THE GENERAL ASPECT OF THE LUNAR SURFACE.
We have now reached that stage of our subject at which it behoves us to
repair to the telescope for the purpose of examining and familiarising
ourselves with the various classes of detail that the lunar surface
presents to our view.
That the moon is not a smooth sphere of matter is a fact that manifested
itself to the earliest observers. The naked eye perceives on her face
spots exhibiting marked differences of illumination. These variations of
light and shade, long before the invention of the telescope, induced the
belief that she possessed surface irregularities like those that
diversify the face of the earth, and from analogy it was inferred that
seas and continents alternated upon the lunar globe. It was evident,
from the persistence and invariability of the dusky markings, that they
were not due to atmospheric peculiarities, but were veritable variations
in the character or disposition of the surface material. Fancy made
pictures of these unchangeable spots: untutored gazers detected in them
the indications of a human countenance, and perhaps the earliest map of
the moon was a rough reproduction of a man’s face, the eyes, nose and
mouth representing the more salient spots discernible upon the lunar
disc. Others recognised in these spots the configuration of a human
form, head, arms and legs complete, which a French superstition that
lingers to the present day held to be the image of Judas Iscariot
transported to the moon in punishment for his treason. Again, an Indian
notion connects the lunar spots with a representation of a roebuck or a
hare, and hence the Sanskrit names for the moon, _mrigadhara_, a
roebuck-bearer, and _’sa’sabhrit_, a hare-bearer. Of these similitudes
the one which has the best pretensions to a rude accuracy is that first
mentioned; for the resemblance of the full moon to a human countenance,
wearing a painful or lugubrious expression, is very striking. Our
illustration of the full moon (Plate III.) is derived from an actual
photograph;[4] the relative intensities of light and shade are hence
somewhat exaggerated; otherwise it represents the full moon very nearly
as the naked eye sees it, and by gazing at the plate from a short
distance,[5] the well-known features will manifest themselves, while
they who choose may amuse themselves by arranging the markings in their
imagination till they conform to the other appearances alluded to.
We may remark in passing that by one sect of ancient writers the moon
was supposed to be a kind of mirror, receiving the image of the earth
and reflecting it back to terrestrial spectators. Humboldt affirmed that
this opinion had been preserved to his day as a popular belief among the
people of Asia Minor. He says, “I was once very much astonished to hear
a very well educated Persian from Ispahan, who certainly had never read
a Greek book, mention when I showed him the moon’s spots in a large
telescope in Paris, this hypothesis as a widely diffused belief in his
country: ‘What we see in the moon,’ said the Persian, ‘is ourselves; it
is the map of our earth.’” Quite as extravagant an idea, though perhaps
a more excusable one, was that held by some ancient philosophers, to the
effect that the spots on the moon were the shadows of opaque bodies
floating in space between it and the sun.
[Illustration: PLATE III.
FULL MOON.]
An observer watching the forms and positions of the lunar face-marks,
from night to night and from lunation to lunation, cannot fail to notice
the circumstance that they undergo no easily perceptible change of
position with respect to the circular outline of the disc; that in fact
the face of the moon presented to our view is always the same, or very
nearly so. If the moon had no orbital motion we should be led from the
above phenomenon to conclude that she had no axial motion, no movement
of rotation; but when we consider the orbital motion in connection with
the permanence of aspect, we are driven to the conclusion—one, however,
which superficial observers have some difficulty in recognising—that the
moon has an axial rotation equal in period to her orbital revolution.
Since the moon makes the circuit of her orbit in twenty-seven days and
one-third (more exactly 27d. 7h. 43m. 11s.) it follows that this is the
time of her axial rotation, as referred to the stars, or as it would be
made out by an observer located at a fixed position in space outside the
lunar orbit. But if referred to the sun this period appears different;
because the moon while revolving round the earth is, with the earth,
circulating around the sun. Suppose the three bodies, moon, earth, and
sun, to be in a line at a certain period of a lunation, as they are at
full moon: by the time the moon has completed her twenty-seven days’
journey around the earth, the latter will have moved along twenty-seven
days’ march of its orbit, which is about twenty-seven degrees of
celestial longitude: the sun will apparently be that much distant from a
straight line passing through earth and moon, and the moon must
therefore move forward to overtake the sun before she can assume the
full phase again. She will take something over two days to do this;
hence the solar period of her revolution becomes more than twenty-nine
days (to be exact, 29d. 12h. 44m. 2s. ·87). This is the length of a
solar day upon the moon—the interval from one sunrise to another at any
spot upon the equator of our satellite, and the interval between
successive reappearances of the same phase to observers on the earth.
The physical cause of the coincidence of times of rotation and
revolution was touched upon in a previous chapter.
We have said that the moon continuously presents to us the same
hemisphere. This is generally true, but not entirely so. Galileo, by
long scrutiny, familiarised himself with every detail of the lunar-disc
that came within the limited grasp of his telescopes, and he recognised
the fact that according as the position of the moon varied in the sky,
so the aspect of her face altered to a slight degree; that certain
regions at the edge of her disc, alternately came in sight and receded
from his view. He perceived, in fact, an _apparent_ rocking to and fro
of the globe of the moon; a sort of balancing or _libratory_ motion.
When the moon was near the horizon he could see spots upon her uppermost
edge, which disappeared as she approached the zenith, or highest point
of her nightly path; and as she neared this point, other spots, before
invisible, came into view, near to what had been her lower edge. Galileo
was not long in referring this phenomenon to its true cause. The centre
of motion of the moon being the centre of the earth, it is clear that an
observer on the surface of the latter, looks down upon the rising moon
as from an eminence, and thus he is enabled to see more or less over or
around her. As the moon increases in altitude, the line of sight
gradually becomes parallel to the line joining the observer and the
centre of the earth, and at length he looks her full in the face: he
loses the full view and catches another side face view as she nears the
horizon in setting. This phenomenon, occurring as it does, with a daily
period, is known as the _diurnal libration_.
But a kindred phenomenon presents itself in another period, and from
another cause. The moon rotates upon her axis at a speed that is
rigorously uniform. But her orbital motion is not uniform, sometimes it
is faster, and at other times slower than its average rate. Hence, the
angle through which she moves along her orbit in a given time, now
exceeds, and now falls short of the angle through which she turns upon
her axis. Her visible hemisphere thus changes to an extent depending
upon the difference between these orbital and axial angles, and the
apparent balancing thus produced is called the _libration in longitude_.
Then there is a _libration in latitude_ due to the circumstance that the
axis of the moon is not exactly perpendicular to the plane of her orbit;
the effect of this inclination being, that we sometimes see a little
more of the north than of the south polar regions of our satellite, and
_vice versâ_.[6]
The extent of the moon’s librations, taking them all and in combination
into account, amounts to about seven degrees of arc of latitude or
longitude upon the moon, both in the north-south and east-west
directions. And taking into account the whole effect of them, we may
conclude that our view of the moon’s surface, instead of being confined
to one half, is extended really to about four-sevenths of the whole area
of the lunar globe. The remaining three-sevenths must for ever remain a
_terra incognita_ to the habitants of this earth, unless, indeed, from
some catastrophe which it would be wild fancy to anticipate, a period of
rotation should be given to the moon different from that which it at
present possesses. Some highly fanciful theorists have speculated upon
the possible condition of the invisible hemisphere, and have propounded
the absurd notion that the opposite side of the moon is hollow, or that
the moon is a mere shell; others again have urged that the hidden half
is more or less covered with water, and others again that it is peopled
with inhabitants. There is, however, no good reason for supposing that
what we may call the back of the moon has a physical structure
essentially different from the face presented towards us. So far as can
be judged from the peeps that libration enables us to obtain, the same
characteristic features (though of course with different details)
prevail over the whole lunar surface.
The speculative ideas held by the philosophers of the pre-telescopic
age, touching the causes which produced the inequalities of light and
shade upon the moon, received their _coup de grâce_ from the revelations
of Galileo’s glasses. Our satellite was one of the earliest objects, if
not actually the first, upon which the Florentine turned his telescope;
and he found that the inequalities upon her surface were due to
differences in its configuration analogous to the continents and
islands, and (as might then have been thought) the seas of our globe. He
could trace, even with his moderate means, the semblance of
mountain-tops upon which the sun shone while their lower parts were in
shadow, of hills that were brightly illuminated upon their sides towards
the sun, of brightly shining elevations, and deeply shadowed
depressions, of smooth plains, and regions of mountainous ruggedness. He
saw that the boundary of sunlight upon the moon was not a clearly
defined line, as it would be if the lunar globe were a smooth sphere, as
the Aristotelians had asserted, but that the terminator was uneven and
broken into an irregular outline. From these observations the Florentine
astronomer concluded that the lunar world was covered not only with
mountains like our globe, but with mountains whose heights far surpassed
those existing upon the earth, and whose forms were strangely limited to
circularity.
Galileo’s best telescopes magnified only some thirty times, and the
views which he thus obtained, must have been similar to those exhibited
by the smaller photographs of the moon produced in late years by Mr. De
la Rue and now familiar to the scientific public. Of course there is in
the natural moon as viewed with a small telescope a vivid brilliancy
which no art can imitate, and in photographs especially there is a
tendency to exaggeration of the depths of shade in a lunar picture. This
arises from the circumstance that various regions of the moon do not
impress a chemically sensitized plate as they impress the retina of the
eye. Some portions, notably the so-called “seas” of the moon, which to
the eye appear but slightly duller than the brighter parts, give off so
little _actinic_ light that they appear as nearly black patches upon a
photograph, and thus give an undue impression of the relative brightness
of various parts of the lunar surface. Doubtless by sufficient exposure
of the plate in the camera-telescope the dark patches might be rendered
lighter, but in that case the more strongly illuminated portions, which
after all are those most desirable to be preserved, would be lost by the
effect which photographers understand as “solarization.”
In speaking of a view of the moon with a magnifying power of thirty, it
is necessary to bear in mind that the visible features will differ
considerably with the diameter of the object-glass of the telescope to
which this power is applied. The same details would not be seen alike
with the same power upon an object-glass of 10 inches diameter and one
of 2 inches. The superior illumination of the image in the former case
would bring into view minute details that could not be perceived with
the smaller aperture. He who would for curiosity wish to see the moon,
or any other object, as Galileo saw it, must use a telescope of the same
size and character in all respects as Galileo’s: it will not do to put
his magnifying power upon a larger telescope. With large telescopes, and
low powers used upon bright objects like the moon, there is a blinding
flood of light which tends to contract the pupil of the eye and prevent
the passage of the whole of the pencil of rays coming through the
eye-piece. Although this last result may be productive of no
inconvenience, it is clearly a waste of light, and it points to a rule
that the lowest power that a telescope should bear is that which gives a
pencil of light equal in diameter to the pupil of the eye under the
circumstances of brightness attendant upon the object viewed. In
observing faint objects this point assumes more importance, since it is
then necessary that all available light should enter the pupil. The
thought suggests itself that an artificial enlargement of the pupil, as
by a dose of belladonna, might be of assistance in searching for faint
objects, such as nebulæ and comets: but we prefer to leave the
experiment for those to try who pursue that branch of astronomical
observation.
A merely cursory examination of the moon with the low power to which we
have alluded is sufficient to show us the more salient features. In the
first place we cannot help being struck with the immense preponderance
of circular or craterform asperities, and with the general tendency to
circular shape which is apparent in nearly all the lunar surface
markings; for even the larger regions known as the “seas” and the
smaller patches of the same character seem to repeat in their outlines
the round form of the craters. It is at the boundary of sunlight on the
lunar globe that we see these craterform spots to the best advantage, as
it is there that the rising or setting sun casts long shadows over the
lunar landscape, and brings elevations and asperities into bold relief.
They vary greatly in size, some are so large as to bear an estimable
proportion to the moon’s diameter, and the smallest are so minute as to
need the most powerful telescopes and the finest conditions of
atmosphere to perceive them. It is doubtful whether the smallest of them
have ever been seen, for there is no reason to doubt that there exist
countless numbers that are beyond the revealing powers of our finest
telescopes.
From the great number and persistent character of these
circumvallations, Kepler was led to think that they were of artificial
construction. He regarded them as pits excavated by the supposed
habitants of the moon to shelter themselves from the long and intense
action of the sun. Had he known their real dimensions, of which we shall
have to speak when we come to describe them more in detail, he would
have hesitated in propounding such a hypothesis; nevertheless it was, to
a certain extent, justified by the regular and seemingly unnatural
recurrence of one particular form of structure, the like of which is,
too, so seldom met with as a structural feature of the surface of our
own globe.
The next most striking features, revealed by a low telescopic power upon
the moon, are the seemingly smooth plains that have the appearance of
dusky spots, and that collectively cover a considerable portion—about
two-thirds—of the entire disc. The larger of these spots retain the name
of _seas_, the term having been given when they were supposed to be
watery expanses, and having been retained, possibly to avoid the
confusion inevitable from a change of name, after the existence of water
upon the moon was disproved. Following the same order of nomenclature,
the smaller spots have received the appellations of _lakes_, _bays_ and
_fens_. We see that many of these “seas” are partially surrounded by
ramparts or bulwarks which, under closer examination, and having regard
to their real magnitude, resolve themselves into immense mountain
chains. The general resemblance in form which the bulwarked plains thus
exhibit to the circular craters of large size, would lead us to suppose
that the two classes of objects had the same formative origin, but when
we take into account the immense size of the former, and the process by
which we infer the latter to have been developed, the supposition
becomes untenable.
Another of the prominent features which we notice as highly curious, and
in some phases of the moon—at about the time of full—the most remarkable
of all, are certain bright lines that appear to radiate from some of the
more conspicuous craters, and extend for hundreds of miles around. No
selenological formations have so sorely puzzled observers as these
peculiar streaks, and a great deal of fanciful theorizing has been
bestowed upon them. As we are now only glancing at the moon, we do not
enter upon explanations concerning them or any other class of details;
all such will receive due consideration in their proper order in
succeeding chapters.
We thus see that the classes of features observable upon the moon are
not great in number: they may be summed up as _craters_ and their
central cones, _mountain chains_, with occasional isolated peaks,
_smooth plains_, with more or less of irregularity of surface, and
_bright radiating streaks_. But when we come to study with higher powers
the individual examples of each class we meet with considerable
diversity. This is especially the case with the craters, which appear
under very numerous variations of the one order of structure, viz., the
ring-form. A higher telescopic power shows us that not only do these
craters exist of all magnitudes within a limit of largeness, but
seemingly with no limit of smallness, but that in their structure and
arrangement they present a great variety of points of difference. Some
are seen to be considerably elevated above the surrounding surface,
others are basins hollowed out of that surface and with low surrounding
ramparts; some are merely like walled plains or amphitheatres with flat
plateaux, while the majority have their lowest point of hollowness
considerably below the general level of the surrounding surface; some
are isolated upon the plains, others are aggregated into a thick crowd,
and overlapping and intruding upon each other; some have elevated peaks
or cones in their centres, and some are without these central cones,
while the plateaux of others again contain several minute craters
instead; some have their ramparts whole and perfect, others have them
breached or malformed, and many have them divided into terraces,
especially on their inner sides.
In the plains, what with a low power appeared smooth as a water surface
becomes, under greater magnification, a rough and furrowed area, here
gently undulated and there broken into ridges and declivities, with now
and then deep rents or cracks extending for miles and spreading like
river-beds into numerous ramifications. Craters of all sizes and classes
are scattered over the plains; these appear generally of a different
tint to the surrounding surface, for the light reflected from the plains
has been observed to be slightly tinged with colour, The tint is not the
same in all cases: one large sea has a dingy greenish tinge, others are
merely grey and some others present a pale reddish hue. The cause of
this diversity of colour is mysterious; it has been supposed to indicate
the existence of vegetation of some sort; but this involves conditions
that we know do not exist.
The mountains, under higher magnification, do not present such diversity
of formation as the craters, or at least the points of difference are
not so apparent; but they exhibit a plentiful variety of combinations.
There are a few perfectly isolated examples that cast long shadows over
the plains on which they stand like those of a towering cathedral in the
rising or setting sun. Sometimes they are collected into groups, but
mostly they are connected into stupendous chains. In one of the grandest
of these chains, it has been estimated that a good telescope will show
3000 mountains clustered together, without approach to symmetrical
order. The scenery which they would present, could we get any other than
the “bird’s eye view” to which we are confined, must be imposing in the
extreme, far exceeding in sublime grandeur anything that the Alps or the
Himalayas offer; for while on the one hand the lunar mountains equal
those of the earth in altitude, the absence of an atmosphere, and
consequently of the effects produced thereby, must give rise to
alternations of dazzling light and black depths of shade combining to
form panoramas of wild scenery that, for want of a parallel on earth, we
may well call unearthly. But we are debarred the pleasure of actually
contemplating such pictures by the circumstance that we look _down_ upon
the mountain tops and into the valleys, so that the great height and
close aggregation of the peaks and hills are not so apparent. To compare
the lunar and terrestrial mountain scenery would be “to compare the
different views of a town seen from the car of a balloon, with the more
interesting prospects by a progress through the streets.” Some of the
peculiarities of the lunar scenery we have, however, endeavoured to
realize in a subsequent Chapter.
A high power gives us little more evidence than a low one upon the
nature of the long bright streaks that radiate from some of the more
conspicuous craters, but it enables us to see that those streaks do not
arise from any perceptible difference of level of the surface—that they
have no very definite outline, and that they do not present any sloping
sides to catch more sunlight, and thus shine brighter, than the general
surface. Indeed, one great peculiarity of them is that they come out
most forcibly where the sun is shining perpendicularly upon them; hence
they are best seen where the moon is at full, and they are not visible
at all at those regions upon which the sun is rising or setting. We also
see that they are not diverted by elevations in their path, as they
traverse in their course craters, mountains, and plains alike, giving a
slight additional brightness to all objects over which they pass, but
producing no other effect upon them. To employ a commonplace simile,
they look as though, after the whole surface of the moon had assumed its
final configuration, a vast brush charged with a whitish pigment had
been drawn over the globe in straight lines radiating from a central
point, leaving its trail upon everything it touched, but obscuring
nothing.
Whatever may be the cause that produces this brightness of certain parts
of the moon without reference to configuration of surface, this cause
has not been confined to the formation of the radiating lines, for we
meet with many isolated spots, streaks and patches of the same bright
character. Upon some of the plains there are small areas and lines of
luminous matter possessing peculiarities similar to those of the
radiating streaks, as regards visibility with the high sun, and
invisibility when the solar rays fall upon them horizontally. Some of
the craters also are surrounded by a kind of aureole of this highly
reflective matter. A notable specimen is that called _Linné_, concerning
which a great hue and cry about change of appearance and inferred
continuance of volcanic action on the moon was raised some years ago.
This object is an insignificant little crater of about a mile or two in
diameter, in the centre of an ill-defined spot of the character referred
to, and about eight or ten miles in diameter. With a low sun the crater
alone is visible by its shadow; but as the luminary rises the shadow
shortens and becomes all but invisible, and then the white spot shines
forth. These alternations, complicated by variations of atmospheric
condition, and by the interpretations of different observers, gave rise
to statements of somewhat exaggerated character to the effect that
considerable changes, of the nature of volcanic eruptions, were in
progress in that particular region of the moon.
In the foregoing remarks we have alluded somewhat indefinitely to high
powers; and an enquiring but unastronomical reader may reasonably demand
some information upon this point. It might have been instructive to have
cited the various details that may be said to come into view with
progressive increases of magnification. But this would be an all but
impossible task, on account of the varying conditions under which all
astronomical observations must necessarily be made. When we come to
delicate tests, there are no standards of telescopic power and
definition. Assuming the instrument to be of good size and high optical
character, there is yet a powerful influant of astronomical definition
in the atmosphere and its variable state. Upon two-thirds of the clear
nights of a year the finest telescopes cannot be used to their full
advantage, because the minute flutterings resulting from the passage of
the rays of light through moving strata of air of different densities
are magnified just as the image in the telescope is magnified, till all
minute details are blurred and confused, and only the grosser features
are left visible. And supposing the telescope and atmosphere in good
state, there is still an important point, the state of the observer’s
eye, to be considered. After all it is the eye that sees, and the best
telescopic assistance to an untrained eye is of small avail. The eye is
as susceptible of education and development as any other organ; a
skilful and acute observer is to a mere casual gazer, what a watchmaker
would be to a ploughman, a miniature painter to a whitewasher. This fact
is not generally recognized; no man would think of taking in hand an
engraver’s burin, and expecting on the instant to use it like an adept,
or of going to a smithy and without previous preparation trying to forge
a horse-shoe. Yet do folks enter observatories with uneducated eyes, and
expect at once to realize all the wonderful things that their minds have
pictured to themselves from the perusal of astronomical books. We have
over and over again remarked the dissatisfaction which attends the first
looks of novices through a powerful telescope. They anticipate
immediately beholding wonders, and they are disappointed at finding how
little they can see, and how far short the sight falls of what they had
expected. Courtesy at times leads them to express wonder and surprise,
which it is easy to see is not really felt, but sometimes honesty
compels them to give expression to their disappointment. This arises
from the simple fact that their eyes are not fit for the work which is
for the moment imposed upon them; they know not what to look for, or how
to look for it. The first essay at telescopic gazing, like first essays
generally, serves but to teach us our incapability.
To a tutored eye a great deal is visible with a comparatively low power,
and practised observers strive to use magnifying powers as low as
possible, so as to diminish, as far as may be, the evils arising from an
untranquil atmosphere. With a power so small as 30 or 40, many
exceedingly delicate details on the moon are visible to an eye that is
familiar with them under higher powers. With 200 we may say that every
ordinary detail will come out under favourable conditions; but when
minute points of structure, mere nooks and corners as it were, are to be
scrutinised, 300 may be used with advantage. Another hundred diameters
almost passes the practical limit. Unless the air be not merely fine,
but superfine, the details become “clothy” and tremulous; the extra
points brought out by the increased power are then only caught by
momentary glimpses, of which but a very few are obtained during a
lengthy period of persistent scrutiny. We may set down 250 as the most
useful, and 350 the utmost effective power that can be employed upon the
particular work of which we are treating. Could every detail on the moon
be thoroughly and reliably represented as this amount of magnification
shows it, the result would leave little to be wished for.
But it may be asked by some, what is the absolute effect of such powers
as those we have spoken of, in bringing the moon apparently nearer to
our eyes? and what is the actual size of the smallest object visible
under the most favourable circumstances? A linear mile upon the moon
corresponds to an angular interval of 0·87 of a second; this refers to
regions about the centre of the disc; near the circumference the
foreshortening makes a difference, very great as the edge is approached.
Perhaps the smallest angle that the eye can without assistance
appreciate is half a minute; that is to say, an object that subtends to
the eye an arc of less than a half a minute can scarcely be seen.[7]
Since there are 60 seconds in a minute, it follows that we must magnify
a spot a second in diameter upon the moon thirty times before we can see
it; and since a second represents rather more than a mile, really about
2000 yards, on the moon, as seen from the earth, the smallest object
visible with a power of 30 will be this number of yards in diameter or
breadth. To see an object 200 yards across, we should require to magnify
it 300 times, and this would only bring it into view as a point; 20
yards would require a power of 3000, and 1 yard 60,000 to effect the
same thing. Since, as we have said, the highest practicable power with
our present telescopes, and at ordinary terrestrial elevations, is 350,
or for an extreme say 400, it is evident that the minutest lunar object
or detail of which we can perceive as a point must measure about 150
yards: to see the form of an object, so as to discriminate whether it be
round or square, it would require to be probably twice this size; for it
may be safely assumed that we cannot perceive the outline of an object
whose average breadth subtends a less angle than a minute.
Arago put this question into another shape:—The moon is distant from us
237,000 miles (mean). A magnifying power of a thousand would show us the
moon as if she were distant 237 miles from the naked eye.
2000 would bring her within 118 miles.
4000 ” ” ” 59 ”
6000 ” ” ” 39 ”
Mont Blanc is visible to the naked eye from Lyons, at the distance of
about 100 miles; so that to see the mountains of the moon as Mont Blanc
is seen from Lyons would require the impracticable power of 2500.