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The strange case of pseudotwilight on the Moon.

Soon after concluding his landmark observations of twilight on Venus in the spring of 1790, the results of which irrefutably established that the planet has an appreciable atmosphere, (1) it occurred to the Hanoverian bureaucrat Johann Hieronymous Schroter (1745-1816) at Lilienthal, near Bremen, that if 'there were a twilight on the moon, as there is on Venus and our Earth, it could not, considering the greater rarity of its atmosphere, be so considerable: and that the vestiges of it, allowing for the brightness of the luminous part of the moon, ... and in some measure the reflected light of our Earth, could only be traced on the limb, particularly at the cusps.' (2)

This possibility struck him forcibly in 1792, when he recollected that two years earlier he had perceived such an appearance at the eastward edge [westward in modern parlance] near the cusp points, 'though I did not then reflect on the cause of it,' as he later wrote. (3)

Detection of such a delicate phenomenon, he realised, would critically depend on the strength of earthshine and the brightness of the sky; and of course, the phase and altitude of the Moon, a suite of circumstances not often encountered. At 5h40m on the evening of 1792 February 24, two days and twelve hours after new Moon, such an opportunity occurred to test the hypothesis. 'The air being perfectly clear,' he tells us, 'I prepared my seven-feet reflector, magnifying seventy-four times, in order to observe the first clearing-up of the dark hemisphere, which was illuminated only by the light of our earth, and more especially to ascertain whether in fact this hemisphere, which, as is well known, is always somewhat more luminous at the limb than in the middle, would emerge out of our twilight at many parts at once, or first only at the two cusps. Both these points appeared now, most distinctly and decidedly, tapering in a very sharp, faint, scarce any where interrupted, prolongation; each of them exhibiting, with greatest precision, its farthest extremity faintly illuminated by the solar rays, before any part of the dark hemisphere could be distinguished. But this dark hemisphere began soon after to clear up at once at its border, though immediately only at the cusps, where, but more particularly at their points,' he took note of 'a luminous margin, above a minute [of arc] in breadth, of a very pale grey light, which, compared with that of the farthest extremities of the cusps themselves, was of a very different colour, and relatively as faint as the twilight I discovered on the dark hemisphere of Venus, ... when compared with the light immediately derived from the sun. This light however, faded away so gradually towards the east [west], as to render the border on that side perfectly undefined, the termination losing itself imperceptibly in the colour of the sky.' (4)


'I examined this light with all possible care,' he tells us, 'and found it of the same extent at both points, and fading away at both in the same gradual proportion. But I also, with the same caution, explored whether I could distinguish any part of the limb of the moon farther towards the east [west in present day terms]; since if this crepuscular light had been the effect of the light reflected from our globe, it would undoubtedly have appeared more sensibly at the parts most remote from the glare of the illuminated hemisphere. But, with the greatest exertion of my visual powers, I could not discover any part of the, as yet, wholly darkened hemisphere, except one single speck, being the summit of the mountainous ridge Leibnitz, which was them strongly illuminated by the solar light; and indeed eight minutes elapsed before the remainder of the limb became visible; when not only separate parts of it, but the whole displayed itself at once.' (5)

Emboldened by his Venus results, and convinced deception played no part in the phenomenon, he ruled out the action of earthlight and confidently asserted: 'Thus, as my successful observations on the twilight of Venus led me to the discovery of that of the moon, so did these latter reciprocally confirm the former: and thus, which ever way we contemplate the subject, must we be struck with the coincidence that prevails throughout.' (6)

It was a plausible inference but nevertheless flawed. For like his Venus mountain hypothesis, it owed more to pluralism and analogy with the known and familiar, than to his telescope. A thesis embedded in William Herschel's statement that his observations of the Moon were made 'with a view to find whether any effects of a lunar Atmosphere could be perceived.' (7)


Discussing the subject in his Celestial Objects, T. W. Webb (1806-1885) alluded to Schroter's inferences as 'supported and in part exceeded by Gruithuisen [1774-1852] who frequently saw--or imagined --fogs and clouds resting on the surface', beside traces of twilight. The brothers Henry at Paris reported what they presumed to be a twilight effect. Neither was Hermann Joseph Klein (1844-1914), Director of the Cologne Observatory, disposed 'to deny the probability of such obscurations, especially in deep places.' W. Beer (1797-1850) and J. H. Madler (1794-1874) on the other hand, rejected Schroter's interpretation, but admitted it was within their remit to concede the possible existence of a thin atmosphere. Webb himself imagined he had caught a glimpse of the effect on 1855 June 20, but as he says, 'only doubtfully from want of better optical means'. (8)


'Traces were very evident of what, to all appearance, is a somewhat rare atmosphere,' declared English observer F. Dennett writing in the English Mechanic, describing what he saw at the south cusp at 06h 45min, 1877 March 17 (age of the Moon 2d 15h 51min). The terminator passed through the Mare Crisium, the high summits of its western shoreline being illuminated. (9) Examining the 3.9 day old Moon with the 6.5-inch refractor of the Brockhurst Observatory on 1912 May 20, double star expert W. S. Franks (1851-1935) noted a feeble thread of light running westwards along the earthlit limb from the Leibnitz Mts., to a bright peak and beyond, and a similar appearance in the north which extended the cusp to a row of four or five bright peaks further along the limb. (10) In more recent times many observers have remarked upon the phenomenon.

Significant as these observations are, the most detailed and reliable known to the writer are those by Harold Hill (1920-2005), the well known British selenographer who over a period of six decades devoted himself to close visual scrutiny of lunar morphology, in particular to the south polar limb, which he frequently studied at very late stages of illumination, and where he 'witnessed comparable effects under morning illumination on many occasions when southern libration has been favourable--the most recent being on 4.4.92 at colongitude 294[degrees].4 when dim extensions to the limb were measured to be 350 arcseconds from fully lit features.' He noted the effect as very 'atmospheric'. (11) It seems probable then that Schroter and others had similar experiences; nevertheless questions remain.

Hill made careful drawings (here reproduced) with annotations, of what he saw on 1984 July 25, and again on 1985 September 12. On the first occasion a narrow strip of dim light along the limb between the crater Cabaeus and the mountain M1 caught his attention. He attributed the appearance to oblique solar lighting on the surface beyond the crater, but wondered why it did not show under more favourable southern libration. He found this quite incomprehensible. The second drawing, made in exceptionally favourable conditions of libration, seeing and transparency over the period 04:45-04:55hrs UT, shows 'an extremely attenuated cusp extending well beyond the Moon's prime meridian; the dusky ill-defined narrow strips which were situated along the strongly-lit earthshine limb obviously belonged to the averted hemisphere. M5 [mountain] was readily identified by its brilliant eastern arrete or spur and by its equally brilliant long western slope and the narrow extension seen in drg. (88)--peculiar to this particular lighting. (12) Beyond this portion of M5 was a bright mass described as difficult to define. Neither the upper slopes of the Ml massif nor M3 were visible. A similar observation but with the sun's selenographic latitude in the southern hemisphere would be highly informative in this regard.'

Modern science has destroyed the mystique and dispelled the inference; the Moon has no atmosphere, at least of the density posited by Schroter and others, hence no twilight. How then are his eyepiece impressions to be explained? Was his 'pyramidal glimmering light' or crepuscular extension of the bright cusps fact or fiction? Natural enough to believe he was mistaken and so define it as a visual fallacy, even a wish fulfilment. To do so however, is too arbitrary. Imperfect as his observational record is, it is at least consistent, and within the parameters of his optical means provides an accurate and honest picture of what he believed he saw. (13) If we consider the careful reportage of Harold Hill, whose reliability as an observer is legendary, all doubt is swept aside and a truth is revealed; the truth of appearances, but not necessarily of reality. Direct sunlight scattered from adjacent mountains may indeed be involved although a study of the drawings by Hill and Schroter reveals no obvious source of reflection.


The eye and the mind in combination can deceive and confuse even the most experienced observers, especially if the feature under review is at or near the limit of vision as in the present instance: there is no shortage of example in the observational history of the subject. This is not to dismiss the observation or to disparage its claimants, but simply to approach such reports with caution to avoid rejection of a possible reality that has yet to be understood.

Significantly this pseudo-twilight or crepuscular light resonates in comments by the crew of Apollo 10 who in reference to earthshine noted, 'On several revolutions, we were able to observe the lunar surface lighted by earthshine. The surface appeared black until spacecraft sunset. However, after a few moments of eye adaptation, the surface appeared to be a bluish white, and peaks on the lunar horizon were clearly visible. We experienced no difficulty in recognizing major features and were able to observe a surprising amount of textural detail within the larger craters. Rays and halos were clearly visible. There is a definite earthshine terminator. As we approached this terminator, the shadows lengthened, and low slopes were accentuated just as along the sunshine terminator. Beyond the earthshine terminator, the lunar surface was black. No features could be detected by starshine, but the horizon could be seen easily as a curved line dividing the star-studded sky and absolute blackness'. (14)

It is not without interest therefore, to remark, in view of recent discussions within the Lunar Section, that many decades earlier the controversial American astronomer William H. Pickering (1858-1938) gave direction to this line of thought; 'It is often stated that no twilight exists upon the Moon,' he wrote in 1895, 'that all the shadows are absolutely black; and that no light is found within them. This latter statement is unquestionably a mistake. We always see light inside of the lunar shadows in Arequipa, when the lunar crescent is sufficiently slender, so that the eye is not dazzled by its light. And this light is, moreover, by no means extremely faint, but is quite sufficient to show the main details of the interior of the craters,--such, for instance, as the central peaks, long before they are struck by the first ray of sunlight. In some craters this phenomenon is much more marked than in others. Thus on March 20, 1893, the interior floor of Stevinus, with its central peak, was clearly seen, although well within the shadow; while the interior of Snellius, at the same distance from the terminator, was perfectly black. The same phenomenon regarding these two craters had been noted also at a previous lunation. Stevinus seems particularly adapted to this observation, although other craters also show it. This does not prove the existence of a lunar atmosphere, however, since the light may be due, at least in part, to reflection from surfaces illuminated by direct sunlight; and indeed, I am inclined to attribute the appearance largely to this cause. It seemed desirable, however, to mention it, since the statement is so frequently made that the lunar shadows are always perfectly black.' (15) This example is a perfect illustration of the fact that there is seldom anything in the visual astronomy of the Moon that is without precedent.

The Moon is a big place and though knowledge of its physical condition has grown, there is much still to comprehend. Visual observers and CCD imagers alike may well find the challenge thus offered of more than ordinary interest as Harold Hill hinted. For the old-fashioned debates that swirled around the subject are not just of historic interest. They contain cautionary tales about how we make sense of what we see, allowing us to indulge reflections on basic questions concerning how we come to know--perhaps, more accurately, how we come to believe we know.


(1) Philosophical Transactions, 309-361:309-337 (1792). A summary account features in R. Baum, The Haunted Observatory (New York; Prometheus, 2007), 142-156

(2) ibid., 338

(3) ibid.

(4) ibid., 338-339

(5) ibid., 340

(6) ibid., 353

(7) Dreyer J. L. E. (ed.), The Scientific Papers of Sir William Herschel, Vol. 1 (London: 1912), xci.

(8) Webb T. W., Celestial Objects for Common Telescopes, 4th ed. (London: Longman, Green & Co., 1881), 78-79

(9) Dennett F., Letter no. 12417, 'Notes on Our Satellite', English Mechanic 25 (1877 April 6), 89

(10) Franks W. S., Notes on the Moon (MSS)

(11) Hill H., Additional note to drawing 90, South Polar Region Series 2, Zone D

(12) Here Hill refers to what he saw on 1984 October 21. Mean colongitude 227[degrees].4, selenographic latitude +0[degrees].77. He says, 'A curious western appendage to M5 is visible from this stage of lighting onwards.'

(13) See H. N. Russell, 'The Atmosphere of Venus', Ap.J 9 (1899), 284-299 on this subject.

(14) Stafford T. P., Cernan E. A. & Young J. W., 'Visual Observations' in Analysis of Apollo 10 Photography and Visual Observations, (Washington DC: 1971), NASA SP-232, p.3

(15) Pickering W. H., 'Investigations in Astronomical Photography', Annals of the Astronomical Observatory of Harvard College, Vol. XXXII.--Part 1, Ch.3, 81-82. (Cambridge, Mass: 1895)

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Received 2009 September 8; accepted 2009 October 28
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Author:Baum, Richard
Publication:Journal of the British Astronomical Association
Date:Dec 1, 2010
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