ECLIPSES OF THE MOON--GENERAL PRINCIPLES.
In dealing with eclipses generally, but with more especial reference to
eclipses of the Sun, in a previous chapter, it was unavoidable to mix
up in some degree eclipses of the Moon with those of the Sun. There
are, however, distinctions between the two phenomena which make it
convenient to separate them as much as possible. Eclipses of the Moon
are, like those of the Sun, divisible into "partial" and "total"
eclipses, but those words have a different application in regard to
eclipses of the Moon from what they have when eclipses of the Sun are in
question. A little thought will soon make it clear why this should be
the case. A partial eclipse of the Sun results from the visible body of
the Sun being in part concealed from us by the solid body of the Moon,
and so in a total eclipse there is total concealment of the one object
by the other.
But when we come to deal with partial and total eclipses of the Moon,
the situation, is materially different. The Moon becomes invisible by
passing into the dark shadow cast by the Earth into space.
[Illustration: FIG. 13.--THEORY OF AN ECLIPSE OF THE MOON.]
Fig. 13 will make this clear without the necessity of much verbal
explanation. S represents the Sun, E the Earth, and _mn_ the orbit of
the Moon. It is obvious that whilst the Moon is moving from _m_ to _n_
it becomes immersed in the Earth's shadow. But before actually reaching
the shadow the Moon passes through a point in its orbit at which it
begins to lose the full light of the Sun. This is the entrance into the
"penumbra" (or "Partial shade"). Similarly, after the eclipse, when the
Moon has emerged from the full shadow it does not all at once come into
full sunshine, but again passes through the stage of penumbral
illumination,[112] and under such circumstances (to speak in the style
of Old "Oireland") the invisible Moon is very often not invisible, and
the part partially eclipsed is often not eclipsed, and when the Moon is
totally eclipsed it is frequently still visible. Of course the general
idea involved in all cases of a body passing into the shadow of another
body is that the body which so passes disappears, because all direct
light is cut off from it. In the case, however, of a lunar eclipse this
state of things is not always literally accomplished, and very often
some residual light reaches the Moon (of course from the Sun) with the
result that traces of the Moon may often be discerned. The laws which
govern this matter are very ill-understood. The fact remains that if we
examine a series of reports of observed eclipses of the Moon extending
over many centuries (and records exist which enable us to do this) we
shall find that in some instances when the Moon was "totally" eclipsed
in the technical sense of that word, it was still perfectly visible,
whilst during other eclipses it absolutely and entirely disappeared from
view. Such eclipses are sometimes spoken of as "black" eclipses of the
Moon, but the phrase is not a happy one. Many instances of both kinds
will be found mentioned in the chapter on historical lunar
eclipses.[113]
[Illustration: FIG. 14.--CONDITIONS OF ECLIPSES OF THE MOON.]
The different conditions of eclipses of the Moon are illustrated by Fig.
14 which must be studied with the aid of the remarks made in a former
chapter concerning the apparent movements of the Sun and Moon and their
nodal passages. Suffice it to state here that in Fig. 14 AB represents
the ecliptic, and CD the Moon's path. The three black circles are
imaginary sections of the Earth's shadow as cast when the Earth is in
three successive positions in the ecliptic. If when the Earth's shadow
is near A the Moon should be at E, and in Conjunction with the Earth
the Moon will escape eclipse; if the Conjunction takes place with both
the Earth's shadow and the Moon a little further forward, say at F, the
Moon will be partially obscured; but if the Moon is at or very near its
node, as at G, it will be wholly involved in the Earth's shadow and a
total eclipse will be the result. In the case contemplated at G in the
diagram, the Moon is concentrically placed with respect to the shadow,
but the eclipse will equally be total even though the two bodies are not
concentrically disposed, so long as the Moon is wholly within the cone
of the Earth's shadow.[114]
Just as in the case of the Sun so with the Moon there are certain limits
on the ecliptic within which eclipses of the Moon _may_ take place,
other (narrower) limits within which they _must_ take place, and again
other limits beyond which they _cannot_ take place. Reverting to what
has been said on a previous page[115] with respect to these matters when
an eclipse of the Sun is in question it is only necessary to substitute
for the word "Conjunction," the word "Opposition"; and for 18½° and 15¼°
of longitude the figures 12½° and 9¼°. The limits in latitude will be
1° 3' and 0° 52' instead of 1° 34' and 1° 23'. These substitutions
made, the general ideas and facts stated with regard to the conditions
of an eclipse of the Sun will apply also to the one of the Moon.
It is to be noted that whereas eclipses of the Sun always begin on the
W. side of the Sun, eclipses of the Moon begin on the E. side of the
Moon. This difference arises from the fact that the Sun's movement in
the ecliptic is only apparent (it being the Earth which really moves),
whilst the Moon's movement is real.
Eclipses of the Moon, though more often and more widely visible than
eclipses of the Sun, do not offer by any means the same variety of
interesting or striking phenomena to the mere star-gazer, and it was
long thought that they were in a certain sense of no use to science.
Now, however, astronomers are inclined to utilise them for determining
the diameter of the Moon by noting occultations[116] of stars by the
Moon, the duration of a star's invisibility behind an eclipsed Moon
being a measure of the lunar diameter when such an observation is
properly transformed and "reduced." Observations of the heat radiated
(or rather reflected) by an eclipsed Moon have also been made with the
interesting result of showing that during an eclipse the Moon's power to
reflect solar heat to the Earth sensibly declines.
The duration of an eclipse of the Moon is dependent on its magnitude.
Where the eclipse is total the darkness, or what counts for such, may
last for nearly 4 hours, though this is an extreme limit rarely
attained. An eclipse of from 6 to 12 digits (to use the old-fashioned
nomenclature which has been already explained) will continue from 2½ to
3½ hours. An eclipse of 3 to 6 digits will last 2 or 3 hours, and a
smaller eclipse only 1 or 2 hours. The visual observations to be made in
connection with partial or total eclipses of the Moon chiefly relate to
the appearances presented by our satellite when immersed in the Earth's
shadow. On such occasions, as has been already stated, it frequently
happens that the Moon does not wholly disappear, but may be detected
either with a telescope or even without one. It may exhibit either a
dull grey appearance, or more commonly a pinkish-red hue to which the
designation "coppery" is generally applied. Perhaps the most remarkable
instance of this was the eclipse of March 19, 1848.
Mr. Forster who observed the phenomenon at Bruges thus describes[117]
what he saw:--"I wish to call your attention to the fact which I have
clearly ascertained, that during the whole of the late eclipse of March
19 the shaded surface presented a luminosity quite unusual, probably
about three times the intensity of the mean illumination of the eclipsed
lunar disc. The light was of a deep red colour. During the totality of
the eclipse the light and dark places on the face of the Moon could be
almost as well made out as on an ordinary dull moonlight night, and the
deep red colour where the sky was clearer was very remarkable from the
contrasted whiteness of the stars. My observations were made with
different telescopes, but all presented the same appearance, and the
remarkable luminosity struck everyone. The British Consul at Ghent, _who
did not know there was an eclipse_, wrote to me for an explanation of
the blood-red colour[118] of the Moon at 9 o'clock."
In striking contrast to this stands the total eclipse of Oct. 4, 1884,
which is described by Mr. E. J. Stone[119] as "much the darkest that I
have ever seen, and just before the instant of totality it appeared as
if the Moon's surface would be invisible to the naked eye during
totality; but such was not the case, for with the last appearance of the
bright reflected sunlight there appeared a dim circle of light around
the Moon's disc, and the whole surface became faintly visible, and
continued so until the end of totality."
A total eclipse of the Moon which happened on January 28, 1888, was
observed in many places under exceptionally favourable circumstances as
regards weather. The familiar copper colour is spoken of by many
observers. The Rev. S. J. Perry makes mention[120] of patches of colour
even as bright as "brick red, almost orange in the brighter parts," and
this, 20 minutes before the total phase began. Mr. Perry conducted on
this occasion spectroscopic observations for the first time on an
eclipsed Moon, but no special results were obtained.
Various explanations have been offered for these diversities of
appearance. Undoubtedly they depend upon differences in the condition of
the Earth's atmosphere, such as the unusual presence or unusual absence
of aqueous vapour; but it cannot be said that the laws which control
these diversities are by any means capable of being plainly enunciated,
notwithstanding that the explanation generally in vogue dates from as
far back as the time of Kepler. He suggested that the coppery hue was a
result of the refraction of the Earth's atmosphere which had the effect
of bending the solar rays passing through it, so that they impinged upon
the Moon even when the Earth was actually interposed between the Sun and
the Moon. That the outstanding rays which became visible are red may be
considered due to the fact that the blue rays are absorbed in passing
through the terrestrial atmosphere, just as both the eastern and western
skies are frequently seen to assume a ruddy hue when illuminated in the
morning or evening by the solar rays at or near sunrise or sunset.
Owing to the variable meteorological condition of our atmosphere, the
actual quantity of light transmitted through it is liable to
considerable fluctuations, and no wonder therefore that variations occur
in the appearances presented by the Moon during her immersion in the
Earth's shadow.
It has been suggested that if the portion of the Earth's atmosphere
through which the Sun's rays have to pass is tolerably free from aqueous
vapour, the red rays will be almost wholly absorbed, but not the blue
rays; and the resulting illumination will either only render the Moon's
surface visible with a greyish blue tinge, or not visible at all. This
will yield the "black eclipse"--to recall the phrase quoted elsewhere.
If, on the other hand, the region of the Earth's atmosphere through
which the Sun's rays pass be highly saturated, it will be the blue rays
which suffer absorption, whilst the red rays will be transmitted and
will impart a ruddy hue to the Moon. Finally, if the Earth's atmosphere
is in a different condition in different places, saturated in some parts
and not in others, a piebald sort of effect will be the result, and some
portions of the Moon's disc will be invisible, whilst others will be
more or less illuminated. Further illustrations of all these three
alternatives will be found amongst the eclipses of the Moon recorded in
the chapter[121] devoted to historical matters.
A few instances are on record of a curious spectacle connected with
eclipses of the Moon which must have a word of mention. I refer to the
simultaneous visibility of the Sun and the Moon above the horizon, the
Moon at the time being eclipsed. At the first blush of the thing this
would seem to be an impossibility, remembering that it is a cardinal
principle of eclipses, both of the Sun and of the Moon, that the three
bodies must be in the same straight line in order to constitute an
eclipse. The anomalous spectacle just referred to is simply the result
of the refraction exercised by the Earth's atmosphere. The setting Sun
which has actually set has apparently not done so, but is displaced
upwards by refraction. On the other hand, the rising Moon which has not
actually risen is displaced upwards by refraction and so becomes, as it
were, prematurely visible. In other words, refraction retards the
apparent setting of one body, the Sun, and accelerates the apparent
rising of the other body, the Moon. The effect of these two
displacements will be to bring the two bodies closer by more than 1° of
a great circle than they really are, this being the conjoint amount of
the double displacements due to refraction.
Amateur observers of eclipses of the Moon will find some pleasure, and
profit as well, in having before them on the occasion of an eclipse a
picture of the Moon's surface in diagrammatic form with a few of the
principal mountains marked thereon; and then watching from time to time
(say by quarters of an hour) the successive encroachments of the Earth's
shadow on the Moon's surface and the gradual covering up of the larger
mountains as the shadow moves forward. The curved lines represent the
gradual progress of the shadow during the eclipse named. This diagram,
ignoring the curved lines actually marked on it, may be used over and
over again for any number of eclipses, simply noting from the _Nautical
Almanac_ or other suitable ephemerides the points on the Moon's disc at
which the shadow first touches the disc as it comes on, and last touches
the disc as it goes off. The _Almanac_ indicates these points by
stating that the eclipse begins, or ends, as the case may be, at a point
which is so many degrees from the N. point of the Moon measured round
the Moon's circumference by the E. or by the W. as the case may be.
One other point and we have disposed of eclipses of the Moon. The shadow
which we see creeping over the Moon during an eclipse is, as we know,
the shadow cast by the Earth. If we notice it attentively we shall see
that its outline is curved, and that it is in fact a complete segment of
a circle. Moreover that the circularity of this shadow is maintained
from first to last so far as we are able to follow it. What is this,
then, but a proof of the rotundity of the earth? This shape of the
Earth's shadow on the Moon during a lunar eclipse was suggested as a
proof of the rotundity of the Earth by two old Greek astronomers,
Manilius and Cleomedes, who lived about 2000 years ago, and is one more
illustration of the great powers of observation and the general
acuteness of the natural philosophers of antiquity.
FOOTNOTES:
[Footnote 112: The time occupied by the Moon in passing through the
penumbra, before and after a lunar eclipse, will generally run to
about an hour for each passage. It will occasionally happen that the
Moon gets immersed in a penumbra but escapes the dark shadow. Such
an event will not be announced in the almanacs under the head of
"Eclipses."]
[Footnote 113: See p. 197 (_post_).]
[Footnote 114: The shadow is spoken of as being in the form of a
cone because it is necessarily such on account of the light-giving
disc of the Sun being so enormously larger in diameter than the
light-receiving sphere of the Moon. This idea can be pursued by any
reader with the aid of a lamp enclosed in a glass globe and an
opaque sphere such as a cricket ball.]
[Footnote 115: See p. 19 (_ante_).]
[Footnote 116: As to occultations see chap. xxi. (_post_).]
[Footnote 117: _Month. Not._, R.A.S., vol. viii. p. 132. March,
1848.]
[Footnote 118: A very striking chromolithograph of the lunar
eclipses of Oct. 4, 1884, and Jan. 28, 1888, showing the contrast
of--(1) an almost invisible grey Moon, and (2) a reddish-pink Moon,
will be found in the German astronomical monthly, _Sirius_, vol.
xxi. p. 241. Nov. 1888.]
[Footnote 119: _Month. Not._, R.A.S., vol. xlv. p. 35.]
[Footnote 120: _Month. Not._, R.A.S., vol. xlviii. p. 227. March
1888.]
[Footnote 121: p. 197 (_post_).]