CHAPTER VII.

AURORA-LIKE PATCHES ON THE PARTIALLY-ECLIPSED MOON.

[Sidenote: Aurora-like patches on the partially-eclipsed moon, Feb. 27, 1877.]

In anticipation of the total eclipse of the Moon on the 27th February, 1877, several articles appeared in the leading journals of the day describing, for the public benefit, the appearances which might be expected during the occurrence of the phenomenon.

[Sidenote: Formerly it was thought the moon was illuminated by auroral light.]

Among these was one by Mr. R. A. Proctor, in which the following passage occurs:—“That dull, or occasionally glowing red colour, shown by the moon when she is fully and even deeply immersed in the shadow of the earth, is a phenomenon whose explanation is not without interest. Formerly it was thought that the moon possessed an inherent light, or _perhaps was illuminated by auroral light_, which only became discernible at the time of total eclipse. Indeed even Sir W. Herschel fell into the mistake of supposing this the only available explanation, having miscalculated the efficiency of the true cause.”

[Sidenote: Author’s notes of the eclipse. Colour-tints described. A crimson-scarlet tint reminded author of an auroral glow.]

This passage was only pointed out to me by a friend after the eclipse had actually taken place, and I had sent him some notes of what I then saw. My notes on the occasion comprised, amongst others, the following remarks:—“The tints of colour also during partial eclipse, owing, no doubt, to the moon’s considerable altitude, were singularly bright and well contrasted. Silver-grey, dusky copper-red, and the same tint clearer and brighter were ranged side by side with a lovely jewel effect. _We noticed also at times a crimson-scarlet tint, deeper and less mixed with yellow than the copper colour._ This last tint reminded me much of a _crimson glow common to the Aurora_, and which I also once distinctly remarked (of course in a weaker degree) in the zodiacal light” (_antè_, p. 68).

[Sidenote: Eclipse, Aug. 23-24, 1877. Sky clear, but eclipsed moon misty and indistinct until total obscuration. Succession of colours.]

On the occasion of the eclipse of August 23-24, 1877, we were favoured at Guildown, in common with many other places, by a singularly clear sky during the progress of the moon’s obscuration and subsequent clearing. In the early part of the evening, however, the moon, from some cause (possibly atmospheric vapour), seemed to have, as the earth’s shadow advanced on its disk, an unexpectedly misty and indistinct appearance, which lasted up to and including total obscuration. Golden yellow, yellow copper, dull copper, ruddy copper, and dull red were successively the principal colours observed at different times and at various portions of the moon’s surface.

[Sidenote: As shadow passed off, indistinctness gave way to a sharpness of the moon’s features as seen through shadow. Two patches of crimson light described.]

After referring to some spectroscopic appearances, my notes then ran on thus:—“As the shadow began to pass off, and the bright sharp crescent of the illuminated portion of the moon to appear, the general aspect of the moon’s disk seemed to me to greatly change. The certain amount of indistinctness noticeable during approach and continuance of totality, gave way to a considerable sharpness of the moon’s features as seen through the shadow. The shadowed part glowed with a richer copper tint, on which were seen dark, almost black, spots and patches.” Then follows a description of these; and the notes continue:—“Two features here struck me—the one a continuation of the upper limb of the illuminated crescent, so that it seemed to form a bead of light just on the centre of the upper edge of the moon; the other _two patches of crimson light_, similar to those I described as having been seen in the last total eclipse. One of these, quite a small one, was just under the elongated bead before described; the other, a much larger and more diffused one, was seen towards the south-west limb of the moon, about midway between it and the centre. The spots or patches were of a decidedly crimson-red, in contrast to the ordinary copper-red of the disk, and were noticed by my friend as well as by myself.”

[Sidenote: Patches well seen in field-glass; lost in small refractor. They gradually deepened in tint.]

These were eye observations. The patches were quite well seen (but not so brightly as with the eye) with a double achromatic field-glass. With a 3¼-inch Cooke refractor and low power, they seemed lost in the general moon tint; but they were then diminishing in brightness. From a comparison of my two sketches, the patches seem to have gradually deepened in tint, and we considered them to have disappeared in a like gradual manner.

[Sidenote: Two sketches taken.]

My first sketch was taken shortly after end of total phase; the second about ten minutes later. I have reproduced the original sketches in preference to any drawing prepared from them (Plate IV. figs. 2 and 3).

The patches did not last long, but were lost as the shadow swept off the moon. I saw nothing of the sort during the approach of or pending totality, nor until a small crescent of the moon began to appear behind the shadow.

I have looked for other accounts of these patches, but cannot find any. Most observers have described the deeper colour of the shadowed moon by the word “copper.” Some extend this colour to red; but there is probably much in the state of the atmosphere affecting this.

[Sidenote: Dec. 3, 1703, moon’s colour described.]

At Avignon, December 3, 1703, the moon appeared, pending eclipse, “extraordinarily illuminated and of a very bright red,” while other and different features were seen at Montpelier.

On March 19, 1848, observers in England, Ireland, and Belgium described the moon’s disk as “intensely bright coppery red.” On the occasion of August 23-24, 1877, before mentioned, an article in one of the public papers described the moon’s disk, during totality, as of a “dull copper colour.”

[Sidenote: Mr. Keye’s observation.]

Mr. Henry Keye, in the Engadine, at a height of 4500 feet above sea-level, and with the purest air, saw the partially covered moon (before totality) as a “dull copper colour.”

[Sidenote: Prof. Pritchard’s. M. Faye’s. Dr. Allnatt’s at Frant.]

Prof. Pritchard, writing from the Oxford University Observatory, says that at 12h 10m (about the time my sketches were taken) there was a good deal of light on the moon’s following limb, and the colour was “more red than copper,” and apparently redder than it had been at a similar distance of time before totality. Mons. Faye reported to the French Academy of Sciences that “a striking phenomenon not previously noticed was that the reddish tinge, resembling that of a fine sunset, was deepest at the margin of the disk, a circumstance which he could not explain.” Dr. Allnatt, writing from Frant, says:—“At totality the moon’s disk presented a most extraordinary appearance: the western limb was comparatively transparent, but the main body appeared as though enveloped in a semi-opaque clot of coagulated blood, through which the lunar features were dimly visible.”

[Sidenote: Observations as to the patches.]

The observations of Prof. Pritchard and Mons. Faye point more immediately to redness; and this is the nearest approach I can find to the patches I noticed. These patches do not seem to me easy of explanation. They could not well be colours or details due to the actual surface of the moon itself. The moon, we are aware, has only a certain portion of the visible disk slightly tinted. The Mare Serenitatis is certainly of a slight green tinge; and to the Palus Somni and certain other districts is attributed a pale red or pink; but these tints could hardly have sufficed to produce the effect seen, as the patches were conspicuous for a bright and decided colour. The positions, moreover, did not correspond; while the ease with which other details of the surface were seen at the time would, if the tints had arisen from the surface itself, probably have enabled the circumstance to be detected.

[Sidenote: Refraction of sun’s rays not a satisfactory explanation.]

The refraction of the sun’s rays by passage through the earth’s atmosphere is, too, not a satisfactory explanation. This, as judged by the appearance of the covered moon immediately before and at totality, gives a disk of shadow deeper in tone in the centre and lightening towards the edges, but in other respects fairly uniform, so that the whole disk seems to partake of the same tint and its graduations; and this is what might have been expected under the circumstances. The patches, on the other hand, were quite local.

[Sidenote: Question of lunar atmosphere.]

The theory of the moon’s possessing no atmosphere whatever is now very generally, but perhaps too readily, received (mainly upon the evidence of the spectroscopic observations of occulted stars[9]), as there still seems a reasonable doubt whether our satellite may not possess an atmosphere, possibly rarefied, but yet sufficiently dense to permit of the formation of cloud or vapour.

[Sidenote: Instance of patch of vapour or cloud on moon’s surface.]

A curious case, in which a patch of vapour or cloud was supposed to be detected on the moon’s surface, is reported by the Rev. J. B. Emmett in a communication to the ‘Annals of Philosophy’ (New Series, vol. xii. p. 81). It is dated “Great Ouseburn, near Boroughbridge, July 5, 1826,” the observation being made with “the greatest care with a very fine telescope.”

On the 12th April 8h, while observing the part of the moon called Palus Mœotis by Nevelius, with an excellent Newtonian reflector of 6 inches aperture, at a particular part of the Palus, which he minutely describes, he saw, with powers 70 and 130, “a very conspicuous spot wholly enveloped in black nebulous matter, which, as if carried forward by a current of air, extended itself in an easterly direction, inclining a little towards the south, rather beyond the margin of Mœotis.” April 13th 8h to 9h, the cloudy appearance was reduced both in extent and intensity, and the spot from which it seemed to issue had become more distinctly visible. On April 17th scarcely a trace of the nebulous matter remained; but so long after as June 10th 8h “a little blackness” remained about the spot. Mr. Emmett suggested “smoke of a volcano or cloudy matter.” A copy of the drawing annexed to the paper is given on Plate X. fig. 10 (black patch on moon). If this observation was (as it certainly appears to be) critical and exact, there must have been a disturbance of the moon’s surface, indicating some sort of cloud- or vapour-supporting atmosphere; and probably, for the purposes of Auroræ, an atmosphere of a very rarefied condition would suffice[10].

[Sidenote: Prof. Alexander’s evidence in favour of a lunar atmosphere.]

According to the ‘New York Tribune,’ at a recent semi-annual meeting of the American Academy of Sciences, Professor Alexander “brought forward a variety of evidence tending to indicate some envelope like an atmosphere for the moon. The evidence was principally drawn from observations during eclipses. The explanations usually offered for the bright band seen around the moon at such times was fully considered, and shown to be inadequate, though good as far as they would apply. The ruddy band of light is much too broad to be the sun’s chromosphere. It was most apparent in those instances where the moon was nearest the earth. It would best be accounted for by supposing an atmosphere to the moon, a thin remnant of ancient nebulosity, comparable to that which accompanies the earth and gives rise to the appearance of the Aurora Borealis.” Is it not, however, possible that the appearance might have arisen from Auroræ in action within the region of the earth’s own atmosphere during the passage of the sun’s rays through it at the time of the eclipse? The whole subject is difficult of explanation, and should be one of the points for attention on the occasion of the next total Lunar eclipse. It seemed to me appropriate for introduction into the present history of the Aurora, whatever its solution may ultimately be.

[Sidenote: Mars and Jupiter.]

In the case of Mars and Jupiter, whose atmospheres are sufficiently recognized, red- and scarlet-tinted patches are frequently noticed. In Mars this is generally attributed to the geological character of the surface of the planet itself; but I have observed on Mars’s surface during the recent opposition a local rosy tint of a more diffused and indefinite character; and in the case of Jupiter the appearances seem almost always connected with the clouds’ belts, as distinguished from the regions lying nearer to the planet’s surface.

[Sidenote: Prof. Dorna’s “Lunar Aurora.”]

Professor Dorna, of Turin, ascribed a flickering light seen on the reddened disk of the moon during the Lunar eclipse of February 1877 to the action of a _Lunar Aurora_, holding that the refraction of the sun’s rays within the cone of the earth’s shadow was not an adequate explanation (‘L’Opinione Nazionale,’ March 3, 1877).

[Sidenote: Spectroscopic observations bearing on the subject. Mr. Christie’s observations at Greenwich.]

The spectroscope might have afforded some information on the question; but my own telescopes (8¼ and 3¼ in.) were not of sufficient aperture to give a sensible spectrum of a portion of the moon’s eclipsed surface, and my observations were chiefly made on the entire disk with hand-spectroscopes without a slit. Mr. Christie, at the Royal Observatory, Greenwich, made a set of observations during totality, and also during subsequent partial phase, with a single-prism spectroscope. During totality a strong absorption band was seen in the yellow, and the red and blue ends of the spectrum were completely cut off, while the orange was greatly reduced in intensity. The yellow and green were comparatively bright, and seemed to constitute the whole visible spectrum. The absorption band became narrowed as the end of the total phase approached, and during partial phase was reduced to a mere line. The red end of the spectrum was cut off by a dark band commencing about halfway from D to C, in which a black line was suspected. The bands observed were characteristic of the spectrum of light which has passed through a thick stratum of air. In the description of the spectrum of the Aurora in Part II., it will be seen that the conspicuous red and green lines of the Aurora are either coincident with, or very close to, some of these atmospheric lines. It does not appear that Mr. Christie examined the crimson patches specifically, nor that he saw bright lines on any part of the moon’s eclipsed disk.

[Sidenote: Mr. Pratt’s notes of Lunar Eclipse, August 23, 1877.]

Mr. Henry Pratt has also kindly handed me for use his notes of the Lunar eclipse of August 23, 1877, as seen at Brighton on a splendid night. They were made as the phenomenon progressed, are 58 in number, and in many instances only a few minutes, or even seconds, apart. A selection of them is here given:—9h 13m 50s, first contact of shadow. 9h 30m, shadow very dark; no details of disk easily seen. 9h 40m, first appearance of red. 9h 50m, _red_ all over disk, except margin bluish and S. part green tint. 10h 2m, _a sudden brightening of whole disk_, in strong contrast to two minutes previously. 10h 15m, _E. limb much darker_. 10h 35m, _south pole decidedly brightest_. 10h 44m, _S.E. limb much brighter_. 10h 48m, _whole disk much darker_. 10h 51m, _S.E. limb brightening again_. 11h 1m, _N.E. limb brightening_. 11h 3m, _N.E. limb has darkened and brightened three times during last two minutes_. 11h 20m, N. pole has _darkened_. 11h 21m, N. pole has _brightened_. 11h 24m 30s, N. pole darker _red_. 11h 35m, N. pole _bright_. 11h 35m 30s, same _dark_ and _red_. 11h 42m, N.E. limb especially bright for a few seconds, and then _reddened_ and shaded again. 11h 49m, _S. pole reddened_. 12h 1m, _S.W. limb reddest part; S. pole red; N. pole paler red_. 12h 3m 50s, first appearance of E. limb (my first sketch was made shortly after this, and my second about ten minutes later). 12h 21m, a bright patch on N.N.W. separated from N. pole. 12h 24m, _S.W. region is reddest part of eclipse_. 12h 40m, _redness_ of shadow fading out.

With a small Browning star-spectroscope Mr. Pratt saw the red and blue ends of the spectrum cut off, but nothing else. Mr. Pratt adds that the _red_ colour was not an effect of contrast or an optical delusion in any way, as was proved by using at times a limited field containing only the red portion under examination. In reference to the curious brightening and darkening of the disk, and the change from time to time of local colour, he says that with much experience he has seen nothing of the same marked character on other occasions, and that “the whole matter was at the time astonishing to me, but none the less real.” The local red patches seen by me seem also to have been observed by Mr. Pratt.

[Sidenote: Mr. Pratt’s observation on the floor of Plato.]

As an addition to the instances of Tycho, Picard, &c., mentioned in the note on p. 73, Mr. Pratt has also sent me his notes of some observations by him, of “local obscuration of the floor of Plato.” As somewhat condensed, they are as follows:—1872, July 16. While in other parts of the floor spots and streaks were well visible, “the N.W. portion was in such a hazy condition that nothing could be defined upon it.” 1873, Nov. 1. 27 light streaks seen (7 new): the brightness of the streaks was in excess of their usual character, as compared with the craterlets; “an _obliteration_ or _invisibility_ of _all_ the light streaks in the neighbourhood of craterlet no. 1 was very noticeable;” and also “a similar obliteration of the N. end of the streak called the Sector, near craterlet 3.” 1874, January 1. 18 light streaks seen, including 3 new, “some of which outshone other longer known ones. This was curious; for had they been as bright within the last two years as on this occasion I must have noticed them.” Mr. Pratt points out, as worthy of remark, that some months previous to November 1st, 1873, neither craterlets nor streaks on the floor of Plato “had maintained their previous characteristic brightness,”—a fact which he thinks ought to be considered together with the outbreak of brilliancy of both orders on that day, as well as the apparently sudden existence of new ones.

[Sidenote: Observation by Mr. Hirst of a dark shade on the moon.]

The ‘Observatory,’ March 1, 1879, p. 375, contains an account, by Mr. H. C. Russell, of some Astronomical Experiments made on the Blue Mountains, near Sydney, N. S. W. Among these it is noticed that on 21st October, 1878, at 9 A.M., when looking at the moon, Mr. Hirst found that a large part of it was covered with a dark shade, quite as dark as the shadow of the earth during an eclipse of the moon. Its outline was generally circular, and fainter near the edges. Conspicuous bright lunar objects could be seen through it; but it quite obliterated the view of about half the moon’s terminator, while those parts of the terminator not in the shadow were distinctly seen.

No change in the position of the shade could be detected after three hours’ watching. The observation is made, “One could hardly resist the conviction that it was a shadow; yet it could not be the shadow of any known body. If produced by a comet, it must be one of more than ordinary density, although dark bodies have been seen crossing the sun which were doubtless comets.” The diameter of the shadow from the part of it seen on the moon was estimated at about three quarters that of the moon[11].