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].