Talk:Tidal locking

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

Coming in from the page for Mercury, in reference to its tidal locking, leads you to this page. However, by this page's own definition, Mercury does not fit the explanation of tidal locking. Now, I don't debate that it is tidally locked, but my issue is that, as a casual reader entering this page, the clarity of specifically WHAT tidal locking is seems nonexistent. I've been up and down this entry and cannot find anything more specific than "tidal locking is when a body's orbit/rotation ratio is 1:1." This is clearly not the case for Mercury, or the other examples of bodies that possess a spin-orbit resonance other than 1:1, yet they are still referred to as tidally locked. The lunar example the article opens with is a case of tidal locking with synchronous rotation, but that still does not explain what tidal locking is, as clearly things can be tidally locked without having synchronous rotation or otherwise Mercury would not qualify. On top of that, the definition uses tidal locking to define itself, which is of course useless in explaining something to someone who doesn't know what it is to begin with - the second paragraph down states "The effect [of tidal locking] arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked." This reads, in my mind, like someone just said to me "Well, bodies become tidally locked when they become tidally locked," which doesn't actually define anything at all. So two issues I'm hoping can be addressed here for better reader understanding and experience, as a visitor to this page who just encountered these issues myself:

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)

My life exploded shortly after I proposed this edit and I completely forgot about it due to all the chaos... I'm laughing at myself for even asking but do we still have the go-ahead to make this change? Fatninjawalrus (talk) 05:11, 6 January 2022 (UTC)

The one suggestion I have is to break out the statement in parentheses from an already lengthy and dense first sentence. Thus:

Tidal locking 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. (This can also be called gravitational locking, captured rotation, and spin–orbit locking.) In the case where a tidally locked body possesses synchronous rotation, the object also takes just as long to rotate around its own axis as it does to revolve around its partner.

Praemonitus (talk) 14:39, 6 January 2022 (UTC)

I recently discovered that, to an approximation, a tide-lock moon in a mildly eccentric orbit will always present its face to the empty focus of the orbit. In particular, this is true of Io's obit around Jupiter and Luna's orbit around Earth. This is relevant the discussion on the movement of Earth from a Lunar observer.

I'm not competent to add a paragraph and explanation but it seems relevant to the page.

References are

+ https://space.stackexchange.com/questions/61755/what-use-if-any-does-the-empty-focus-of-an-elliptical-orbit-have-in-orbital

+ BBC TV programme on volcanic worlds (The Solar System) HaydonBerrow (talk) 15:08, 28 October 2024 (UTC)

Stack Exchange is not a reliable source for wikipedia articles (user-generated content). Schazjmd (talk) 15:13, 28 October 2024 (UTC)

More context in Introduction

I think it would be helpful to have more context in the Introduction. I'm not, however, qualified to write it. AFAIK, tidal locking is important in the search for habitable planets, especially those around Red Dwarves? It'd be nice if someone more qualified could write a few sentences in the Introduction spelling out the broader context of this mechanical phenomena. briardew (talk) 17:01, 12 September 2022 (UTC)

The lede is just summarizing what is in the article body, per WP:LEDE. You probably mean the red dwarf article, where it is briefly mentioned. Praemonitus (talk) 20:47, 12 September 2022 (UTC)

Axial parallelism

Tidal locking on the left. Axial parallelism is similar to the image on the right, except that with axial parallelism the satellite would be spinning fast

See the new article axial parallelism. It can be considered the "opposite" of tidal locking; I am not aware of any other forms of orbit for spinning bodies. Any help to build out the new article would be appreciated; it would be ideal if we could connect the two topics together in the way they are explained. Onceinawhile (talk) 07:58, 4 December 2022 (UTC)

"In the case where a tidally locked body possesses synchronous rotation…"

The lede describes a type of tidal locking as "In the case where a tidally locked body possesses synchronous rotation…" Is there any other type of tidal locking? As I understand it, the word tidal indirectly implies synchronicity. Onceinawhile (talk) 11:32, 4 December 2022 (UTC)

Yes. See paragraph 3 of the lede. Praemonitus (talk) 15:17, 4 December 2022 (UTC)

Earth as seen from the Moon...

Under the section about our Moon, the text states:

"When the Earth is observed from the Moon, the Earth does not appear to move across the sky. It remains in the same place while showing nearly all its surface as it rotates on its axis."

However the Moon undergoes libration and appears to wobble in the sky, so if you were to stand on the nearside of the Moon and crane your head up at the Earth, the Earth would move back and forth in the sky, along with moving up and down too (the lunar axis is not perfectly perpendicular to the lunar orbit). Inky Bendy (talk) 17:44, 15 February 2023 (UTC)

How noticeable would that be? Are you aware of a reliable source that comments on that? Without a reliable source, saying anything about that movement in the article would be original research. Donald Albury 18:57, 15 February 2023 (UTC)

His point is valid. There is a preprint available with some relevant information:

• Gorkavyi, Nick; Krotkov, Nickolay; Marshak, Alexander (2022), "Earth Observations from the Moon surface: dependence on lunar libration", Atmospheric Measurement Techniques, doi:10.5194/amt-2022-158.{{citation}}: CS1 maint: unflagged free DOI (link)

"... This causes the center of the Earth move in the Moon’s sky in a rectangle measuring 13.4°× 15.8°."

Unfortunately, it doesn't appear to have been published in a peer-reviewed journal yet. Praemonitus (talk) 00:01, 16 February 2023 (UTC)

This article in Sky&Telescope has a couple of time-lapse animations of the movement. I am impressed at how large the movement is. From positions close enough to the edge of Earth-side, Earth would set and rise each 'month'. Donald Albury 01:04, 16 February 2023 (UTC)

The moon is tide-locked to the empty focus of its orbit around the Earth. The second focus is approx 42,201 km from the earth's centre and the radius of the earth is 6,371 km so that's almost 14 radii from side to side. HaydonBerrow (talk) 15:16, 28 October 2024 (UTC)

Perhaps instead it could say something like the following:

"When the Earth is observed from the Moon, the planet moves around within a rectangular region determined by the Moon's libration. The Earth remains within this area while showing nearly all its surface as it rotates on its axis."

Praemonitus (talk) 04:49, 16 February 2023 (UTC)

I believe it would be more correct to say that the Earth viewed from the Moon moves in an elliptical path with a major axis of 15.8° and a minor axis of 13.4°. I am sure this is covered in some textbook or other scholarly work about the Moon, but I haven't found one yet. I also suspect that the major and minor axes vary over long periods of time due to variations in the Moon's orbital eccentricity and the precession and nutation of its rotation, but that may be more detail than is needed here. Added note: Very long term, the gradual increase of the distance between and Earth and Moon would also affect the size of the ellipse. Donald Albury 18:46, 16 February 2023 (UTC) 18:51, 16 February 2023 (UTC)

Or possibly a lissajous curve. But we need to stay away from WP:OR. I think a more detailed explanation really belongs on the libration article. Here it needs to be kept brief, because it's off topic. Praemonitus (talk) 20:00, 16 February 2023 (UTC)