the attachment and down. In fact, by the time it gets down to the bottom, it's travelling at the same speed it was before. All assuming no friction. This is why we can have torque periodically slowing it down, but by itself it just causes an oscillation that repeats on and on without locking. Now if we also add friction, say in the spring, analogously to friction between rocks in the moon, it's clear what happens. It will fly around, slowly losing a tiny piece of speed with each revolution, until at last it's lost so much that it can't crest the top of the rotaton, and falls back down on the same side, swings around on the bottom a few times, and eventually stops, hanging straight down. It's locked.
 * So, regarding your points, (1) The superfluid will slow from the torque, but later speed up, and won't lock. (2) Yeah I kind of agree. The subtle thing is that a rock on the surface is not the same as the bulge (the bulge hardly moves at all, in fact). Hopefully the analogy above clears up this difference. What I wrote in the article needs fixing. Deuar 22:15, 23 May 2006 (UTC)
 * I disagree. You're missing the fact that the bluge isn't fixed on the sfc - it moves. But because of inertia its always a bit behind the rotation. Your analogy does not work. Furthermore, I'm a bit disturbed that you are essentially working this out in arrears... you should know this already, if you're editing it against objections William M. Connolley 22:32, 23 May 2006 (UTC)
 * Perhaps you misunderstand the analogy. The weight is a point on the surface, the surface is rotating. That's why the weight goes around. The bulge is always pointing roughly towards the planet, which is "down" in the analogy, but of course the surface rotates past the bulge all the time. The bulge occurs in the place where the extension of the spring is greatest, not necessarily where our test weight is at any time. And, the analogy does allow for a bulge that is slightly towards the direction of rotation because of inertia. If the weight goes around clockwise, the greatest extension of the spring will be at around 7 o'clock (not 6) because of the weight's inertia. It takes a little while for the spring to start significantly pulling it back up.
 * I'd also like to point out that my editing was before any of your objections, and in fact inspired by complaints about the previous version! (see first post in this thread). I have not edited the explanation since you raised doubts. Deuar 09:49, 24 May 2006 (UTC)
 * Sure. What I'm pointing out is that the previous explanation was correct, even if some people found it hard to follow. The current one isn't. Friction doesn't get rid of angular momentum. Now, you seem to be saying, lets leave your version in place while you try to work out how to fix it. Thats not good. William M. Connolley 10:47, 24 May 2006 (UTC)
 * I don't know why you're trying to put words in my mouth now. That's not good :-) I believe we were merely discussing the physics. Deuar 16:50, 26 May 2006 (UTC)


 * Before we invest time & energy into a rewrite, could we reach consensus on what the scientific views are? Does anyone have a WP:RS astrophysics textbook or whatnot?  --Nmagedman 11:31, 23 May 2006 (UTC)
 * Time for a brain strain indeed. Deuar 21:35, 23 May 2006 (UTC)

I decided that I preferred the old version of the mechanism section, so I've restored it. William M. Connolley 13:39, 24 May 2006 (UTC)
 * I had hoped you could provide a more objective reason than your personal dislike, but hey, the old version is mostly correct so it can be worked with equally well. Friction is needed to tidally lock and should be mentioned. A frictionless superfluid body would rotate around, with a bulge, but without ever slowing. Flowing around without slowing is the defining feature of a superfluid after all. Deuar 16:50, 26 May 2006 (UTC)

I think it's the ambiguity of the phrase "slow down" that caused the confusion. The angular speed of the moon is not monotonous and never slows down or speeds up at all times. A better description of that is it is a dissipating periodic function of time. In other words, the angular speed at which the moon passes through a fixed angle decreases, while slows down and speeds up as it revolves through different angles at different time. (1) When there is no friction and the moon is perfectly elastic, its speed is purely periodic. It slows down and speed up in regular intervals. William is correct that only external torque changes angular momentum, not friction. But he didn't realize that the same torque increases as well as decreases the angular momentum at different times. Its speed when passing the same angle remains the same in every period. (2) When there is friction, the kinetic energy (assuming no additional potential energy is converted from the kinetic energy) is dissipated and the angular speed diminishes after each "period". The speed approaches zero at time infinity. The current lunar libration is the manifestation is this remnant damping oscillation. Waltzingfeet (talk) 22:19, 19 January 2009 (UTC)

Prolate
I was just curious about the use of the term "prolate". I had understood that tidal forces deform a body so that it is oblate. Please correct me if I am wrong. --CKozeluh 15:52, 9 June 2006 (UTC)
 * Check out spheroid. Deuar 21:38, 9 June 2006 (UTC)
 * That's just it; spheroid, oblate, equatorial bulge, and moon all suggest that the planets and moons are oblate. Do tidal forces act counter to rotational forces, bringing an orbiting body closer to perfect sphericity? CKozeluh 19:22, 15 June 2006 (UTC)
 * Usually the body is flattened (in the "north-south" direction) by centrifugal force due to its rotation in the "north-south" direction, and then also becomes prolate in one of the perpendicular directions (as in "towards the planet") from tidal forces. The end result is an ellipsoid. Deuar 19:35, 15 June 2006 (UTC)
 * The same question occurred to me while reading the article. Now I've read this exchange, and I still don't see that the question was settled.  I'm further puzzled by Deuar's reference to "rotation in the north-south direction".  I'm out of my element here, but isn't north-south defined as the poles of the rotational axis?  So rotation in a north-south direction seems to be an oxymoron.  Educate me, please. 24.58.11.50 03:38, 5 February 2007 (UTC)
 * Umm, sorry about the confusion caused by my sloppy writing above; I meant flattened in the north-south direction (wording now fixed). We have a combination of two basic deformations:
 * 1) Oblate flattening at the poles due to rotation.
 * 2) Prolate elongation of the tidal bulge at the equator.
 * On the scale of things, this prolate part is negligible unless teh body in question is orbiting close to another large mass. Also, this "prolate" part of the deformation travels around the equator with every revolution, unless our body is tidally locked. Deuar 14:05, 5 February 2007 (UTC)
 * This discussion is frankly a mess. Whether or not the rotation deformed the body it would be irrelevant to the tidal effect.  The Tidal effect should cause the body to become prolate if it in fact stretches the body along the axis of separation.  How the article came to say the tidal effect caused the body to become oblate is a mystery to me.  —Preceding unsigned comment added by 192.25.142.225 (talk) 20:52, 30 September 2010 (UTC)

Timescale for tidal locking
Timescale formula is missing the mass of the satellite m_s. —Preceding unsigned comment added by 24.16.137.250 (talk) 06:03, 16 June 2009 (UTC)

The following sentence is misleading:

The moon has oriented itself so that its heavy side with much more extensive maria, which we know as the near side, is oriented towards the Earth.

This is misleading. The moon would have tidally locked to the Earth fairly quickly (probably less than a thousand years). However, the maria would likely have formed over a longer period. In other words, by the time the maria formed, the moon would already have been tidally locked. To imply that the maria formed before the moon tidally locked is misleading at best and factually inaccurate at worst.

A reference to the time to tidal locking is given by a formula here: http://groups.google.com/group/rec.arts.sf.science/msg/e05283a619187a8f?dmode=source&hl=en --B.d.mills 10:20, 3 July 2006 (UTC)
 * That's a really good point. I've now found that kind of formula elsewhere, e.g. this paper, so it's not a discussion group fluke. I get 90,000 years at current earth-moon distance, but this must have been much quicker in reality since they were once closer, and the rate decreases as orbital radius to the sixth.


 * I find that short timescale totally amazing. It may itself be worth remarking in the article.


 * Regarding the maria, it's obviously as you say, but I would find it very odd if they just happenned to form on the earth-facing side. My best guess is that the equilibrium locked position must have migrated at some stage either gradually or in a jump to orient the heavy hemisphere towards Earth. This could use some kind of literature search... Deuar 20:34, 3 July 2006 (UTC)


 * The crust of the moon is thicker on the far side than the near side. (Ref: http://ottawa.rasc.ca/articles/hanmer_simon/geology/moon3/index.html) The moon is also believed to have had a magma ocean immediately after it formed. Evidence for this comes from the presence of certain minerals found by the Apollo and Luna missions. If the moon did tidally lock in under a thousand years, it would likely have locked with the crust still molten. This explains why the crust is thicker on one side than the other; when it solidified the moon was already tidally locked. It also explains the presence of the maria on the near side. With the thinner crust there, it would have been easier for basalt to flow on the surface after an impact. So the presence of the maria on the near side are actually a consequence of tidal locking and not a cause.


 * By the way, my calculations suggest that the moon tidally locked a lot quicker than a thousand years, I simply used that as a safe figure in case my rough calculations were flawed. The moon is believed to have formed very close to the Earth, maybe about 20,000 km from the centre of the Earth. At that distance, my calculations suggest that the tidal locking could take only a few months. --B.d.mills 11:22, 4 July 2006 (UTC)
 * I follow the argument up to and including "when it solidified the moon was already tidally locked", but I don't see what being Earth-facing has to do with crust thickness. The difference in gravity due to the Earth is completely miniscule. Deuar 14:07, 4 July 2006 (UTC)
 * The moon's crust is thicker on the far side, but an explanation of the mechanisms that may cause it will require further research. --B.d.mills 12:26, 5 July 2006 (UTC)

I find In fact the tidal locking was so rapid, that the frictional energy produced must have melted the moon's surface, and so must have occured even before the highland terrain solidified. somewhat doubtful. It has no source. And according to the discussion above, it was molten at the time anyway William M. Connolley 19:54, 4 July 2006 (UTC)
 * That makes sense. It's gone. Deuar 20:15, 4 July 2006 (UTC)


 * Let's better say, that there could have been maria everywhere on the Moon surface, but since one face points to the Earth all the time, the other face is much more exposed to asteroid impacts and the maria disappeared between asteroid impact craters during bilions of years, making the outer side even more light-weight and the tidal lock even more strong... Does this sound more probable?
 * It is just not possible to guess now, whether the Moon has been beside the Earth from the early times, when its surface was molten...
 * ~ Semi, July 5, 2006
 * See the discussion above on the thickness of the lunar crust. The thin crust on the near side allowed magma to flow onto the surface, whereas the thicker crust on the far side blocked the flow of magma. Your conjecture about not being able to guess the age of the surface is incorrect, because the lunar rocks brought back by Apollo and Luna missions have been dated accurately by the decay of the radioactive isotopes that they contain. --B.d.mills 02:34, 8 July 2006 (UTC)

Could the units of measurement be stated for each variable? Mass of the planet in kg etc, and the unit output. I'm trying to use it at the moment and I don't know if the output is in seconds, days or years. —Preceding unsigned comment added by Firstmatekevin (talk • contribs) 15:48, 5 July 2010 (UTC)

Pluto's moons: Nix and Hydra
Would the recently-discovered moons of Pluto (Nix and Hydra) be tidally locked? There's no mention of them in the list. These moons would have a unique tidal lock, because they would be locked to two bodies (Pluto and Charon) and this is worth mentioning in the article. --B.d.mills 12:56, 5 July 2006 (UTC)
 * Well from the locking time formula in this paper (which I should get around to putting in the article, I guess), I get the following time constants for the bodies in the Pluto system to lock:
 * Charon: 300,000 years; Pluto: 4 Myr; Nix: 50 Gyr; Hydra: 300 Gyr.
 * These are necessarily extremely rough, I made the same assumptions as in the paper, plus an initial rotation period of 12 hours for each body. I wouldn't trust these more than being within a factor of 10, and they are more likely over-estimates than under-estimates because of the gradual increase in orbital radii that must have occured in the meantime. So Nix looks like it might or might not be locked, while Hydra is significantly less likely to be locked. Maybe there's a paper somewhere that considers this more rigorously for Nix and Hydra. Deuar 18:23, 5 July 2006 (UTC)

Venus corrected
I have corrected the Venus paragraph.

Could you, please, recalculate the Venus apparent solar day and sidereal day, before reverting my change?

The more propper value seems, that after 13 Venus years and 8 Earth years, the both planets meet on an almost same place, and it takes 12 Venus sidereal days, with a difference, that is negligible, when compared to a relatively short time of radar insight.

The apparent solar day on Venus, as stated elsewhere here, copies an older bug in NASA planet factsheets, where the ea-day value in that cell is mistaken by hours in that column heading...

The apparent Solar day on Venus calculation: if it orbits arround the Sun in 224.70069 ea-days, which is 1.60° ea-daily, and meanwhile spins in counter direction after 243.0185 ea-days, which is 1.48° ea-daily, relative to star background, then the Sun moves apparently only 0.12076° ea-daily. From this, the apparent travel of Sun one "day" arround, as seen from the Venus surface, takes 8.28 Earth-years!

So how is that?

~ Semi, July 5, 2006
 * These calculations are flawed because they do not allow for the retrograde rotation of Venus. With the rotation and the revolution going in opposite directions, the mean solar day is about half a Venus orbit long. --B.d.mills 02:01, 6 July 2006 (UTC)


 * Aha, thank you. I was probably misleaded by some earlier version of Venus article here, that readed such, that the yearly and daily move of Sun are contrary and should subtract. It is no more on the Venus article... So they should actually add (both apparently counter-clock-wise from the surface) and make the value of 3.08° ea-daily, or 116.75 ea-days per whole rotation... Sorry for my previous error above.
 * Then, after 583.9 days, the planets meet on an almost-opposite side of the Sun (ie. after 1.60 earth-years, which is 0.6 of a circle arround from the previous meeting), and if it shows same face to Sun, it shows the opposite side (same as in previous meeting) to the Earth...
 * So I have corrected the article yesterday to say an almost same thing in other words, but related to the sidereal day in ratio 8:13:12... Could this be yet reformulated to say both?
 * Actually, these planets meet on an almost-same place (relative to Sun) just after 5 consecutive meetings, after 8 Earth-years and 13 Venus-years, which is after 12 Venus-sidereal-days (relative to star background) and after 25 Venus apparent-solar-days...
 * During the times, when these planets meet, the Earth moves retrograde from the Venus perspective. Also, when Venus meets Mercury or Jupiter (the only other tidally-important planets from Venus perspective, beside the Sun, and only tidally-important during the meetings), they also move retrograde from the Venus perspective, which all together is the most probable cause of the retrograde spin of the Venus...
 * Also, the Venus is one of few planets, that has got no moon to induce/support the prograde spin. The only other such planet (without a moon) is Mercury, whose rotation is also "relatively strange"... Could this be connected?
 * ~ Semi, July 6, 2006
 * It is pretty doubtful that any of the planets including Earth have significant tidal effect on Venus. That's why these matching periods puzzle people. Earth's tidal effect appears on the face of it to be completely miniscule, not to mention the other planets. The tidal force falls off as radius to the third power, while the rate of tidal locking falls off as radius to the sixth. There were some papers published suggesting that the tidal slowing may be bigger than normal on Venus because of its large atmosphere, but this is rather speculative and is not an accepted explanation by any means. Deuar 18:57, 6 July 2006 (UTC)
 * Venus and Mercury have no moons and rotate slowly. There is a connection - moons that orbit faster than the planet rotates will be accelerated in their orbit by tidal forces, eventually causing the moon to crash into the planet. This is happening with Phobos. So moons have to orbit Mercury and Venus a long way out to avoid being deorbited into the planet. When moons orbit so far out, the Sun can perturb them out of orbit - see Hill sphere. So Venus and Mercury cannot have satellites with stable orbits. --B.d.mills 23:52, 6 July 2006 (UTC)
 * ~ Semi, July 6, 2006
 * It is pretty doubtful that any of the planets including Earth have significant tidal effect on Venus. That's why these matching periods puzzle people. Earth's tidal effect appears on the face of it to be completely miniscule, not to mention the other planets. The tidal force falls off as radius to the third power, while the rate of tidal locking falls off as radius to the sixth. There were some papers published suggesting that the tidal slowing may be bigger than normal on Venus because of its large atmosphere, but this is rather speculative and is not an accepted explanation by any means. Deuar 18:57, 6 July 2006 (UTC)
 * Venus and Mercury have no moons and rotate slowly. There is a connection - moons that orbit faster than the planet rotates will be accelerated in their orbit by tidal forces, eventually causing the moon to crash into the planet. This is happening with Phobos. So moons have to orbit Mercury and Venus a long way out to avoid being deorbited into the planet. When moons orbit so far out, the Sun can perturb them out of orbit - see Hill sphere. So Venus and Mercury cannot have satellites with stable orbits. --B.d.mills 23:52, 6 July 2006 (UTC)
 * Venus and Mercury have no moons and rotate slowly. There is a connection - moons that orbit faster than the planet rotates will be accelerated in their orbit by tidal forces, eventually causing the moon to crash into the planet. This is happening with Phobos. So moons have to orbit Mercury and Venus a long way out to avoid being deorbited into the planet. When moons orbit so far out, the Sun can perturb them out of orbit - see Hill sphere. So Venus and Mercury cannot have satellites with stable orbits. --B.d.mills 23:52, 6 July 2006 (UTC)

locked satellites
An application of the rough formula for locking time (6 Rμ/msmp² years) to the satellites of the outer planets indicates that the current status of most of them can be confidently estimated. Apart from a couple of exceptions, all the inner regular satellites have a locking time of less than several million years, which is barely an instant in astronomical time. Conversely, all of the irregular outer satellites except Triton have locking times in excess of 1 years, and obviously are extremely unlikely to have been locked so far. The exceptions to this clean division are: So it turns out that even allowing for a factor of 10 uncertainty in the above estimates, Pluto's new moons are the only ones for which the locking situation is strongly ambiguous. Accordingly, I have standardised the "(presumably)" tag to "(assumed)" for bodies on the "locked" list in the article, since their lock is pretty certain. Also added Oberon which was missing for some reason. Deuar 20:04, 16 July 2006 (UTC)
 * Polydeuces: 100 Myr. Still only a fraction of solar system age, and even allowing for a factor of 10 uncertainty, so it is fair to assume that it is probably locked by now.
 * Hyperion: Known from observations not to be locked
 * Iapetus: 500 Myr, known from observations to be locked
 * Nix: 350 Myr to 7 Gyr depending on its size: faster locking if it has low albedo
 * Hydra: 1.4 Gyr to 25 Gyr depending on its size like for Nix

Mechanism section
Please could someone clear up the mechanism section - it's a little unclear. A gif animation would make this much clearer - I'd do it myself but I don't quite understand it! Thanks --El Pollo Diablo (Talk) 10:41, 9 October 2006 (UTC)
 * You're too right, it was unclear. I have gingerly made a rewrite which imho improves the situation − gingerly, because this has been tried before, not necessarily with good results. Diagrams would indeed be very useful... Deuar 17:37, 22 October 2006 (UTC)

Trivia to be added - Edmund Halley was the first to note the effect. He found that the revolution period of the moon is getting shorter and concluded that that moon is accelerating. 202.156.12.12 04:15, 17 December 2006 (UTC)Rajesh

but being precise...
Pluto's not a planet. --Taraborn (talk) 20:21, 30 January 2009 (UTC)

For the purposes of the article, Pluto's planetary status doesn't matter. Planet status under the new definition is not a function of size or mass...but of location. Just pretend that the Pluto-Charon system were located between Mercury and Venus. If that were the case, even Pluto's small mass would suffice to clear its orbit, and (surprise) it would be a Planet by the new definition and orbit-clearing formula. It is the bizarre insistence that location matter (in the definition of planet) that riles up so many people. If something is over 1,000 miles in diameter and suddenly appeared in the inner Solar System, we'd say "that's a planet"... so close your eyes and pretend. Chesspride 172.164.20.73 (talk) 21:46, 9 February 2016 (UTC)
 * We already do have something over 1,000 miles in the diameter in the inner Solar System which most people don't consider a planet. It's quite close by: it's called the Moon. Are you going to call it a planet? If not, then location obviously matters. Double sharp (talk) 12:11, 25 November 2021 (UTC)

Definition of tidal locking
Per
 * "A widely spread misapprehension is that a tidally locked body permanently turns one side to its host (e.g. in Neron de Surgy & Laskar 1997; Joshi et al. 1997; Grießmeier et al. 2004; Khodachenko et al. 2007). Various other studies only include the impact of eccentricity on tidal locking, neglecting the contribution from obliquity (Goldreich& Soter 1966; Goldreich 1966; Eggleton et al. 1998; Trilling 2000; Showman & Guillot 2002; Dobbs-Dixon et al. 2004; Selsis et al. 2007; Barnes et al. 2008). As long as ‘tidal locking’ denotes only the state of d&omega;p/dt = 0, the actual equilibrium rotation period, as predicted by the CTL model of Lec10, may differ from the orbital period, namely when e ≠ 0 and/or &psi;p ≠ 0. Only if ‘tidal locking’ depicts the recession of tidal processes in general, when e = 0 and &psi;p = 0 in Lec10’s model, then &omega;p = n. As given by Eq. (23), one side of the planet is permanently orientated towards the star if both e = 0 and &psi; = 0 [2]. In this case, habitability of a planet can potentially be ruled out when the planet’s atmosphere freezes out on the dark side and/or evaporates on the bright side (Joshi et al. 1997). As long as e and &psi;p are not eroded, however, the planet can be prevented from an &omega;p = n locking. In the CPL model, however, the equilibrium rotation state is not a function of &psi;p, thus ‘tidal locking’, denoting d&omega;p/dt = 0, indeed can occur for &psi;p ≠ 0."

An example of this is Mercury, which follows the definition of tidal locking but is not in synchronous rotation in large part due to its high eccentricity. In other words, the description in the lede section denotes the more general definition of orbital locking, which includes non-zero eccentricity and/or non-zero obliquity. It depends on whose definition you want to use; my preference is to cover the wider case since it's more interesting.

Praemonitus (talk) 17:59, 20 April 2018 (UTC)


 * Both the book and the paper you mentioned in your edit summary give similar definition of "tidal locking".


 * The Barnes (2010) book states on p.248: "... assume that, over the course of an orbit, there is no net transfer of angular momentum between the planetary rotation and orbit, a situation called tidal locking".


 * The Heller, Leconte, Barnes (2011) paper in Section 5 says "'Tidal locking' is used when there is no more transfer of angular momentum over the course of one orbit, i.e. <...> &omega;p = n" (this notation means synchronized rotation as used in the paper).


 * There's also some brief discussion related to the terminological ambiguity in Section 3.3 of the paper (from which you quoted above), but the authors' posited candidates for the definition of "tidal locking" are either A) "d&omega;p/dt = 0" (which is a pretty strict condition), or B) "recession of tidal processes in general" which is an even stricter condition with eccentricity and obliquity vanishing, and synchronized rotation appearing as a result. Note that, as is clear from Section 3.3, the authors there play with their own notions of what the meaning of the term "tidal locking" could be, and are quick to acknowledge that this is quite different from other definitions that are "widely spread" in literature.


 * My interpretation of what they tried to say in that paragraph, is that their main gripe is with the notion that "a tidally locked body permanently turns one side to its host" – which is the case strictly only when eccentricity and obliquity are both zero. Clearly, there can be librations, etc. present in the motion of the planetary body – which still has its spin and its orbit synchronized, but no longer has a strict condition of one side being permanently turned to its host (if you define "one side" in the strictest sense).


 * Now as for your claim of the "harmonic ratio" statement being supported by the book and the paper – this is not true: this statement is found in neither of the two. Nowhere in the paper do they state anything related to the term "harmonic ratio". Same holds for the book, where the closest they come to anything "harmonic" is a harmonic oscillator.


 * In addition, please note that the term "harmonic ratio" itself is not a commonly used term in scientific literature – and as a result it lacks a clear and widely accepted definition. Noteworthy, as I mentioned above, that neither the 2010 book nor the 2011 paper use this term at all.


 * cherkash (talk) 21:33, 20 April 2018 (UTC)
 * Fair enough, although your edit about tidal locking means being locked into a 1:1 ratio with the orbital period is likewise discounted by the authors. The problem seems to lie in the varying definitions used in the literature. Perhaps then we need to introduce the term 'pseudo-synchronous rotation' as used in the Heller et al. (2011) discussion, and state that this will eventually devolve to synchronous rotation as e and &psi;p change to zero with time? Praemonitus (talk) 22:05, 20 April 2018 (UTC)


 * Yes, possibly – feel free to introduce this term. I've fixed the lead for the time-being based on the sources discussed here. cherkash (talk) 00:53, 21 April 2018 (UTC)

The Equilibrium Explanation
Why are simple ratios of rotation:orbits preferred?

The ratios are friendly to oscillation in the sense that they lead to repetition of the system within a few orbits. eg. Mercury has a 3:2 ratio. In 2 orbits it is back where it started from in terms of rotation, position, velocity. However, repetition of this information is the definition of equilibrium. Mercury repeats itself every two years it is locked in equilibrium.

If the ratio is slightly different than a simple integer ratio, the ratio will change until it is a match and then stops changing. This is a damped oscillation and should not take millions of years.

I am unclear if this is a duplication of the information in the article. I don't think it is but it should be.45.72.152.47 (talk) 19:13, 11 January 2019 (UTC)
 * I think it's just a historical legacy. Tidal locking is based on rotations per complete orbits, so the orbital part is given as an integer. Praemonitus (talk) 20:53, 11 January 2019 (UTC)

Minimal tlock estimates for various locked and non-locked bodies in solar system
I think it would be awesome to have some estimates of tlock for moons that are not tidally locked (yet) to their parent body. And estimates of how long would it take for some other moons to lock if they were spinning at some reasonable speed (like 1 per day) initially. Definitively there are some bodies that take few millions years and some that take many billions of years, so knowing some examples in a table or in section about formula would be useful.81.6.34.246 (talk) 11:13, 17 January 2019 (UTC)

Intro section
I don't doubt that the current introduction to the article is scientifically accurate. However, a user-unfriendly phrase like, "an integer ratio of orbits have been completed such as 3:2 for Mercury" in the very first sentence flies in the face policies and guidelines in places like the Manual of Style/Technical language and Make technical articles understandable. Astronomers, mathematicians and physicists are undoubtedly among Wikipedia's readers, but it is an unfortunate mistake to begin an article as if they are the only readers. The second sentence verges toward incomprehensibility for an average reader. I'm college-educated, and I only dimly perceive what it means. A phrase like "net transfer of angular momentum" is fine for someone well-versed in physics or mathematics, but not for an unspecialized general reader. That phrase, along with "orbital eccentricity and obliquity"--terminology that will stop most readers in dead their tracks--belongs in the article body, not the lead. Linking those terms to their own articles, though proper, is not a reason to have them in the lead, where they will turn off most readers, except those determined to slog through such specialized terminology. I ask that you offer some more reader-friendly revisions here in Talk, so the article introduction can be put in a form that will be useful to the majority of readers, rather than a minority who are already familiar with such jargon. DonFB (talk) 22:44, 1 July 2019 (UTC)

Take particular note of What Wikipedia is Not, which gives excellent advice:


 * "A Wikipedia article should not be presented on the assumption that the reader is well-versed in the topic's field. Introductory language in the lead (and sometimes the initial sections) of the article should be written in plain terms and concepts that can be understood by any literate reader of Wikipedia without any knowledge in the given field before advancing to more detailed explanations of the topic."

DonFB (talk) 23:05, 1 July 2019 (UTC)

Praemonitus, I saw your latest revisions immediately after posting an addition to this Talk section. I appreciate that you have markedly improved the introductory section and welcome any additional changes that serve the goal of reader-friendliness. DonFB (talk) 23:14, 1 July 2019 (UTC)


 * Thanks. While I too want the information to be presented in a clear, comprehensible manner, the critical factor for me is in maintaining scientific accuracy. I appreciate that you are okay with the revisions I made. Praemonitus (talk) 03:05, 2 July 2019 (UTC)

In response to your comments:


 * My guess is that 100% of physicists will understand "net transfer of angular momentum" and that 1% of everyone else will get it. This kind of arcane terminology is not appropriate for the introduction.


 * Well "angular momentum" is the correct term. I'm not sure what else you'd call it. There's a "Conservation of Angular Momentum" concept in physics, which is what is happening throughout this process. Angular momentum is a kind of rotational "inertia" that is being shifted between the orbits and rotations of the bodies to produce the locked state. How would you explain that more clearly? Perhaps by starting the paragraph with a simplified explanation of "angular momentum"? How about:
 * "The rotational and orbital inertia of these bodily masses are described numerically as their angular momentum."


 * Could this simply say "gravity"? Is it necessary to use fancy (and unfamiliar/inaccessible) 'gravity gradient' jargon? Readers will understand gravity; they'll scratch their heads when they see 'gravity gradient'--what the heck is that? Could the sentence simply say: "The effect arises when gravity between the bodies induces friction that slows down the rate of rotation, especially of a smaller body, eventually resulting in tidal locking" ?


 * It can't just say "gravity", because it's the difference in gravitational force across the body that creates the tides; hence the gradient term. But "gravitational gradient" is just a synonym for tidal force, so that part can be safely removed. Praemonitus (talk)


 * This sentence says very nearly the same thing as the preceding sentence. I think the sentence could safely be migrated, with any appropriate editing/tweaking, to an appropriate location within the body of the article. My suggestion just above covers, in simple language, the subject of 'rotation'.


 * Mmm, no not really. The "no net transfer" part is key to understanding the tidal locking state. You're not in a tidal lock unless things aren't changing in some fashion; that thing being the net transfer of angular momentum.

Thanks. Praemonitus (talk) 05:26, 2 July 2019 (UTC)


 * Lost internet, couldn't answer earlier.
 * I think your definition of angular momentum is helpful; however, I don't see the necessity at all of including in the Introduction "angular momentum" or "no net transfer", "gravitational gradient", etc. Those are terms found in a peer-reviewed science paper, not the introduction to a general interest encyclopedia article. If the full WP article were confined to the length of the Introduction, maybe such terms would have to be included. But we're not forced to put all such uncommon terms in the Introduction; they can be placed in the body, including the definition you've suggested.
 * Do you object to the use of "friction" or "slowing" in the Introduction? Angular momentum decreases as a result of friction, does it not? Are 'friction' and 'slowing' inaccurate oversimplications, or are they legitimate descriptions of reality? Does gravity create the 'gravity gradient'? Hence, gravity/gravitation is a perfectly acceptable word to use in the Introduction to identify the source of locking. The science journal terms are not necessary to provide a basic, correct description of the phenomena; they serve only to obfuscate simple concepts for general readers. They are clearly not in keeping with the policy text I quoted above; here's a reminder: "Introductory language in the lead...of the article should be written in plain terms". "Angular momentum", "gravitational gradient", "orbital eccentricity and obliquity" are decidedly not plain terms.
 * We've made some progress, and I appreciate your willingness to make some changes, but there is still more to be done to make the Introduction a lot more useful--and understandable--to the typical reader. Late now; I'll continue tomorrow (later today, actually). DonFB (talk) 10:59, 2 July 2019 (UTC)
 * I suppose you could use tidal friction, although friction isn't an accurate term. (It's more of an exchange of rotational/orbital inertia.) Tidal acceleration would be better. "Slowing" isn't accurate since the reverse is true and you can spin up a body. I hate using terms like that; they seem sloppy and ambiguous.
 * One could define "orbital eccentricity and obliquity" by a negative, as: either a significantly non-circular orbit or else a rotation axis well out of alignment with the orbital plane. But you can't do away with the meaning of these terms and still have a proper, accurate introduction. This is a technical subject and requires some understanding of what's going on physically. Praemonitus (talk) 15:03, 2 July 2019 (UTC)
 * "Tidal deceleration" might be useful to convey the issue to the uninitiated reader. Attic Salt (talk) 15:14, 2 July 2019 (UTC)
 * Also, I think we need to mention dissipation. This being necessary (as I understand it) for the orbits and rotations to settle into a stable locked state. Attic Salt (talk) 15:32, 2 July 2019 (UTC)
 * Dissipation is a factor for close orbits, in that it changes the rate at which tidal locking occurs. It's not necessary though for tidal locking to happen. Praemonitus (talk) 17:11, 2 July 2019 (UTC)
 * Okay, as per my understanding, and this source:, dissipation is needed to bring two objects into tidal lock. I'm not sure, but we both might be saying this. Attic Salt (talk) 17:51, 2 July 2019 (UTC)
 * True, it's probably not possible to have a tidal interaction without some amount of dissipation. Praemonitus (talk) 18:30, 2 July 2019 (UTC)
 * Well, interaction is one things, but tidal locking is another. For locking to be established, you need dissipation, otherwise, you just have oscillation.
 * Possibly. In the case of Mercury it's mostly due to a permanent deformation. Praemonitus (talk) 19:22, 2 July 2019 (UTC)

Praemonitus: I have seen "friction" used in non-academic sources. Is there not friction when tidal bulges are "dragged" from one place to another on the body? Regarding slowing: ok, tidal effects can accelerate as well as decelerate. This article is about locking. Does locking involve acceleration or deceleration? My guess is: deceleration. Is "slowing" another word for deceleration? Do you object to phrasing along the lines of: "slowing rotation until the body becomes tidally locked"? I repeat: is "gravitational gradient" caused by gravity?

Here's your sentence:

"This effect arises when the tidal force, or gravitational gradient, between the co-orbiting bodies, acting over a sufficiently long period of time, results in a sufficient net transfer of angular momentum to reach a state of tidal locking"

Here's mine:

"The effect arises when gravity, acting over a sufficiently long period of time, slows a body's rotation until it becomes tidally locked." (Or perhaps use "gravitational interaction")

More needs to be said to offer a thorough explanation, but do you claim my sentence does not describe the phenomenon?

Regarding my comment that two consecutive sentences seemed to say nearly the same thing. Here are the relevant phrases in those sentences: Sentence 1): "results in a sufficient net transfer of angular momentum to reach a state of tidal locking". Sentence 2): "the state where there is no more net transfer of angular momentum". Is this not redundant? One says: "sufficient"; the other says "no more". These sentences can be merged without much difficulty, I believe. Instead of using (and needlessly repeating) "angular momentum", the wording can be "slowing" (a reminder: this article is about locking, which--correct me if I'm wrong--refers to slowing rotation, not speeding up). "Orbital eccentricity and obliquity" can be substituted with "elongated" or "oval shaped" and "tilted on its axis" or wording very close to that.

Of course, there is a technical aspect to this subject, in some respects, highly technical. On the other hand, the English language is more than capable of communicating simple concepts (gravity, bulge, drag, friction, slow) using those plain, familiar and useful words. We are not strait-jacketed into using only the words found in academic sources and PhD dissertations. Think a little outside the box of your academic background and training (I'm assuming) and write as if you were explaining the subject to someone without such a background--which, in fact, is the readership of this site. All of the technical terms you prefer can be used in the article body; they are not needed or desirable in the Introduction, as WP guidelines that I've referred to and quoted explicitly explain. DonFB (talk) 19:26, 2 July 2019 (UTC)


 * No, the sentences are different because the first sentence describes how the system first reached that state, whereas the second is the test that shows it is remaining in a stable state. It's comparable to hiking to a cabin, then deciding to stay there rather than moving onward; the two acts are separate but connected. I suppose you could communicate the second by saying there is no net speeding up or slowing down after a complete revolution.
 * I suspect that in most cases the rotation is slowing, but there are cases where it needs to speed up. Retrograde orbits, for example, can lead to speeding up of an orbital period and so require increasing rotation to stay synchronous. But, yeah, slowing is probably good enough for most cases. Praemonitus (talk) 20:12, 2 July 2019 (UTC)


 * Refactor. Praemonitus, don't insert comments in the middle of another editor's comments. Place below, and refer to relevant text as appropriate. (Per Talk Page Guidelines: "Generally, you should not break up another editor's text by interleaving your own replies to individual points; this confuses who said what".)


 * The distinction you are attempting to make, as currently worded, between "how the system first reached that state" and "the test that shows it is remaining in that state" is invisible, at least to me, as a non-expert reader. Further, why try to make such a fairly subtle distinction in the Introduction? If a distinction is to be made between "reaching" and "remaining", you could do it with a simple statement that locking can be changed/undone/temporary [due to some cause] without repeating big chunks of the same jargony text in consecutive sentences.


 * In the Introduction, are you willing to accept this wording? -
 * "The effect arises when gravity, acting over a sufficiently long period of time, slows a body's rotation until it becomes tidally locked."


 * In the Introduction, are you willing to accept this wording? -
 * "In the special case where an orbit is nearly circular and the body's rotation axis is not tilted, such as for the Moon...."


 * Can the last sentence of the Introduction read:
 * "This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit, because..." [explain in plain English why "there can be some back and forth"]. What causes the "back and forth"? Variation in gravity effects? Variability in mass of a body?
 * DonFB (talk) 21:28, 2 July 2019 (UTC)


 * Para. 1: Well it's more awkward to respond that way, but I'll try.


 * Para 2.: Because it answers the question: "What does it mean to be tidally locked?" I.e. the whole point of the article. Synchronous rotation is only one case of tidal locking, so it needs a more general definition. I have to say I'm not really clear why you're not getting this, but it needs to be in there so I'll just keep saying as such.


 * Para 3.: I think I'd prefer "gravitational interaction".


 * Para 4.: I'd say "not significantly tilted". The Moon still has a 5° inclination.


 * Para 5.: For an elliptical orbit, the orbital velocity is not a constant. This means the rotation and orbital position get out of sync as the planet speeds up or slows down, until it finally gets back in sync after a full orbit. This happens even with the Moon (see libration). There's also the inclination of the object that can cause it to rock back and forth.


 * Thanks. Praemonitus (talk) 21:56, 2 July 2019 (UTC)


 * I'm not clear if your comment about "general definition" relates to my observation that two consecutive sentences are virtually indistinguishable. What do you want to say about "general definition" and where in the Intro do you want to say it?
 * Do you accept my proposed text....?
 * "The effect arises when gravitational interaction, acting over a sufficiently long period of time, slows a body's rotation until it becomes tidally locked."
 * Do you accept this proposed text...?
 * "In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as for the Moon...."
 * Regarding the following text....
 * "This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit...." What is the distinction between "rotation" and "spin rates"? What factors are "synchronized"? Or can the text instead say:
 * "This does not mean the exact same portion of a body always faces its partner. There can be some shifting due to variations in orbital velocity and inclination of a body's rotation axis."
 * DonFB (talk) 22:27, 2 July 2019 (UTC)


 * You did a good job of distinguishing the special case of synchronous rotation in the opening two sentences. But I still see a problem with the two sentences later in the Intro that virtually repeat each other, with excessive use of overly technical terms. DonFB (talk) 22:36, 2 July 2019 (UTC)


 * I'd like to resolve the last issue first. I'll try to simplify the two sentences:
 * As the two bodies gravitationally interact over many millions of years, tidal acceleration forces changes to their orbit and rotation rates. Once one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked.
 * Does that make sense now? Note that it is not saying that one face of the tidally-locked object always faces the other; only that it's rotation rate remains fixed. I.e. the tidal acceleration is no longer having a net effect. It could be spinning three times over the course of an orbit.
 * Your other wording seems okay, I think, but I'd like to see it all put together to check that nothing is being lost or confused in the process. Praemonitus (talk) 04:43, 3 July 2019 (UTC)


 * I like your rewrite of those two sentences. I'm offering a tweak to the first sentence, which contains "tidal acceleration". That phrase is a bit more understandable than some of the other technical terms I highlighted, but I think the phrase can be removed without harm, as shown below in proposed text with all the suggested changes. I've made some copyedits that differ slightly from the suggestions shown above. I'm not showing the first paragraph of the Introduction, which I think we're both satisfied with now:


 * "The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked.


 * Not every case of tidal locking involves synchronous rotation.[3] With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin-orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.[2][3][4] The exact same portion of the body does not always face the partner on all orbits, however. There can be some minor shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis."
 * DonFB (talk) 06:42, 3 July 2019 (UTC)


 * The last two sentences are not accurate; they were meant to apply to the general case rather than just synchronous rotation. But I suppose you could change the "The" to "In this case" and get rid of "minor".
 * I'd like to try and address Attic Salt's concern by adding a sentence at the end of the first paragraph above:
 * An object tends to stay in this state when leaving it would require adding more kinetic energy to the pair; energy that has already been lost through dissipation.
 * Will that work for you? Praemonitus (talk) 14:24, 3 July 2019 (UTC)

The sentence about kinetic energy and dissipation is not objectionable, but I think it is rather pedantic and seems to do no more than offer a kind of restatement of the definition of inertia (a body at rest....). It also seems to want to show off a specialized meaning of an otherwise ordinary word, dissipation. I looked at the Dissipation article, and it is no friend to an averge reader. As far as I'm concerned, the sentence is of insufficient importance to be in the lead, but could be slipped in someplace in the main body.

So, the final sentence, which I attempted to fix by writing about "shifting", needs a clearer version: "This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit, and there can be some back and forth transfer of angular momentum over the course of an orbit" In different, plainer words, what is this saying? Praemonitus, I think you've done excellent translations of the other problematic text in the lead; how about converting this one to ordinary English? I offered the idea of "shifting" exposure (of the hemisphere), but if that's not what it means, what does it mean? DonFB (talk) 05:12, 4 July 2019 (UTC)


 * Para 1.: No its not pedantic. If the dissipation wasn't there, the system would just keep slipping in and out of tidal locking − i.e. oscillating. Would it really be "locking" at that point? Nope. The locking state represents a kind of "local minimum" energy-wise, and the system stays in that state because dissipation soaked off the energy needed to escape. An example: you're driving along in a hilly area when the engine dies. Without friction your speed would be enough to keep going over the next hill. Instead, the friction on your tires dissipates your speed and you roll back and forth until you come to a stop at the bottom of the valley.


 * Para 2.: The wording is (mostly) fine. I just want to make it plain this only applies to the synchronous rotation case. I'll give an alternate example: say a planet is going around as star with an oval (elliptical) orbit. It becomes locked completing two rotations per orbit, and the locking occurs primarily because the planet returns to the same position when it is nearest to its star. (I.e. when the tidal force is at its strongest.) However, during the remainder of its orbit the same face is completely out of sync with its position along the orbit. Can you picture that?


 * Thanks. Praemonitus (talk) 13:51, 4 July 2019 (UTC)
 * I certainly agree with the point Praemonitus makes about dissipation wrt Para 1. Note purely gravitational interaction is just potential, and so without dissipation, as said, the system will just oscillate and not obtain locking. Attic Salt (talk) 13:58, 4 July 2019 (UTC)


 * The last sentence of the Intro says, in part: "there can be some back and forth transfer of angular momentum over the course of an orbit". Above, you gave an example of a change in orientation of a body in synchronous rotation ("face is completely out of sync"). In the last sentence, instead of saying "back and forth transfer of angular momentum", how would you phrase this phenomenon in real-world, plain English terms?
 * Regarding my comment about the pedantic explanation given in the suggested text: "An object tends to stay in this state when leaving it would require adding more kinetic energy to the pair; energy that has already been lost through dissipation."
 * The text could just as well say: "An object tends to remain locked unless it is influenced by more energy." Save discussion of "dissipation" for the article body. Though I question whether this seemingly trivial fact deserves mention in the Introduction.  DonFB (talk) 19:01, 4 July 2019 (UTC)
 * Para 1.: Just as I suggested: change the "The" to "In this case" and get rid of "minor".
 * Para 2–3.: No. It's not trivial, and your proposed wording just leaves a puzzling gap in the explanation.
 * Thanks. Praemonitus (talk) 20:41, 4 July 2019 (UTC)

Proposed wording with both inputs:

"The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. The object tends to stay in this state when leaving it would require adding energy back into the system that has already been lost through heat dissipation."

"Not every case of tidal locking involves synchronous rotation.[3] With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin-orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.[2][3][4] However, in this case the exact same portion of the body does not always face the partner on all orbits. There can be some shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis."

I tweaked a few of the words. Praemonitus (talk) 20:47, 4 July 2019 (UTC)


 * Looks good, but a question or two for clarification on the dissipation sentence. Is the phrase "when leaving it would require" intended to imply that "leaving" can never occur? If not so, wouldn't the wording, "unless energy is added back into the system" indicate that the condition is not necessarily permanent? The phrase "has already been lost" seems, again, to imply that new energy can never be acquired. How about "that was lost" instead? So my version would be: "The object tends to stay in this state unless energy is added back into the system that was lost through heat dissipation." DonFB (talk) 22:48, 4 July 2019 (UTC)
 * Where would this “new energy” come from? Fancy sources, like collisions, aren’t worthy of mention. Attic Salt (talk) 22:54, 4 July 2019 (UTC)


 * Of course, I don't know. So, what about "The body tends to remain in this state as a result of energy loss through heat dissipation." ? DonFB (talk) 23:06, 4 July 2019 (
 * Well the energy loss is in the past, so "loss" should really be "lost". Praemonitus (talk)


 * As an example: if a planet is in, say, in an elliptical orbit with a 2:1 spin-orbit resonance with its host star (2 rotations per orbit), over time the orbit can become more circularized, possibly allowing the planet to drift out of the tidal lock then later on migrate to a synchronous rotation. Another factor may be external perturbations, such as from a Jupiter-like planet. Stuff shifts around over long time periods. Praemonitus (talk) 01:48, 5 July 2019 (UTC)


 * "Lost" is fine with me. With that change, do you agree to the dissipation sentence I suggested? I think we are in agreement about everything else, based on the two paragraphs of text you proposed. Your explanatory comments just above are interesting and clear; I haven't scoured the article to see if such info is in it, but I'd suggest adding that info, in close to that easily-understood wording, in an appropriate place in the article. DonFB (talk) 02:04, 5 July 2019 (UTC)


 * Yes, the article is by no means complete and has much room for improvement. Praemonitus (talk) 14:22, 5 July 2019 (UTC)

I'd drop "losing" and "gaining" of energy, and rewrite the defining sentence as something like:

"Tidal locking occurs between two astronomical bodies when their mutual gravitational forces and internal thermal dissipation cause a synchronisation of rotation of one or both bodies relative to their orbits around each other."

This is possibly verbose, and would need tuning. I also think it is a problem that nowhere in the article is tidal heating mentioned. Attic Salt (talk) 13:12, 5 July 2019 (UTC)


 * One potential problem with your wording is that the 'synchronization' term may get conflated with synchronous rotation. That's something I keep trying to avoid here. Praemonitus (talk) 14:32, 5 July 2019 (UTC)
 * Okay, apologies, I'm just catching up on this. It is unfortunate that the work "synchronise" is reserved for a 1:1 spin-orbit resonance. And, having said that, I note that the Mercury artile refers to the "3:2 spin-orbit resonance". While it is perhaps not my favorite terminology, we might say something like:


 * "Tidal locking is a resonance between the orbit and rotation of one astronomical body about another caused by gravitational forces and internal thermal dissipation."


 * I'd prefer to say "planet" rather than "astronomical body", but Wikipedians get technical about what is and is not a "planet" (Pluto, for example). The problem is, that tidal locking doesn't apply to pairs of fluid bodies (like most stars). Attic Salt (talk) 14:55, 5 July 2019 (UTC)


 * It's not just moons or planets. Close binary star systems can become locked. Praemonitus (talk) 16:24, 5 July 2019 (UTC)
 * That's what the article asserts, yes, but the example is weak (defined by magnetic fields). My proposed sentence, is the main point of my comment, however, as I acknowledge technical exceptions and include "astronomical body" rather than "planet". So, what about the proposed sentence? Attic Salt (talk) 16:27, 5 July 2019 (UTC)
 * I'd probably say "stable resonance". Praemonitus (talk) 16:33, 5 July 2019 (UTC)


 * Works for me:


 * "Tidal locking is a stable resonance between the orbit and rotation of one astronomical body about another caused by gravitational forces and internal thermal dissipation."


 * Attic Salt (talk) 18:10, 5 July 2019 (UTC)


 * Ideally that would be the first sentence of the article, followed by the synchronous rotation case. The second paragraph could go down into the body at that point. Praemonitus (talk) 18:44, 5 July 2019 (UTC)
 * Yes, but if we use this first sentence, then the above drafts of the first paragraph need to be adjusted.

My one paragraph introduction:

"Tidal locking is a stable resonance between the orbit and rotation of one astronomical body about another caused by gravitational forces and internal thermal dissipation. Over long periods of time, often, millions of years, gravity and dissipation change the orbits and rotation rates of interacting bodies. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. For an object in a nearly circular orbit and having a rotation axis that is roughly aligned with its orbital axis, tidal locking results in the same hemisphere facing its partner.[2][3][4] The Moon is in such 1:1 spin-orbit resonance -- it keeps one face pointing (very nearly) straight at the Earth as it orbits the Earth every month. Not every case of tidal locking involves synchronous rotation.[3] Mercury, for example, has a 3:2 spin-orbit resonance -- it completes three rotations for every two orbits around the Sun."

Attic Salt (talk) 20:13, 5 July 2019 (UTC)
 * There's redundancy in the first three sentences and it barely mentions libration. I suppose the "very nearly" could be linked to libration. Praemonitus (talk) 01:26, 6 July 2019 (UTC)
 * Please feel free to revise. Attic Salt (talk) 01:44, 6 July 2019 (UTC)


 * The following comments are by User:DonFB.
 * I support the following existing (published) text of the first paragraph of the Introduction:
 * "Tidal locking (also called gravitational locking or captured rotation), in the most well-known case, occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. This is known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body.[1] However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon."
 * I don't support the recent proposal, just above, which begins: "Tidal locking is a stable resonance..." because the very first sentence begins and ends with scientific jargon. ("Tidal locking is a stable resonance"..."internal thermal dissipation").  I don't doubt its academic accuracy; I object to its reader-unfriendliness, a point I've been making in this thread, a point made clearly in the Manual of Style about the appropriate way to write an Introduction, including articles on technical subjects.


 * I support, with one qualification, the following text, previously proposed by Praemonitus in this discussion, for the second paragraph of the Introduction:
 * "The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. The object tends to stay in this state when leaving it would require adding energy back into the system that has already been lost through heat dissipation."
 * The qualification relates to the dissipation sentence, which I think needs clarification.
 * Earlier, I proposed the following revision:
 * "The body tends to remain in this state as a result of energy loss lost through heat dissipation."
 * This revision seems to have been supported by Praemonitus.
 * I support the following text, previously proposed by Praemonitus in this discussion, for the third paragraph of the Introduction:
 * "Not every case of tidal locking involves synchronous rotation.[3] With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin-orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.[2][3][4] However, in this case the exact same portion of the body does not always face the partner on all orbits. There can be some shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis."
 * These three proposed paragraphs contain only two terms that could be considered jargon: synchronous rotation and spin-orbit resonance. In both cases, the sentence immediately preceding or following the term helpfully defines it in context. DonFB (talk) 04:29, 6 July 2019 (UTC)

I get the sense that this is going to go around for a while longer. Don, you need to recognise that "The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked." is not an accurate sentence. While you might regard "thermal dissipation" as jargon, it is part of the process that causes tidal locking -- "gravitational interaction" does not, on its own, slow a body's rotation. Gravity can act on another body and cause oscillation, but the "slowing" part is caused by dissipation. Please consider reading Tidal heating and Tidal acceleration. You might also consider reading Harmonic oscillator. Attic Salt (talk) 12:44, 6 July 2019 (UTC)
 * The way I was thinking about this is the gravitational interaction creates the thermal dissipation, so the former is ultimately the root cause. The thermal dissipation as such is mentioned in the last sentence. But perhaps we can move that up a bit as follows:
 * The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. The object tends to stay in this state when leaving it would require adding energy back into the system.
 * Will this work? Praemonitus (talk) 14:35, 6 July 2019 (UTC)


 * Works for me, but I'll offer a suggestion:
 * "The object tends to stay in this state, because leaving it would require adding energy back into the system, which could happen, for example, if a Jupiter-like planet perturbs the object." I suggest the example, because the phrase "when leaving it would require" seems ambiguous: does it denote that the object can never leave, or does phrase imply a conditional situation (it can leave, if....)?
 * Attic Salt: I read the other articles you suggested (and, previously, Resonance and Orbital Resonance), but they don't offer any particular insight regarding my effort to convert the Introduction of this article from its overly dense, science jargon condition to an accessible, reader-friendly opening, as strongly encouraged for all articles in the Manual of Style and associated guidelines, and my own sense of what Wikipedia should do. DonFB (talk) 20:31, 6 July 2019 (UTC)


 * Para 1.: In my mind it's covered by the words "tends to", so I think we should leave that example out. Actually I think it would be more accurate to say:
 * The object tends to stay in this state when leaving it would require adding energy back into the system or else a change to the orbital configuration, which could happen, for example, if a massive planet perturbs the object.
 * I changed 'because' to 'when' because it's not a guarantee that enough energy has been lost to avoid a transition to a different state. Praemonitus (talk) 05:08, 7 July 2019 (UTC)


 * I don't disagree with you on the basic wording. To nitpick: the use of "else" anticipates a verb form, which is absent. The fix could be: "require adding energy back into the system or else changing the orbital..." Simply eliminating "else" would avoid the issue, and leave it as: "require adding energy back into the system or a change...."
 * A question about the information (which could affect the final wording): Your sentence makes a distinction between "adding energy" and orbital change due to a massive object. The massive object actually belongs in the category of "adding energy" does it not?
 * This could result in:
 * "The object tends to stay in this state after which [when] leaving it would require adding energy back into the system, for example, if a massive planet perturbs the object. Or, the object's orbit may migrate over time so as to undo the gravitational lock."
 * DonFB (talk) 08:52, 7 July 2019 (UTC)


 * No, the other is probably more likely:


 * "The object tends to stay in this state when leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object."


 * The point of the first sentence is that it is at a local energy minimum, thanks to the thermal dissipation. I can't say if energy is transferred to the object during the perturbation. It may just be lowering the energy barrier needed to leave the local minimum. Best not to speculate. Praemonitus (talk) 16:07, 7 July 2019 (UTC)


 * Ok, to try to tie up loose ends, what's an example of an event or circumstance that results in "adding energy back into the system"? DonFB (talk) 04:10, 8 July 2019 (UTC)
 * Do we really need to specify that? I think the point it probably isn't going to happen, at least not very readily. Attic Salt suggested an impact. I know a close binary star can exchange mass when its Roche lobe is full, potentially spinning up its locked partner. Do we care about that though? For the lead, I suggest we just leave it unanswered. Praemonitus (talk) 04:38, 8 July 2019 (UTC)

Ok, we have now: For the 2nd paragraph, Praemonitus' two most recent proposals, combined:


 * "The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. The object tends to stay in this state when leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object."

For the 3rd paragraph, this proposal:


 * "Not every case of tidal locking involves synchronous rotation.[3] With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin-orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.[2][3][4] However, in this case the exact same portion of the body does not always face the partner on all orbits. There can be some shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis."

I support both of these paragraphs, and the existing, published 1st paragraph. I propose that we make the edit. DonFB (talk) 04:56, 8 July 2019 (UTC)


 * Fine, let's get this done. There are other articles awaiting. Praemonitus (talk) 14:29, 8 July 2019 (UTC)
 * Done, and thanks for helping. References placed where previously located, though text changes cause some movement. Adjust, if necessary.
 * Yep, other articles awaiting translation of their Introductions from scientificese to plain language useful to everyday readers, our prime audience. DonFB (talk) 01:49, 9 July 2019 (UTC)
 * Add: The 1,562 byte reduction is due to deletion of hidden comments I previously inserted. DonFB (talk) 01:57, 9 July 2019 (UTC)

Locking of the larger body section
The section discusses how the tidal influence of the Moon is increasing the length of the Earth's day. Until today, that section said, "Earth's day lengthens, on average, by about 15 microseconds every year." That was edited to read, "Earth's day lengthens, on average, by about 15 microseconds every century." The source cited for that sentence, here, states, "after 1000 days our earth clock loses about 2.3 seconds." That figure does not support either the previous or current version of the rate of loss given in the article. This needs to be resolved. - Donald Albury 20:05, 13 January 2020 (UTC)

See https://en.wikipedia.org/wiki/Leap_second#Slowing_rotation_of_the_Earth — Preceding unsigned comment added by 74.188.144.252 (talk) 10:45, 30 January 2020 (UTC)


 * See my question at Talk:Leap second. Frankly, I find the figure of lengthening of the day by 25.6 seconds per century quoted there to be compativble with the number of leap seconds that have been added to UTC in recent decades. - Donald Albury 18:55, 30 January 2020 (UTC)
 * The source says 2.3 milliseconds per day per century, which I restored. Gap9551 (talk) 15:40, 23 March 2020 (UTC)