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How old is the idea of the far side of the Moon? got me thinking that as soon as we see the moon librate we have to come to terms with there being even more of it we can't see.

The Moon's libration is a natural consequence of its orbit being elliptical; while it rotates around its own axis at steady angular rate (360° every ~29.5 days), its angular rate of revolution around the Earth is not so constant; advancing faster than average when it's closer to us and slower than average when further.

This allows us to "peek around the corner" and see extra bits on the left and right side during the month.

The uneven angular rate of revolution is a consequence of Keplerian orbits, so it could have been predicted. In fact, even before orbital mechanics it's possible just plotting the Moon's position among the stars would show this variation in speed.

So I'd like to ask the following:

Question: Was lunar libration first observed or first predicted? In either case, who was the responsible party?

This answer to Moon's rotation and revolution says

Anyways, if what you are asking instead is how and when we realised the exact mechanism by which the Moon gained its synchronous rotation, then the answer is that the process was first correctly described by british astronomer George Darwin (this is Charles Darwin son by the way) in 1879. But the main idea was really present as early as Isaac Newton's work on his lunar theory. So not only the fact that the Moon has the same rotational period as its orbital period was known long ago but also the exact way as to why this happened has been scientifically understood at least for 142 years now. This is 90 years before any human landed on the Moon and 78 years before even the Sputnik exited our world at the dawn of space exploration.

which is quite helpful, but it's not enough to definitively answer my current question.

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The variable speed of the Moon on the celestial sphere has been known since ancient times. The Babylonians made ~7 centuries of daily astronomical observations from around 700 BC. That data was the basis of the astronomical tables of Hipparchus, which Ptolemy used in creating the formulae of celestial motions in the Almagest.

However, actually observing the Moon's libration would have been tricky before the development of the telescope. Thomas Harriot was the first person to make a drawing of the Moon through a telescope, on 26 July 1609, over four months before Galileo Galilei.

According to Wikipedia, the lunar libration in longitude was discovered by Johannes Hevelius in the early 1640s & published in 1647. Hevelius was the last astronomer to do significant work without telescopes, although he did eventually use them.

William Gilbert

made the first attempt to map the surface markings on the Moon in the 1590s. His chart, made without the use of a telescope, showed outlines of dark and light patches on the Moon's face.

Wikipedia says

Galileo Galilei is sometimes credited with the discovery of the lunar libration in latitude in 1632, although Thomas Harriot or William Gilbert might have done so before.

Newton presented a fairly detailed analysis of lunar motion in the Principia, but some of his calculations were rather inaccurate due to various approximations that he made in order to simplify the calculations. Lunar theory is hard!

Prior to Newton, it's unlikely that anyone thought of the Moon being affected by tidal locking. Of course it was known that the tides were connected to the cycles of the Moon, but there wasn't really any solid theory on how that worked.

Bear in mind that before Newton, it was standard to consider celestial mechanics and terrestrial mechanics to be governed by separate systems of laws. As I mentioned here, Newton's outstanding insight was that a single scheme could describe both terrestrial motion and celestial motion.

So prior to Newton, it was assumed that the Moon's motion was due to celestial influences. The notion that the Earth influenced the Moon was a very radical idea.

Kepler's laws, as published in his Astronomia Nova were purely empirical. It was reasonable to assume that other Solar System bodies also moved in elliptical orbits, but we had to wait several decades for Newton's Principia to give that idea a solid theoretical basis. Similarly, it was reasonable to assume that the Moon's rotation on its axis (relative to the stars) was fairly uniform (like the Earth's), but until Newton there was no solid theory stating that the rotation of celestial bodies ought to obey the law of conservation of angular momentum.

Thus the observations of lunar libration came before the necessary theory.

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  • $\begingroup$ looks great, thanks! $\endgroup$
    – uhoh
    Nov 24 at 19:53
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To understand the answer provided below, we need to reframe the original question slightly. Perhaps a better way to put "was libration first predicted" is to say, outright, libration was first observed, then a theory was advanced explaining its occurrence. Although the observation of libration is documented in 1632 by Galileo, and independently observed and documented in 1647 by Hevelius, the question opens up an initially unseen view of history regarding who first developed a theory for libration. Reading relevant portions of Newton's preserved correspondence, clearly Newton was working in depth on the libration problem by about 1670, or so. From about 1670 and, more or less, the decade following, the matter of lunar libration was the subject of discussion among Newton and Huygens, as well as with Halley, and Mercator, among others. The history of events shows that to no small degree, in later life Newton was trying to pin down the primacy of his work regarding libration. There seems to have been an air of intrigue regarding when libration had been first explained, others were trying to claim they were first in line. The general view of the Ptolemeo-Aristotelian cosmos is changed by Newton's presentations in Principia Mathematica, and regarding libration, Newton wanted a solid and honest cornerstone establishing the date of his work as being the first. But apparently there is no extant evidence that lunar libration had been predicted and then confirmed by observation.

Alan Gabby discusses the discovery of lunar libration by Hevelius and development of the theory regarding it by Newton, in an essay titled Innovation and Continuity in the History of Astronomy: The Case of the Rotating Moon. This essay is within a larger anthology, Studies in Philosophy and the History of Philosophy, Volume 24, Revolution and Continuity: Essays in the History and Philosophy of Early Modern Science, edited by Peter Barker and Roger Ariew. The publisher is Catholic University Press (1991).

Gabby gives the following account on page 96 and following pages:

Galileo gave his account of parallactic lunar in the Dialogo of 1632, and in correspondence of 1637-38 (albeit probably unknown to Newton) announced further parallactic libratory effects. Libration in latitude and longitude was discovered by Hevelius, who produced the first maps of libration in Selenographia (1647). In 1648 Hevelius hit on the idea of explaining longitudinal libration by having the Moon face always its eccentric point, not the Earth. This idea occurred independently to Riccioli, who published it in Almagestum novum (1651), together with libration maps and a general survey of the subject. However, Riccioli was unhappy with the eccentric hypothesis, since the observed librations were not quite those predicted from the accepted value for lunar eccentricity, a weakness that Hevelius recognized, though he favored retaining the hypothesis until something better came along.

Gabby gives a detailed account of Newton's involvement in the development of an understanding of lunar libration and its occurrence. Mercator, in his Institutionum astronomicarum libri duo, (1676) provides the earliest version of Newton's libration theory as told to him by Newton, both in person (presumably in discussions with Mercator at Royal Society meetings) and later by letter. Newton apparently had ready a text for publication on the subject in 1673. As Gabby states,

Newton’s letter to Oldenburg for Huygens of 23 June 1673...strongly suggests that the theory itself was in existence by that date at the latest.

Clearly, Newton had the theory at hand yet before mid-1673, but details of the time are unavailable. However, Newton's own recollection of the inception of the theory was documented by Newton some 40 years later in the third edition of his Principia Mathematica. Those details come, somewhat, from a draft of that edition indicating Newton may have been uncertain, precisely, of the date. Without his preserved correspondence, and that of others, as well as the communications with, and publication by, Mercator, those details possibly would have remained unclear. His notation in the draft was for deletion of "anno 1675" regarding the date of which he had given Mercator details of the libration theory. Gabby takes particular note,

Whether Newton’s deletion of “anno 1675” signifies an uncertain recollection of the date of something that took place at least forty years previously, or a decision to mention only the date of publication, it seems safe to link Newton’s lunar libration theory, his first published contribution to astronomy, with the years 1673—75.

But did Newton have another reason for establishing this date for the primacy of his work? Why would Newton, now in his old age, be so insistent on the veracity of this date? Gabby makes the following comment:

What prompted these persistent efforts in Newton’s old age to get right for first publication the wording of what we now realize is a priority claim? It seems to me that Newton had read or heard about (I cannot say when) the theory of lunar libration in longitude that [Giovanni Domenico] Cassini had published in 1693. Cassini’s theory seems to date also from circa 1675 and also employed, though none too clearly, the model of a moon rotating uniformly about an axis inclined at 7.5° to the orbital axis (and at 2.5° to the ecliptic axis), and moving nonuniformly in its orbit. No doubt it was G. D. Cassini (whose striking contributions to the observational study of ultra-lunar rotations Newton recognized and used) who was the implied foreign rival claimant to the libration of the moon to whom Newton wished to deny priority in the third edition of the Principia...


This was a very interesting question. Nevertheless, things are not always as they seem... One cannot prove the negative, but apparently there is no record of a prediction of lunar libration, and then an observation to confirm it. Rather, first lunar libration was observed, and then a theory for its occurrence was advanced.

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    $\begingroup$ Thank you for an amazing, thorough and insightful answer, and welcome to Stack Exchange! I had no idea that this would turn out to be such an interesting story. In case you don't know, of the almost 200 Stack Exchange sites there is also History of Science and Mathematics SE which you may find interesting, at which I also have several unanswered questions many of which are related to Astronomy. $\endgroup$
    – uhoh
    Nov 24 at 21:08
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According to Wood Galileo was the first to observe libration in longitude and latitude and make notes of it, described in his Siderus Nuncius.

Given that exact observations, and comparisons require detailed photos or images, it seems unlikely that this libration was observed earlier. An interesting read on the discussion among the leading scientists around 1600 is found in this article by Jarosław Włodarczyk. The difficulties described there to obtain even a decent grasp of a map of the moon at that time even with (the first) telescopes makes it seem unlikely it was discovered earlier.

The absence of any talk of confirmation, especially texts or correspondence where e.g. Kepler is involved, makes me believe it hasn't been predicted previously - at least not by anyone who Kepler, Galileo or Gilbert had knowledge of - despite the knowledge of the Moon's variable speed on its orbit.

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  • $\begingroup$ Thanks! Is there anything in your sources to indicate if Galileo found it surprising i.e. they were not aware of libration having been predicted earlier? $\endgroup$
    – uhoh
    Nov 24 at 11:45
  • $\begingroup$ Judge yourself: people.reed.edu/~wieting/mathematics537/SideriusNuncius.pdf - I'm not sure I can read surprise in it. $\endgroup$ Nov 24 at 13:41
  • $\begingroup$ And actually there's a digital copy of the Latin original: digital.libraries.ou.edu/histsci/books/1466.pdf $\endgroup$ Nov 24 at 13:46
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    $\begingroup$ Stack Exchange answers should address the question as-asked. "Judge yourself" is not an appropriate response. $\endgroup$
    – uhoh
    Nov 24 at 15:02
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    $\begingroup$ According to Wikipedia, the libration in longitude was discovered by Johannes Hevelius in the early 1640s & published in 1647. $\endgroup$
    – PM 2Ring
    Nov 24 at 18:05

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