Abstract
After the digression of the preceding chapter concerned with the electromagnetic properties of the lunar surface, let us return to the basic question raised at the commencement of this part (Chapter 19): namely, what are all the constituents of moonlight?
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Bibliographical Notes
In view of the interest shown in recent years in transient luminous phenomena on the lunar surface attributed now to luminescence, let it be recalled that the first observer who noted and described such phenomena was nobody lesser than Sir William Herschel, in 1783, then at the beginning of his great career. Apparently he saw first a red glow in (or near) Aristarchus on May 4, 1783; and again 1787. An account of his earlier observations was never published; but when his collected works (The Scientific Papers of Sir William Herschel, ed. by J. L. E. Dreyer, London 1912) appeared, theireditor’s preface contained extensive extracts from Herschel’s correspondence; and among them was a letter to J. H. de Magellan (a Portugese scientist then resident in London) who had asked Herschel for details of the reported “lunar volcano” of 1783.
As quoted by Dreyer, a part of Herschel’s reply reported that, ... “May 4, 1783. I perceived in the dark part of the Moon a luminous spot. It had the appearance of a red star of about the 4th magnitude. It was situated in the place of Hevelii Mons Porphyrites (i.e., Aristarchus); the instrument with which I saw it was a 10-feet Newtonian reflector of 9 inches aperture. Dr. Lind’s lady who looked in the telescope immediately saw it, tho’ no person had mentioned it, and compared it to a star. Dr. Lind tried to see it in an achromatic of 3½ feet of Dollond’s but could not perceive it, tho’ with difficulty perceive it in the refractor.”
Herschel saw such phenomena again in April 1887; and in a note entitled ‘An account of Three Volcanoes on the Moon’ (Herschel, 1787) he reported that ...
“April 19, 1787, 10 h 36 m Sidereal Time: I perceive three volcanoes in different places of the dark part of the new moon. Two of them are either already nearly extinct, or otherwise in a state of going to break out; which perhaps may be decided next lunation. The third shows an actual eruption of fire, or luminous matter. I measured the distance of the crater from the northern limb of the Moon, and found it to be 3′57″.3. Its light is much brighter than the nucleus of the comet which M. Méchain discovered at Paris the 10th”.
“April 20, 1787, 10 h 36 m Sidereal Time: The volcano burns with greater violence than last night. I believe its diameter cannot be less than 3″, by comparing it with that of the Georgian planet [i.e., Uranus]. As Jupiter was near at hand, I turned the telescope to his third satellite, and estimated the diameter of the burning part of the volcano to be equal to at least twice that of the satellite. Hence we may compute that the shining or burning matter must be above three miles in diameter. It is of an irregular round figure, and very sharply defined on the edges. The other two volcanoes are much farther towards the center of the Moon, and resemble large, pretty faint nebulae, that are gradually much brighter in the middle; but no well defined luminous spot can be discerned in them. These three spots are plainly to be distinguished from the rest of the marks upon the Moon; for the reflection of the sun’s rays from the earth is, in its present situation, sufficiently bright, with a ten-feet reflector, to show the Moon’s spots, even the darkest of them : nor did I perceive any similar phenomena last lunation, though I then viewed the same places with the same instrument.”
“The appearance of what I have called the actual fire or eruption of a volcano, exactly resembled a small piece of burning charcoal, when it is covered by a very thin coat of white ashes; and it had a degree of brightness about as strong as that which such a coal would be seen to glow in faint daylight”.
“All the adjacent parts of the volcanic mountain seemed to be faintly illuminated by the eruption and were gradually more obscure as they lay at a greater distance from the crater”.
Moreover, in a letter of April 26th, 1787, to Sir Joseph Banks (then President of the Royal Society) accompanying his above communication, Herschel stated “... Inclosed I have sent you an account of three volcanoes in the moon, one of which is now actually burning with great violence, and probably disgorging an immense lava” (cf., C. A. Lubbock, The Herschel Chronicle, Cambridge 1933, pp. 217–218). So convinced was he of the reality of this phenomenon that, on May 20th, he sent the following message to a page of his royal patron, King George III: “Last month I discovered three volcanos on the Moon, and saw the actual eruption, or fire, of one of them, yesterday I examined the same place again and found that one of these volcanos is not yet quite extinct. Will you do me the favour to acquaint the King with these circumstances; and if his Majesty would wish to see the Moon, the best time for viewing the Crater, which continues still to be considerably luminous, will be this evening between 9 and 10 o’clock. I will be at Windsor in good time to see the King’s ten-foot telescope brought out and prepared, if it should please his Majesty to have it done”.
We do not know what his Majesty’s pleasure may have been in this matter; but more important to us may be what the contemporary scientific opinion thought of Herschel’s discovery. His accuracy as an observer was held already by then in such high regard that the existence of these volcanoes was not immediately disputed; but neither was it readily accepted. Lalande probably spoke for many when he wrote to Herschel from Paris that “... the volcano on the Moon has been visible these last few days; but there are astronomers who are inclined to believe that Mount Aristarchus, which is naturally very brilliant, might very well reflect the light of the Earth in such a manner as to produce this bright appearance across the ashen light of the Moon” (cf. again Lubbock, 1933, op. cit.). How Herschel took on to this suggestion we do not, unfortunately, know; the two astronomers may have discussed it when Lalande visited Herschel at Slough in August of the same year. At least Lalande’s missive located for us the region on the Moon where the volcano was supposed to glow in May 1787. Herschel himself never identified the active region with Aristarchus itself, but described its position to be located 3′57″.3 from the northern limb of the Moon (the center of Aristarchus being only 3′35″ so distant). If we assume that this was in a direction perpendicular to the limb for the libration at that time, it coincides pretty well with the limb distance of the active spots which were located there more recently by Greenacre and Barr (Greenacre, 1963) and are marked on Figure 22–3.
In fact, the modern reader may readily discern that a simple change of a few expressions and turns of the text should enable us paraphrase, in Herschel’s language, the more recent observations of Greenacre and Barr (“a burning charcoal thinly covered by ashes, three miles across on the Moon”) as well as those of Kopal and Rackham (“large, pretty faint nebulae, that are gradually brighter in the middle, but no well-defined luminous spot can be discerned in them”). As none of the observations which Herschel reported to have made were ever found wrong in any essential respect -though many had to await posthumous verification — we have no reason to believe that the same should not be true of his lunar observations as described in the preceding paragraphs; and if so, there seems but little doubt that the first indications of the luminescence on the dark side of the Moon were noted already by Herschel not less than 180 years ago.
Of subsequent observers who recorded from time to time the occurrence of similar phenomena (for appropriate references cf., e.g., Flamm and Lingenfelter, 1965) it should be merely noted that stellar magnitudes assigned by them to such bright spots possess little or no quantitative meaning; for the definition of the stellar magnitude as we know it to-day (i.e., corresponding to an intensity ratio of the fifth root of 100) was not proposed till 1850 by Pogson; and further decades had yet to elapse before such magnitudes could be properly measured. To Herschel, the term “stellar magnitude” carried still but little more quantitative meaning than it did to Hipparchos two thousand years before.
In order to appreciate the fact that Herschel’s reference to a luminous spot on the dark side of the Moon in 1783 having the appearance of a red star of about the 4th magnitude” must have grossly exaggerated the situation, suffice it to recall that one square second of earthlit lunar ground corresponds in brightness to a star of + 13.5 visual magnitude. Accordingly, (as one square second at the mean distance of the Moon corresponds approximately to one square mile of its surface) the doubling of the normal surface brightness of each square second would give it the brightness of a star of only 13.5 – 0.75 = 12.7 vis. magn.; and ten square miles of such ground would then possess an integrated magnitude of 12.7 – 2.5 = 10.2.
In more recent times, the subject of the lunar luminescence as such was originally opened up by Khan (1946), Kaplan (1946) and Herzberg (1946), seeking luminescence of such traces of tenuous gas as may surround the lunar globe; but it was Kozyrev (1956) who first realized clearly that the virtual absence of any such atmosphere is bound to relegate the lunar “auroral zone” on to the solid surface.
Kozyrev’s quest for the indications of such luminescence by the line-depth method commenced in the autumn of 1955, with the aid of a three-prism spectrograph attached to the 50-inch reflector of the Crimean Astrophysical Observatory, giving a dispersion of 50 Å/mm at Hγ, with a spectral resolution of about 2.5 Å; and his 0.05 mm slit-width intercepted on the lunar surface a strip of little less than 4 km across. In order to secure accurate observations of luminescent intensity, he restricted his measurements to the H and K lines of Ca 11 — two of the strongest absorption lines in that part of the solar spectrum where the intensity of the adjacent continuum rapidly falls off. In this way he studied the spectrum of the moonlight scattered from the plains of Mare Serenitatis and Imbrium; from the craters Aristarchus, Copernicus, Plato, Schickard and Wood’s spot. Only Aristarchus (together with the neighbouring Herodotus) gave positive results, as described in the text.
The only other specific observations aimed at investigation of lunar luminescence by the line-depth method before 1960 were those of Dubois (1957, 1959) — in point of fact, Dubois (1959, p. 20) stated that his first observations of this effect were undertaken between 1949 November — 1952 March; and that a report (unpublished) based upon them was read before the Société Française de Physique (section du Sud-Ouest) on November 21, 1952. His best observations were made subsequently with the high-dispersion solar spectrographs at Arcetri and Utrecht. He employed slit widths as small as 0.2 Å, which allowed him to measure the depths of many more lines than Kozyrev; but in his publications he quoted luminescence measurements only on the Fraunhofer lines D, F, E, G, and Ha. The characteristics of his spectrographs prevented him from observing too far in the violet — a fact which rendered comparison with Kozyrev’s results difficult. However, Dubois’s observations referred to some 90 different points of the lunar surface; and he claimed that about half of these showed luminescence at the time of observation, of fractional intensity ranging from 3 to 25 per cent.
Unlike Kozyrev, Grainger (1962) used a grating spectrograph giving a dispersion of 5 A/mm and spectral resolution of more than 0.1 Å; the registration was photoelectric, with a separate channel to compensate the seeing. The size of the slit intercepted on the lunar surface a rectangular area of approximately 1 × 10 miles in size. The photoelectric spectrograph used by Scarfe was described by Griffin (1961).
For photometric evidence of lunar luminescence from lunar eclipse studies cf., Danjon (1920); Dubois (1944, 1955); Link (1946); Link and Siroky (1951); Bakharev (1953); Cimino and Fortini (1953); Cimino, Fortini and Gianuzzi (1955); Fortini (1954, 1955); Cimino (1957); Cimino and Fresa (1958); Matsushima (1965); Cohen (1965); or Sanduleak and Stock (1965).
For Polarimetric evidence of lunar luminescence cf., Gehrels (1964) or Gehrels, Coffeen and Owings (1964).
For transient colour changes which may be attributable to luminescence, cf., e.g., Blackwell and Ingham (1961), Kopal and Rackham (1963; 1964a, b), or Greenacre (1963, 1965). Cf. also Kopal (1964b); Middlehurst (1964), Botley (1964), or Flamm and Lingenfelter (1965) for luminous phenomena on the lunar night side.
For spectroscopic investigations of lunar luminescence by the line-depth method, cf. Link (1951); Kozyrev (1956); Dubois (1956, 1957, 1959); Grainger (1962); Grainger and Ring (1962); Spinrad (1964); Scarfe (1965) or Myronova (1965).
Solar-lunar particle relations underlying lunar luminescence have recently been studied by Cameron (1964), Anand, Oster and Sofia (1964); Dodson and Hedeman (1964); Bigg (1963, 1964); or Aronowitz and Milford (1965) who considered the magnetic shielding of the lunar surface from the solar wind as a function of the lunar magnetic moment; while the magneto-hydro-dynamic wake of the Moon in solar wind was recently investigated by Ness (1965).
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Kopal, Z. (1966). Luminescence of the Lunar Surface. In: An Introduction to the Study of the Moon. Astrophysics and Space Science Library. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-6320-2_22
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