# Talk:Tidal locking

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To-do list for Tidal locking: edit·history·watch·refresh· Updated 2019-07-06

 Here are some tasks awaiting attention: Expand : The impact of heat dissipation on tidal locking.Interactions that can end a tidal lock.Mention of tidal heating.Libration from ellipticity and obliquity.

## Well done

To all persons responsible for this article - it explains this phenomenon very clearly and simply. Take a bow, the lot of you. ElectricRay (talk) 13:24, 6 January 2014 (UTC)

• Good start, but there is a lot that is still unexplained. Like how does a planet like Mercury obtain a "tidal lock" to the Sun, having no surface fluids? Why is not every orbital element ultimately in tidal lock (or some other form of resonance) with its larger, nearest neighbor? (For example, why does the Earth have day/nite, yet Mercury does not?) ALL orbits are elliptical so I remain skeptical. Need some expert to follow up on this. There is much more to address in this comments section. --2600:6C48:7006:200:D84D:5A80:173:901D (talk) 02:54, 22 February 2018 (UTC)
• I think you're possibly mixing up the Earthy concept of ocean tides with tidal force in general. There's no mention of surface fluids in the article; just bulges and the forces acting on them. Ocean tides are just one example. The article does mention that tidal locking takes a long period of time to occur. Also, Mercury does experience day/night because it is rotating (slowly) with respect to the Sun. Praemonitus (talk) 03:49, 22 February 2018 (UTC)

## Final Configuration

"The simple picture of the moon stabilising with its heavy side towards the Earth is incorrect, however, because the tidal locking occurred over a very short timescale of a thousand years or less, while the maria formed much later."

Why does it matter when the maria formed? If the density of the moon shifts to a different side following tidal locking, wouldn't the moon re-orient itself subsequently? Afterall, it is a similar adjustment that causes the tidal locking in the first place, following perturbations in the shape of the moon. Could someone address this? —Preceding unsigned comment added by Blueil77 (talkcontribs) 00:21, 13 January 2008 (UTC)

## Misleading Sentence: Both bodies sychronize

It results in the orbiting bodies synchronizing their rotation so that one side always faces its partner

The above sentence implies that both bodies face their partner. But when one body is tremndously more massive than the other, isn't it only the tiny one whose rotation gets synchronized:

• moon and earth - moon faces earth, but earth rotates 28 extra times per lunar rotation/revolution
• mercury and sun - mercury's rotation synchronized in a 2:3 harmonic to its orbit period, while suns rotates considerably faster (?)

--Ed Poor

I think the fact that it says, "one side", implies that only one of the pair will show the same face at all times. Maybe this could be changed so it's more explicit?
The discussion, the way it is formed, is so unclear to me. The moon does rotate once everytime it circles the earth, correct? Doesn't that mean the same side of the moon always faces the sun, not the earth? One side of the moon is always in the dark to us and one side is always in the sun? Please help me here...
Cvgittings (talk) 05:03, 10 December 2010 (UTC)
--Don
It is mentioned later-on that Earth is still slowing down its rotation due to its interaction with the moon. —Preceding unsigned comment added by Blueil77 (talkcontribs) 00:25, 13 January 2008 (UTC)
The usage of the term "synchronized" implies that the moon just happens to spin at the right rate to always face its same side to the earth, which is not the case. It is tidally locked because one side is more dense, which is the side that always faces us. Would it be correct to describe an "orbiting" ball tethered by a string to a focal point, synchronized? In my opinion it wouldn't be.
All I am saying is that its misleading to what is actually* happening with tidal locking between the earth and moon. Its not a coincidence, like describing it as synchronized suggests.
No, "synchronize" in this case means that a body's the rotational period becomes equal its own orbital period. It does not imply a coincidence. --JorisvS (talk) 10:05, 1 May 2013 (UTC)

## Tau Boötis is known to be locked to the close-orbiting giant planet Tau Boötis Ab

This sounds like BS. Could someone please provide a reference to this statement, or at least describe how one could obseverve that an extra-solar planet is indeed tidally locked? Lunokhod 23:52, 24 January 2007 (UTC)

Presumably a tidal locking timescale has been estimated, and it's much much less than the known age of the system? By the way, what's BS stand for (sorry)? Deuar 15:32, 25 January 2007 (UTC)
Then perhaps we should move this entry from "List of known tidally locked bodies" to "Bodies likely to be locked". Either there has been some amazing advancement in imaging extra solar planets that I am ignorant of, or this is only an inference! Also, if the planet is a gas giant, then the Q and k2 will be much different than for a solid object. Perhaps this could be discussed in the article? A quick google search suggests that the Q of jupiter is 1 billion, as opposed to ~100 for the Earth. And this would increase the tidal locking timescale accordingly. Lunokhod 15:54, 25 January 2007 (UTC)
Go for it, I reckon. Deuar 16:21, 25 January 2007 (UTC)

## Mercury

So...Mercury isn't tidal locked to the Sun? Then why is it on the list? --MPD T / C 02:43, 1 March 2007 (UTC)

I think that tidal locking is not the same as "synchronous rotation", even though the intro seems to say so. Perhaps it is better to say that tidal locking is a process where tidal torques leave one body on a spin-orbit resonance. Synchronous (1:1) is the lowest energy configuration. Lunokhod 13:04, 1 March 2007 (UTC)

## Tidal braking

In the UTC article, there's a [[tidal locking|tidal braking]] link. Can someone please provide a definition for tidal braking in this article, even if they are the same? The "Locking of the larger body" paragraph in the "Mechanism" section seems an appropriate place to do this. Thanks. Xiner (talk, email) 01:22, 11 March 2007 (UTC)

Actually, I think the tidal acceleration article seems to discuss this in more detail. I've linked it to there instead. Deuar 15:55, 19 March 2007 (UTC)

## Unclear Description of Orbital Resonance

Under "Mechanics," the description of orbital resonance is rather unclear. This is what it says:

Rotation-Orbit resonance: Finally, in some cases where the orbit is eccentric and the tidal effect is relatively weak, the smaller body may end up in an orbital resonance, rather than tidally locked. Here the ratio of rotation period to orbital period is some well-defined fraction different from 1:1. A well known case is the rotation of Mercury—locked to its orbit around the Sun in a 3:2 resonance.

It does not specify why or how this happens. I don't know myself, but I'm guessing it happens because the smaller body's rotation does not change when it reaches the aphelion of its orbit, thus causing it to skip ahead. However, my guess does not explain

A. why the opposite does not happen at the perihelion, thereby nullifying the effect, or

B. why this would cause resonance to occur in well-defined ratios such as 3:2 (in the case of Mercury).

I would find it very helpful if a little more research were done and this paragraph were revised. I myself would not know where to look, and the page on orbital resonance does not seem to describe rotational resonance at all.

## Tidal locking and developing life

I removed the following from the "planets" section:

Tidally locked planets may present problems for developing life, as one side of the planet will always be facing away from the star and the other side will always face toward it; in the absence of significant heat redistribution by atmospheric winds or hydrospheric currents, this would result in constant temperature extremes. [citation needed] On the other hand, tidally locked large satellites of gas giants rotate with respect to the central star, providing places for developing life that avoid extremes in temperature.[citation needed]

Besides that being mere speculation, and being without any references, it also mentions "large satellites of gas giants" which are clearly not planets. If this text should be on this page at all, then please in a separate section, and with references. Jalwikip (talk) 14:10, 19 November 2007 (UTC)

It's back with a reference to a badly written anonymous paper on arxiv.org. Deleting. Oumot (talk) 00:17, 13 December 2017 (UTC)

## Isn't the animation confusing?

The animation shows two bodies orbiting a central body at the same rate, yet they are at different distances from the central body...what is the point of this?, It might lead some people to think that this is a real orbital configuration...Jellyboots (talk) 20:55, 29 January 2009 (UTC)

## 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)

## Final configuration

I took this out. It has no sources and I don't think I believe it. Does anyone speak for it?

There is a tendency for a moon to orient itself in the lowest energy configuration, with the heavy side facing the planet. Irregularly shaped bodies will align their long axis to point towards the planet. Both cases are analogous to how a rounded floating object will orient itself with its heavy end downwards. In many cases this planet-facing hemisphere is visibly different from the rest of the moon's surface.
The orientation of the Earth's moon might be related to this process. The lunar maria are composed of basalt, which is heavier than the surrounding highland crust, and were formed on the side of the moon on which the crust is markedly thinner. The Earth-facing hemisphere contains all the large maria. The simple picture of the moon stabilising with its heavy side towards the Earth is incorrect, however, because the tidal locking occurred over a very short timescale of a thousand years or less, while the maria formed much later. The maria are instead formed from heavier lunar magma that responded to the tidal lock by gravitating towards the Earth.

William M. Connolley (talk) 18:39, 9 March 2010 (UTC)

I speak for it and so does Gravity-gradient stabilization. Tidal locking forces are proportional to the spin angular velocity difference between the bodies orbital velocities. As the locking body slows down so the locking force reduces. This continues until when you take the limit no locking force exists. So, at the end there is no phase lock information remaining. Between the moon and Earth there is a fixed phase (empirical). This phase lock is occurring and it cannot be from tidal locking. As William describes above the moon cannot have symmetrical weight. Hence the heavy side faces the gravitational attractor i.e. the Earth.

We should add a section on phase locking which is as a result of Gravity-gradient stabilization.

User:pcrengnr —Preceding undated comment added 16:56, 24 March 2016 (UTC)

## Side?

Since when does a spherical object have "sides"? --77.109.223.37 (talk) 07:56, 15 June 2010 (UTC)

Since there are defined features on that sphere, that's when. —Preceding unsigned comment added by 129.186.253.87 (talk) 19:45, 27 August 2010 (UTC)

In the section "Timescale", I think the second ("simplified") formula given for tlock

${\displaystyle t_{\textrm {lock}}\quad \approx \quad 6\ {\frac {a^{6}R\mu }{m_{s}m_{p}^{2}}}\quad \times 10^{10}\ {\textrm {years}}}$

is incorrect, if the first formula (${\displaystyle t_{\textrm {lock}}\approx {\frac {wa^{6}IQ}{3Gm_{p}^{2}k_{2}R^{5}}}}$ ) is right. Namely, the following conclusion drawn from the second formula

One conclusion is that other things being equal (such as Q and μ), a large moon will lock faster than a smaller moon at the same orbital radius from the planet because ${\displaystyle m_{s}\,}$ grows much faster with satellite radius than ${\displaystyle R}$.

contradicts the first formula: There, the satellite mass only appears in the numerator of the formula given (via the "Inertia momentum" term). If we assume the satellite mass to roughly increase at the third power of its radius (i.e. assuming constant density, which seems a plausible assumption), we get five powers of R in both the numerator and the denominator of the fraction. Thus, tidal locking should essentially be independent of the satellite's mass, all other things (except the radius) being equal. Can someone clarify this for me? --Roentgenium111 (talk) 21:43, 23 August 2010 (UTC)

♦ I agree that there is something wrong with the ("simplified") formula. Earth's moon was tidally locked by the time of the Lunar Cataclysm, but plugging in the values for the Earth/Moon system in the ("simplified") formula gives a time to lock of 2.3 × 1031 years which we know cannot be true. The time to lock the Earth/Moon system must be less than the difference between the time of formation of the moon (~4,450 million years ago) and the time of the Lunar Cataclysm (~3,900 million years ago) which is approximately 550 million years. -az — Preceding unsigned comment added by Sciencebookworm (talkcontribs) 17:33, 9 March 2011 (UTC)

I don't know how you got that high a figure! I calculated 3.8 million years for the moon tidally locking to Earth. (using the same simplified formula). If anything the simplified formula seems to underestimate the tidal locking times, by a factor of 100, particularly for planets. When I multiply the results by 100, I get - 384 millions years for the moon - 5.1 billion years for a planet in the habitable zone of Epsilon Indi - 543 billion years for the Earth to the sun. The results I get after multiplying the simplifed formula in the article by 100 are very similar to the results from other formulae for calculating tidal locking time. For example, 'Tidal Locking Time in years = (ρ*((a/0.0483)^6))/(M^2)' - where a = distance in AU from primary, M - Mass of primary, as fraction of the sun (eg -Epsilon Indi M=0.762), ρ (density of satellite, Earth taken for Epsilon Indi= 5512 Kg/m^3 or for the moon= 3346 Kg/m^3). Using that formula I get - 4.1 billion years for Epsilon Indi- 433 billion years for the Earth - 8.4 million years for the Moon to tidally lock to Earth, almost the same values. Or, using the formula taken from 'Peale et al - 1977'. 'Tidal Locking Time in Seconds = (a/(0.027*(M^(1/3))))^6/486' - using CGS units where M= Stellar Mass (grams) a= orbital distance (cm), gives a tidal locking time of 477 billion years for the Earth to the sun and 5 billion years for an exo-Earth around Epsilon Indi, again almost the same values, if you multiply the result by 100. I think a lot of this article is more or less photocopied from a book called "Habitability and cosmic catastrophes" By "Arnold Hanslmeier" "2009" so the formula probably must be accurate, in some way. — Preceding unsigned comment added by 86.185.215.187 (talk) 01:20, 29 November 2011 (UTC)
Your comment seems correct, the formulas are not contradictory. I derived the simplified formula from the first one. One needs to use ${\displaystyle k_{2}\approx 3/2/(19\mu /(2\rho gR)),g=Gm_{s}/R^{2},\rho =m_{s}/(4/3\pi R^{3}),I=4m_{s}R^{2}/10}$. Approximation for the Love number seems OK, since it is usually much smaller than 1[1]. If I input μ for rocky planet, 3×1010 Nm−2 and original ω = 1/3600/24 rad, Q = 100, I get 3.8 million years. The prefactor ${\displaystyle 6*10^{10}\;\mathrm {kg} ^{2}/\mathrm {m} ^{6}\mathrm {s} ^{2}\mathrm {year} }$ should be probably different to give more realistic results, but from the direct calculation, I obtained ${\displaystyle 3*10^{10}\;\mathrm {kg} ^{2}/\mathrm {m} ^{6}\mathrm {s} ^{2}\mathrm {year} }$, which is very similar. My personal guess is, that Q = 100 from the referenced article, is in CGS units and it is not valid in SI units. But the problem can be elsewhere. Irigi (talk) 09:09, 8 October 2014 (UTC)

Another issue with the Timescale portion as it stands, the tidal locking formula description shows a value for the mass of the satellite, but that value does not appear anywhere within the formula given. As the mass of the planet is given as a squared value, is this assuming the mass of the planet and satellite are the same? — Preceding unsigned comment added by 205.166.76.15 (talk) 18:25, 18 July 2014 (UTC)

## Kant?

I'm dubious about [1]. "researched" appears a little over the top - speculated, or reasoned, might be closer. But the text too is uncertain: according to [2] Immanuel Kant, who took great interest in scientific issues, reasoned on the basis of pure theory that the action of oceanic tides must slow down the earth's rotation which isn't what the retarding forces of tides on satellite bodies says William M. Connolley (talk) 17:12, 29 September 2010 (UTC)

It isn't what Immanuel Kant says, either William M. Connolley (talk) 17:14, 29 September 2010 (UTC)

I agree with you and am removing the sentence. Not only is the way the claim is phrased dubious, the source does not show he was the first either. —Lowellian (reply) 03:27, 1 October 2010 (UTC)

## Orbital changes

Can someone expand on this section, specifically what mechanism is responsible for an orbiting body moving farther from its parent as its rotation slows? The explanation that angular momentum is conserved just doesn't do it for me. An explanation of the forces that cause a body to speed up in its orbit like the preceding sections on Bulge dragging and Resulting torque would be nice. 128.32.99.173 (talk) 15:16, 5 November 2010 (UTC)

Sorry, but it is *precisely* the conservation of angular momentum that explains why this happens. We have various phenomena, and we have the principle that angular momentum is conserved. We see that the phenomena are aligned with the principle. It is common -- though incorrect perhaps -- to say that "aha, that is why X occurs, it is due to the conservation principle" but as far as a mechanism that aligns the phenomenon with the principle -- there isn't one that is known. In the same way, various phenomena that align with the principle that (global) entropy for a system tends to increase (but local entropy may decrease if it contributes to global entropy) lacks a mechanism. Also, things like Gauss's law -- that the distribution of charge in a large body tends to be such that the positive charge is on the surface (but the net charge for the body is still zero) lacks a mechanism of the detailed sort that you seek. Get used to it. Chesspride 172.164.20.73 (talk) 21:55, 9 February 2016 (UTC)

## Time Scale Formula Useless

The timescale formula given is useless, because without units the value is meaningless. Iæfai (talk) 03:18, 20 April 2011 (UTC)

Pretty sure it is in seconds. — Preceding unsigned comment added by 149.169.221.113 (talk) 18:46, 25 September 2012 (UTC)
If the dimensionality of the formulae were correct (that is, if they were in units of time), you could use whatever units you wanted. However, the "complex" formula gives an answer in units of angle * time, while the "simple" formula gives an answer in units of distance^6 / mass^2 * time. As they are, both formulae are thus completely useless. 76.255.189.2 (talk) 20:51, 24 September 2013 (UTC)

## Rotation of the locked object

It can be quite easily demonstrated that the apparent rotation of the moon about it's axis is actually a necessary result of the moons orbit and not an independent source of rotation, meaning that it is technically incorrect to say that the rotational period of a tidally locked object precisely matches it's orbital period, since it has no rotational period independent of it's orbital motion. Tidal locking actually slows the orbiting object's rotation until it completely stops rotating around it's central axis. DoC352 (talk) 07:24, 9 August 2011 (UTC)

Based on DoC352s' statement above, the following sentence from the first paragraph, "A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner.", is incorrect. As a result I am editing this sentence to say, "With Tidal Locking the smaller body does not rotate around its own axis as it revolves around the larger body. This is completely different than Tidal Resonance. With Tidal Resonance the smaller body does rotate around it's own axis as it revolves around the larger body.". There are some other statements about the Moon rotating further down in the article that I am also removing. If you disagree with these edits please let me understand your reasoning. Cvgittings (talk) 20:01, 9 March 2012 (UTC)

Phancy Physicist (talk) 18:56, 10 March 2012 (UTC)
Unfortunately this explanation is inadequate. Consider for instance a plane flying in a complete circle around earth at a specific altitude. As it flies "straight ahead" much like we might assume the momentum of the moon does, the gravity of earth causes the plane to fly around the curvature of the earth with the bottom of the plane always parallel to the surface of the earth. Does the plane therefore rotate about its own axis with the exact periodicity of its revolution around the earth? Such a conclusion seems to represent a fundamental misunderstanding of the difference between a geometric rotation and translation. When an object geometrically rotates about a distant central point, it maintains its same facing towards the central point without any separate rotation about it's own axis, much as we see with the moon relative to the gravitational pull from the Earth's center of mass. Alternatively, if it could be shown that gravity results in a translation rather than a rotation (i.e. if it can be shown that some force other than gravity acts on the plane to make the nose come "down" as it attempts to fly in a straight line out of the atmosphere, and that gravity from the earth alone would allow the plane to absurdly maintain the same absolute rotation relative to its starting point) as an object maintains forward momentum through the gravitational field, then Phancy Physicist's argument would be perfectly valid. DoC352 (talk) 04:13, 3 March 2013 (UTC)
First of all, the plane is not bound in it's path by gravitational forces. Or not only gravitational forces are keeping it in the sky. Second of all, rotation in mathematics has nothing to do with orbital mechanics. Rotation, as defined in the linked article, is used there as math jargon. Rotation as used here is used as astronomy jargon - i.e. there is a clear distinction between rotation and revolution. Furthermore, the geometrical rotation you're talking about requires a rigid connection between the rotating body and the rotation axis - i.e. all points of the rotating body must maintain the same distance to the point of rotation. With gravity, that's not the case. Finally, if you have a fixed (i.e. non rotating) reference frame, you can decompose the motion of the rotating body in two in two separate rotational motions - one about the central axis, and one about the body's central axis. Tidal locking means that these two motions are synchronized - that is they have a fixed ratio between the periods.95.76.220.229 (talk) 14:26, 7 January 2015 (UTC)Apass

## Effect of composition and structure

In the article it says "μ can be roughly taken as 3×1010 Nm−2 for rocky objects and 4×109 Nm−2 for icy ones.". What would the effect of a more or less substantial atmosphere, of (bodies of) surface or subsurface liquid(s), or of a body being mostly metallic be on this value? --JorisvS (talk) 12:39, 19 September 2012 (UTC)

## Tidal locking in gas giants

Many of the extrasolar gas giants that were detected are very close to their primaries and are assumed to be tidally locked. But how this tidal locking is defined for planets that don't have fixed surface features (i.e. they don't really have a surface)? And what would be the effects on the planet? Visibly, I guess the planet would look pretty much as any other gas giant, with no visible cue that it's tidally locked. Apass 89.137.186.101 (talk) 20:25, 9 November 2012 (UTC)

Defining tidally locked rotation is, in principle, still straightforward for gas giants: Their rotation period must be the same as their orbital period. However, defining what is 'the' rotation period of a gaseous planet is far more tricky: The rotation period of Jupiter's cloud tops seems to be what is usually considered its 'rotation period', but this gives it an equatorial rotation period and a polar rotation period (Jupiter#Orbit_and_rotation). Tides will tend to synchronize a planet's rotational angular momentum to its orbital angular momentum, no matter its composition. Given that a gas giant's 'rotational period' is usually taken to be that of the visible clouds, other effects could affect a hot jupiter's apparent rotational period, possibly resulting in an apparently unlocked state. Strong winds resulting from high temperature differentials would likely be a factor here. --JorisvS (talk) 00:09, 13 November 2012 (UTC)

## Oberon?

How come Oberon is both on "Locked by Uranus" and "Probably locked by Uranus" lists? — Preceding unsigned comment added by 90.151.131.81 (talk) 20:20, 28 March 2013 (UTC)

## Language section

I've tried hard to change the Arabic corresponding language but received errors of usage by another item. The new way of modifying languages has made Wikipedia more complicated that before :(. Please the corresponding article in Arabic is ar:تقييد مدي--Almuhammedi (talk) 11:27, 22 July 2013 (UTC)

Done, though I'm not sure this is the correct page to make such a request. — Reatlas (talk) 12:11, 22 July 2013 (UTC)

## length of lunar month is getting shorter?

Under "Locking of the larger body", it says "Given enough time, this would create a mutual tidal locking between Earth and the Moon, where the length of a day has increased and the length of a lunar month has shortened until the two are the same.". Isn't the length of a lunar month actually getting longer? For example: [2] RDV74 (talk) 04:26, 7 February 2016 (UTC)

## Definition of tidal locking

According to:

Heller, R.; et al. (April 2011), "Tidal obliquity evolution of potentially habitable planets", Astronomy & Astrophysics, 528: 16, arXiv:1101.2156, Bibcode:2011A&A...528A..27H, doi:10.1051/0004-6361/201015809, A27.

"A widely spread misapprehension is that a tidally locked body permanently turns one side to its host." Further, "As long as 'tidal locking' denotes only the state of dωp/dt [rotation rate change] = 0, the actual equilibrium rotation period ... may differ from the orbital period, namely when e [eccentricity] ≠ 0 and/or ψp [obliquity] ≠ 0." This statement differs from the definition in the lead of this article. Praemonitus (talk) 19:51, 7 April 2016 (UTC)

## Regarding bodies A and B in the mechanism section

The mechanism section employs bodies A and B to explain the phenomenon and does a good job at it. The explanation can be improved if there was a figure depicting the two bodies A and B (or marked in the figure on the right - I hope that the person who updated this had such a figure in mind?). Also, what does red line depict in the figure? — Preceding unsigned comment added by Blackholebounce (talkcontribs) 19:06, 3 March 2017 (UTC)

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## Definition of tidal locking

Per Heller, R.; et al. (April 2011), "Tidal obliquity evolution of potentially habitable planets", Astronomy & Astrophysics, 528: 16, arXiv:1101.2156, Bibcode:2011A&A...528A..27H, doi:10.1051/0004-6361/201015809, A27.

"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 dωp/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 ψp ≠ 0. Only if ‘tidal locking’ depicts the recession of tidal processes in general, when e = 0 and ψp = 0 in Lec10’s model, then ωp = n. As given by Eq. (23), one side of the planet is permanently orientated towards the star if both e = 0 and ψ = 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 ψp are not eroded, however, the planet can be prevented from an ωp = n locking. In the CPL model, however, the equilibrium rotation state is not a function of ψp, thus ‘tidal locking’, denoting dωp/dt = 0, indeed can occur for ψp ≠ 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. <...> ωp = 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) "dωp/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 ψp 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)

## References

1. ^ B. Gladman; et al. (1996). "Synchronous Locking of Tidally Evolving Satellites". Icarus. 122: 166. Bibcode:1996Icar..122..166G. doi:10.1006/icar.1996.0117. Explicit use of et al. in: |author= (help) (See pages 169-170 of this article. Formula (9) is quoted here, which comes from S.J. Peale, Rotation histories of the natural satellites, in J.A. Burns, ed. (1977). Planetary Satellites. Tucson: University of Arizona Press. pp. 87–112.)
2. ^ https://en.wikipedia.org/wiki/Lunar_month#Cycle_lengths

## 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

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)

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)

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.[3] 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: [4], 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.[5] 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?

"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#Source does not support statement in "Slowing rotation of the Earth" section. 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)

## Tidal locking/spin-orbit resonance as too vague a definition

1. A more concise definition of the actual title the article is defining, tidal locking, within the opening intro and summary, for increased accessibility to readers. Layman's terms, what determines when a body is tidally locked, and what specifically is it? The article uses synchronous rotation to define it by example, but that is clearly an optional aspect of tidal locking, since there are bodies that are tidally locked that are not in synchronous rotation. This makes the current definition vague and confusing. Proposed tidal locking definition, if I understood it correctly, in potentially more user friendly form: "Tidal locking is a term used to describe when an astronomical body's rotation rate during the course of a complete orbit does not change. That is to say, the rate of rotation is neither accelerating nor decelerating, but remains constant. (Compare this to Earth, where the rotation rate slows by 1.7ms every century)"

2. It seems as though tidal locking, spin-orbit resonance, and synchronous rotation all have slightly varying but specific definitions and applications, but all redirect here and are lumped under one term within the opening paragraph, which could lead to confusion and inaccuracy. Are the latter two variations? Subclasses? Potential aspects? Do they deserve their own entries? Clearly you can have combinations with varying spin-orbit resonances, as with Mercury to the Sun (tidally locked but non-synchronous rotation, 3:2) and the Moon to Earth (tidally locked with synchronous rotation, 1:1). This lends itself to needing more specificity regarding each.

Fatninjawalrus (talk) 10:51, 29 May 2020 (UTC)

We had a proper definition, but then an editor insisted that we simplify and rewrite the lead, and thus it has been watered down. Here is the proper definition. Praemonitus (talk) 21:09, 29 May 2020 (UTC)
I am the editor who urged that the lede be revised for understandability to the general reader. Currently, the 3rd sentence of the 2nd paragraph provides a definition that seems both appropriately generalized and precise: "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". This sentence could be repositioned closer to the start of the lede section, if its placement now is deemed too far from the beginning. DonFB (talk) 03:09, 30 May 2020 (UTC)
I hate to discredit anyone's hard work here but the old opening was so much more direct and understandable in my opinion. It hits the definition and the cause directly and concisely, links to angular momentum for clarity, and then immediately references the most well known example (the Moon). This helps readers to relate it to a real life example but still ensures that by defining synchronous rotation directly afterwards as a potential aspect of this process, they realize that it will not always be the case. As someone who literally came here to learn about an aspect of orbital mechanics I was less familiar with, I have to say that I'd have preferred an encounter with the original definition and layout any day. The current definition left me confused and frustrated. If you're looking to make it more accessible, would anything about my attempt to define above it help, or is my understanding of the definition still not quite accurate? Fatninjawalrus (talk) 03:38, 30 May 2020 (UTC)
I don't object to a general definition preceding a special-case definition. My original objection, however, still exists to the wording and scientific jargon that made the overstuffed first sentence in the previous version very reader-unfriendly. Since you appear to be new to Wikipedia, you may be unaware that site guidelines strongly encourage the use of plain language in the opening section of articles about technical subjects, reserving more specialized language and scientific terminology for the body of the article--a philosophy I strongly support. Our experiences appear to be rather divergent regarding the usefulness of the old version. DonFB (talk) 04:58, 30 May 2020 (UTC)
Yes, I agree that the first sentence should provide a succinct definition. Something like this:
Tidal locking (also called gravitational locking or captured rotation) between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. The best-known example is when an orbiting body always has the same face toward the object it is orbiting. This case 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.
Praemonitus (talk) 13:18, 31 May 2020 (UTC)
Seconded, the accessibility of the article in large part comes from the strength of the opening couple of sentences and how clear they are. Getting to the definition later just doesn't flow as well. I'm worried about having two sentences back to back with "example" in them like that though, as the next sentence in the article uses "For example" to lead into discussion about the Moon. I'd propose:
Tidal locking (also called gravitational locking or captured rotation) between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the special case in which a tidally locked body possesses synchronous rotation, the body also 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.
to avoid a "best-known example, for example" kind of scenario. I don't quite like how fast it jumps into synchronous rotation but I can't think of something better currently. It just feels like it needs a small sentence after the first one for more clarity and separation of the terms in there. Thoughts?
I did remember the plain language rules and I didn't personally feel the original was overly technical, just perhaps a bit wordy. The overall structure there felt better to me personally as it introduced concepts a little less rapid-fire. But I am indeed new here and so I do greatly appreciate reminders of guidelines - help is welcome. My intent here is simply to improve user experience on this page, and do so without being a pain. Let me know if I overstep. 2606:A000:EB44:8300:65E2:6464:FA3F:59EA (talk) 09:26, 3 June 2020 (UTC)
Well it's not really a special case; that's actually the most stable form, and orbital systems tend to evolve to that low-energy state over long enough periods. Perhaps it would work without the word 'special'? Praemonitus (talk) 14:11, 3 June 2020 (UTC)
I like the version you offered above, introduced by: "something like this". DonFB (talk) 04:49, 4 June 2020 (UTC)
Either sounds good to me, go for it. I only used 'special' to try to clearly differentiate it from the base scenario, since they're not always together and I feel it's important to maintain that distinction. I guess "In the specific variation in which" might work better if you want to go that route.
2606:A000:EB44:8300:842B:D34E:ABAE:14A5 (talk) 04:39, 7 June 2020 (UTC)