Talk:Libration

Diurnal Libration

I finally uploaded and now linked an image of the lunar diurnal libration that I observed in 2018. I also have an anaglyph version and a cross-eye version that actually show that one can create a 3d-image from these two photos, but I thought the animated gif going back and forth between the evening and morning image was best to illustrate the diurnal libration itself.
KJ, 2024-05-06

(Analemma)

Looks like the cross-hairs, in the center of the image, etch out the shape of an "Analemma" over time
—Preceding unsigned comment added by 136.1.1.101 (talk) 20:41, 14 December 2004

Molecular libration

The article would benefit if molecular libration was also explained. --Mfrosz 06:44, 25 July 2006 (UTC)

No, that's a separate topic, now moved to Libration (molecule).
--Jerzy•t 07:06, 6 September 2010 (UTC)

(Adjustments to visible area)

59%

It would be nice if the article stated where this number comes from. I'd have a stab at some approximate calculations, but I'm busy right now. Shinobu 16:14, 14 April 2007 (UTC)

This number doesn't come with any accompanying calculations because I am quite sure it'd be a complex formula. Both NASA and ESA use this number in their moon info publications, so I imagine that should satisfy any question of the number's value. As for how it's calculated, maybe contact a boffin at NASA. Links: ESA - Moon and NASA - Moon. 130.102.44.52 (talk) 04:55, 14 October 2008 (UTC)

It's probably an integral of some sort, knowing the horizontal and vertical variations. Tom Ruen (talk) 15:44, 14 October 2008 (UTC)

50%

Also, because the ratio of the moon's diameter to its distance from earth is non-negligible, at any given moment an earth observer can see slightly less than half the surface. —Preceding unsigned comment added by 131.193.179.146 (talk) 03:10, 4 October 2007 (UTC)

   This is not an important effect: according to the Wolfram Alpha page "Angle subtended by the Moon when viewed from the Earth", that number was less than 0.6 of a degree at the time the server prepared its response to me (it should vary ±10% over the month), and my view of a full moon would have been short of 180° by that small angle. The full moon loses illumination (at 360° per lunar cycle) from about that much of its surface in about 40 minutes. (The effect thus entails making the apparent full moon about 40 minutes longer!) This is not at all relevant to how much of the moon's surface can be mapped without sending something far enuf out: the mapping from earth is cumulative, and the diurnal libration is sufficient to wipe out this parallax-related issue's effect on mapping.
   Taking note of the effect is moderately clever; for those a speck cleverer than that, it may induce the desire for the numbers i've furnished. (I invest my cleverness elsewhere, and for me the numbers' only value is their relevance to what should be in the article.) My read is that interest directly in the cited phenomenon is sufficiently narrow that it applies to professional astronomers (those happy few), perhaps astronomy students, and probably dedicated amateurs: i.e., IMO it is probably not fodder for a general-purpose encyclopedia.
   BTW, "How long is the moon really full?" is a related question (IM-offhand-O more interesting and thus the best reason for considering inclusion of the restricted view of the moon), and the sun subtends (from earth and, shall i say, to the moon) an angle about 10% less than the moon from the earth. (Note that one answer is "only an instant", focusing on monthly maxiumum illumination of either the instantaneous near hemisphere or the median near hemisphere.) Offhand, i think that means that at the moment in each lunar cycle when the three bodies are closest to alignment, the Moon appears full at solar midnight (i.e., at the longitude where sunset is as far in the past as sunrise is in the future), but simultaneously, at the longitudes where sunset or sunrise is occurring, appearing full is either recently over or soon to start (not necessarily respectively; i'm getting tired of getting such details right). That could BTW be worth a passage in a science-fiction novel that involves separated lovers, or precision treatment of lycanthropy!
--Jerzy•t 19:36, 9 & 08:07, 10 September 2010 (UTC)

   My clumsy language (in the last of those 3 'graphs) re a core instant of full moon deserves rewording. It reflects my attempts to stand in, with the math and physics, where only colleagues lacking those tools had previously ventured re the fraction -- perhaps also lacking (as i do) adequate command of much astronomy-specific background.
   My wording "maximum illumination of ... the instantaneous near hemisphere" (which actually echoes the fact that my parents had no idea why the kids'-TV-show demonstration of the three-body explanation of lunar phases didn't also imply monthly lunar and solar eclipses) should have referred centrally to the instant of closest alignment among sun, a moon longitude, and either the earth as a whole or the observer, since the sun (duh) is not emerging from behind the earth and rapidly increasing its illumination. That maximum of illumination would, i'm pretty sure, be so "broad" and thus at the mercy of subjective judgments as to be an observation useless for predicting future observations, and its use as a reference pointless.
   All that said, alignment to the median longitude (or, on reflection, rather the mean, which presumably converges faster in the face of random censoring of rare amd crucial extreme observations by bad weather or inaccessibility of desirable earth longitudes) (in any case, to be recognized relative to surface features) or to the center of the disk, may nevertheless both be workable conventions; the accompanying article should identify the accepted or predominant one. (That convention may already be stated in other articles.)
--Jerzy•t 08:07, 10 September 2010 (UTC)

Consider the right triangle formed by the Moon's center (C), your eye (E), and the limb of its apparent disc (L, the right angle). Clearly the angle ECL is short of a right angle by CEL, the angular radius of the moon in your sky, which is about a quarter degree. —Tamfang (talk) 22:13, 11 September 2010 (UTC)

Physical libration

According to this source, the moon also wobbles slightly about its centre of gravity. Shouldn't something about his be included in the article? Kelisi 01:13, 22 July 2007 (UTC)

The current text of the article (as at 27 January 2019) seems to be misleading. The introduction to the article says that libration is due to 'three mechanisms detailed below, two of which are causing a relatively tiny physical libration...' But as far as I am aware, none of the three mechanisms described are 'physical', because they are all due to the different angles at which the moon is seen from the earth, not to any motion of the moon itself. There are some genuine physical 'wobbles' of the moon, but these are very slight, and at present they are not described in the article.109.150.6.143 (talk) 11:24, 27 January 2019 (UTC)

Latitude libration

I am unhappy about this explanation, and about the analogy with the seasons. Since there is tidal locking, the same polar tilt should be exhibited at all times (all things being equal, i.e. if we ignore the OTHER types of libration listed). In other words, the 59% arises from the other types, with this one not contributing. — Preceding unsigned comment added by 92.2.211.127 (talk) 13:24, 24 October 2012 (UTC)

The tidal lock determines the period of the rotation, but has not yet fixed its axis. —Tamfang (talk) 06:36, 23 April 2023 (UTC)

Moon specific

This article seems to talk an awful lot about the Moon, even though libration is by no means Moon-specific. Researchers just used the librations of Mercury to determine it has a molten core (http://www.news.cornell.edu/stories/May07/margot.mercury.html) So I think the definitions of longitudinal and latitudinal librations should be more general. brithans 2008/11/09 12:34 (UTC)—Preceding undated comment added 17:35, 9 November 2008.

True, I'd support moving this article to Libration of the moon. Tom Ruen (talk) 02:12, 10 November 2008 (UTC)

   Not appropriate, & i'm Rdr'g that title to the corresponding secn of the accompanying article, since astronomical libration is by far the primary topic and the Moon's by far the most significant example. We are a long ways from being prepared to support two articles on astronomical libration! Even additional material would leave a single article (on the more general topic) more convenient for users.
--Jerzy•t 08:17, 6 September 2010 (UTC)

   You must mean libration of Merc relative to sun, with those definitions being exact analogs (but the diurnal only introducing confusion if mentioned).
   (Way cool: Cornell is not serving that page right now, but the liquid core must slip relative to the solid crust, reducing the angular moment of the planet and increasing the restoring rotation in response to the torque on the tidal bulge when libration shifts it away from the Mercury/Sun axis.)
   I would say that the current structure is fine, pending the introduction of article text on (Merc or other bodies), and that once that makes it worthwhile, splitting the def'ns out into the lead secn would be fine but the dominant interest in the Moon's libration would commend still using it for examples w/in the def'ns.
--Jerzy•t 08:17, 6 September 2010 (UTC)

I think more pertinent is the fact that despite this article (per its title) purporting to cover libration in general, it has only the vaguest description of libration in general: "In astronomy, libration is a particular relative motion of orbiting bodies". That description could apply to plenty of other relative motions that are not libration. I wouldn't propose that Libration of the moon needs to be moved to a separate article, but this article does need to describe what libration is in general. I personally came here via links from Jupiter Trojan, which has nothing to do with the moon, so a description of only lunar libration is frustrating. Huttarl (talk) 16:34, 18 November 2010 (UTC)

I definitely think this article should be renamed libration (astronomy). Libration is a type of motion that can be exhibited by astronomical bodies, molecules, people nodding their heads, balance scales, empty rocking chairs, etc. People would expect the article titled "libration" to be about the phenomenon of "libration", not about "libration in astronomy". :-)

I would make Libration just redirect to this article.

Any objections? :-) --Steve (talk) 18:39, 18 November 2010 (UTC)

The word "libration", in general, means "oscillatory rotating motion", such as illustrated here.

Huttarl, I made an image, now just at libration (molecule). It's not directly astronomy-related, but it does illustrate what the word "libration" means: "rotation back and forth". You're welcome to put it back into this article if you think it's helpful. :-) --Steve (talk) 04:57, 22 November 2010 (UTC)

History

The article should mention whoever it was that discovered lunar librations. —MiguelMunoz (talk) 00:30, 4 September 2010 (UTC)

   It came pretty early; i think Brahe first mapped the range of the variability of the longitudinal limbs, but i'm not sure the Arabs didn't note the effect before telescopes.
--Jerzy•t 08:24, 6 September 2010 (UTC)

Distinguish?

   I'm removing

{{distinguish|Liberation|Libation|Vibration}}

-- which rendered as

Not to be confused with Liberation, Libation, or Vibration.

-- bcz that template is not for careless misspellings or typos, but for actual confusions: in this case,

1. seeing or hearing "Liberation", "Libation" or "Vibration",
2. confusing it with "Libration", and
3. thus seeking one of those other topics on the accompanying article.

Liberation & vibration are so familiar that those confusions are too implausible to burden the HatNote with. The "libation" case is not so obvious, but

1. its mention in the "The Death of Socrates" makes it fairly well known,
2. it is also probably reasonably common as an implicit analogy with a toast, and also as a euphemism for, or an ironically formal reference to, the next round of drinks.

Even so, i might have left it, pending discussion, if it had been the first {{distinguish}} entry placed, but in fact the three seem to have been added in the order they appear, with the bad judgment of distinguishing from "Liberation" having set the bar as low as is imaginable, and serving to encourage misunderstanding of what kind and degree of confusion would justify additional entries. So i consider my case against "{{distinguish|Libation}}" decisive, pending anyone's eloquent defense of that HatNote.
--Jerzy•t 06:32, 6 September 2010 (UTC)

You seem to be assuming that only well-read native english speakers (with perfect spelling and hearing abilities) will read this article! I disagree. We want the article to be helpful for non-native English speakers too. Just look at most wikipedia discussion pages (especially in science topics) and you'll see comments by people who clearly speak English poorly but are using English-language wikipedia. I don't know why you would spend your time to make life harder for these people by deleting hatnotes! Anyway, I have seen "vibration" misheard as "libration" and vice-versa even in conversations between two native English speaker scientists. The idea is, someone reading the article certainly needs to know this important information: "Libration and Vibration (and Libation etc) are both legitimate, and different, English-language words". Knowing that, they won't be confused in future conversations, hearing the word vibration (or libation etc) and thinking "this must be the word "libration" I read about on wikipedia!" It's especially important since vibration and libration have similar meanings and are used in similar contexts, that people see the words side-by-side. Anyway, even native English speakers may misspell the word "liberation" and end up on this page. Try reading a typical high-schooler's writing without spell-check and you will surely get a more pessimistic and realistic idea of how frequently people misspell even famous common words like liberation!

Also, if you don't like the order that the words are written, then change it! :-) --Steve (talk) 14:16, 6 September 2010 (UTC)

Gravitational waves?

There doesn't seem to be any discussion as to the cause of lunar libration. Might it be due to gravitational waves, as have recently been proven experimentally? - Paul2520 (talk) 13:25, 18 March 2016 (UTC)

Gravitational waves are exotic, but libration can be understood with Newtonian physics. LunaJim (talk) 00:41, 18 April 2021 (UTC)

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Visible

The 6.7 degrees should be visible to the unaided eye. — Preceding unsigned comment added by 2A00:23C0:7C80:8401:981D:286D:6C7:9A8D (talk) 16:50, 18 June 2018 (UTC)

Harriot might have made the discovery some decades before Galileo. See http://rsnr.royalsocietypublishing.org/content/63/2/163 — Preceding unsigned comment added by 2A00:23C0:7C80:8401:981D:286D:6C7:9A8D (talk) 16:58, 18 June 2018 (UTC)

Thanks for spotting, added caveat. Brandmeister^talk 17:23, 18 June 2018 (UTC)

7.7 degrees is about the same as 6.7 degrees. — Preceding unsigned comment added by 86.179.131.222 (talk) 10:28, 19 June 2018 (UTC)

Libration of the moon as seen from the moon

I do not have a planetarium program but such a program might be able to generate an image of a full year's libration as one graph. The position in the moon's sky of the Earth as seen from the center of the near side of the moon should be recorded by its Cartesian coordinates for 365 or more positions in the year and the points connected by a drawn curve which would be a graph of libration. The citation for the image could be the name of the planetarium program. I for one would like to see the graph. - Fartherred (talk) 17:19, 8 June 2019 (UTC)

Libration point orbit.

I just created a new page for libration point orbit, associated of late with the JWST.

My mission was merely to cover a small, evident gap, but for as long as I attended to this, I came to the impression that this page should be renamed lunar libration and a top level libration page (not necessarily as reductive as a dab) could dispatch the various weakly related libration subpages. It's somewhat problematic in this instance that celestial mechanics is shot through with various and sundry libratory jitterbugs, lunar libration merely being the most parochial.

Just a suggestion based on short contact. — MaxEnt 23:34, 20 September 2022 (UTC)