THE MOON
Our attention is next engaged by the body which is our nearest
neighbour in space and our most faithful attendant and useful servant.
The moon is an orb of 2,163 miles in diameter, which revolves round
our earth in a slightly elliptical orbit, at a mean distance of about
240,000 miles. The face which she turns to us is a trifle greater
in area than the Russian Empire, while her total surface is almost
exactly equal to the areas of North and South America, islands
excluded. Her volume is about 2/99 of that of the earth; her materials
are, however, much less dense than those of which our world is
composed, so that it would take about eighty-one moons to balance the
earth. One result of these relations is that the force of gravity at
the lunar surface is only about one-sixth of that at the surface of
the earth, so that a twelve-stone man, if transported to the moon,
would weigh only two stone, and would be capable of gigantic feats
in the way of leaping and lifting weights. The fact of the diminished
force of gravity is of importance in the consideration of the question
of lunar surfacing.
[Illustration: FIG. 21.--THE TIDES.
A, Spring Tide (New Moon); B, Neap Tide.]
The most conspicuous service which our satellite performs for us is
that of raising the tides. The complete statement of the manner in
which she does this would be too long for our pages; but the general
outline of it will be seen from the accompanying rough diagram (Fig.
21), which, it must be remembered, makes no attempt at representing
the scale either of the bodies concerned or of their distances from
one another, but simply pictures their relations to one another at the
times of spring and neap tides. The moon (M in Fig. 21, A) attracts
the whole earth towards it. Its attraction is greatest at the point
nearest to it, and therefore the water on the moonward side is drawn
up, as it were, into a heap, making high tide on that side of the
earth. But there is also high tide at the opposite side, the reason
being that the solid body of the earth, which is nearer to the moon
than the water on the further side, is more strongly attracted, and
so leaves the water behind it. Thus there are high tides at the two
opposite sides of the earth which lie in a straight line with the
moon, and corresponding low tides at the intermediate positions. Tides
are also produced by the attraction of the sun, but his vastly greater
distance causes his tide-producing power to be much less than that of
the moon. His influence is seen in the difference between spring and
neap tides. Spring tides occur at new or full moon (Fig. 21, A, case
of new moon). At these two periods the sun, moon, and earth, are all
in one straight line, and the pull of the sun is therefore added to
that of the moon to produce a spring tide. At the first and third
quarters the sun and moon are at right angles to one another; their
respective pulls therefore, to some extent, neutralize each other, and
in consequence we have neap tide at these seasons.
[Illustration:
PLATE XI.
The Moon, April 5, 1900. Paris Observatory.]
No one can fail to notice the beautiful set of phases through which
the moon passes every month. A little after the almanac has announced
'new moon,' she begins to appear as a thin crescent low down in the
West, and setting shortly after the sun. Night by night we can watch
her moving eastward among the stars, and showing more and more of
her illuminated surface, until at first quarter half of her disc is
bright. The reader must distinguish this real eastward movement from
the apparent east to west movement due to the daily rotation of the
earth. Its reality can readily be seen by noting the position of the
moon relatively to any bright star. It will be observed that if she
is a little west of the star on one night, she will have moved to a
position a little east of it by the next. Still moving farther East,
she reaches full, and is opposite to the sun, rising when he sets, and
setting when he rises. After full, her light begins to wane, till at
third quarter the opposite half of her disc is bright, and she is seen
high in the heavens in the early morning, a pale ghost of her evening
glories. Gradually she draws nearer to the sun, thinning down to the
crescent shape again until she is lost once more in his radiance, only
to re-emerge and begin again the same cycle of change.
The time which the moon actually takes to complete her journey round
the earth is twenty-seven days, seven hours, and forty-three minutes;
and if the earth were fixed in space, this period, which is called the
_sidereal month_, would be the actual time from new moon to new moon.
While the moon has been making her revolution, however, the earth has
also been moving onwards in its journey round the sun, so that the
moon has a little further to travel in order to reach the 'new
moon' position again, and the time between two new moons amounts to
twenty-nine days, twelve hours, forty-four minutes. This period
is called a _lunar month_, and is also the _synodic period_ of our
satellite, a term which signifies generally the period occupied by any
planet or satellite in getting back to the same position with respect
to the sun, as observed from the earth.
The fact that the moon shows phases signifies that she shines only
by reflected light; and it is surprising to notice how little of the
light that falls upon her is really reflected by her. On an ordinarily
clear night most people would probably say that the moon is much
brighter than any terrestrial object viewed in the daytime, when it
also is lit by the sun, as the moon is. Yet a very simple comparison
will show that this is not so. If the moon be compared during the
daytime with the clouds floating around her, she will be seen to be
certainly not brighter than they, generally much less bright; indeed,
even an ordinary surface of sandstone will look as bright as her
disc. In fact, the reason of her great apparent brightness at night is
merely the contrast between her and the dark background against which
she is seen; a fragment of our own world, put in her place, would
shine quite as brightly, perhaps even more so. It is possibly rather
difficult at first to realize that our earth is shining to the moon
and to the other planets as they do to us, but anyone who watches the
moon for a few days after new will find convincing evidence of the
fact. Within the arms of the thin crescent can be seen the whole body
of the lunar globe, shining with a dingy coppery kind of light--'the
ashen light,' as it is called. People talk of this as 'the old moon
in the young moon's arms,' and weather-wise (or foolish) individuals
pronounce it to be a sign of bad weather. It is, of course, nothing
of the sort, for it can be seen every month when the sky is reasonably
clear; but it is the sign that our world shines to the other worlds of
space as they do to her; for this dim light upon the part of the moon
unlit by the sun is simply the light which our own world reflects from
her surface to the moon. In amount it is thirteen times more than that
which the moon gives to us, as the earth presents to her satellite a
disc thirteen times as large as that exhibited by the latter.
The moon's function in causing eclipses of the sun has already been
briefly alluded to. In turn she is herself eclipsed, by passing behind
the earth and into the long cone of shadow which our world casts
behind it into space (Fig. 19). It is obvious that such eclipses can
only happen when the moon is full. A total eclipse of the moon, though
by no means so important as a solar eclipse, is yet a very interesting
and beautiful sight. The faint shadow or penumbra is often scarcely
perceptible as the moon passes through it; but the passage of the dark
umbra over the various lunar formations can be readily traced, and
is most impressive. Cases of 'black eclipses' have been sometimes
recorded, in which the moon at totality has seemed actually to
disappear as though blotted out of the heavens; but in general this
is not the case. The lunar disc still remains visible, shining with
a dull coppery light, something like the ashen light, but of a
redder tone. This is due to the fact that our earth is not, like
its satellite, a next to airless globe, but is possessed of a pretty
extensive atmosphere. By this atmosphere those rays of the sun which
would otherwise have just passed the edge of the world are caught
and refracted so that they are directed upon the face of the eclipsed
moon, lighting it up feebly. The redness of the light is due to that
same atmospheric absorption of the green and blue rays which causes
the body of the setting sun to seem red when viewed through the dense
layer of vapours near the horizon. When the moon appears totally
eclipsed to us, the sun must appear totally eclipsed to an observer
stationed on the moon. A total solar eclipse seen from the moon must
present features of interest differing to some extent from those which
the similar phenomenon exhibits to us. The duration of totality will
be much longer, and, in addition to the usual display of prominences
and corona, there will be the strange and weird effect of the black
globe of our world becoming gradually bordered with a rim of ruddy
light as our atmosphere catches and bends the solar rays inwards upon
the lunar surface.
In nine cases out of ten the moon will be the first object to which
the beginner turns his telescope, and he will find in our satellite
a never-failing source of interest, and a sphere in which, by patient
observation and the practice of steadily recording what is seen, he
may not only amuse and instruct himself, but actually do work that
may become genuinely useful in the furtherance of the science. The
possession of powerful instrumental means is not an absolute essential
here, for the comparative nearness of the object brings it well within
the reach of moderate glasses. The writer well remembers the keen
feeling of delight with which he first discovered that a very humble
and commonplace telescope--nothing more, in fact, than a small
ordinary spy-glass with an object-glass of about 1 inch in
aperture--was able to reveal many of the more prominent features of
lunar scenery; and the possessor of any telescope, no matter whether
its powers be great or small, may be assured that there is enough work
awaiting him on the moon to occupy the spare time of many years with
one of the most enthralling of studies. The view that is given by even
the smallest instrument is one of infinite variety and beauty; and its
interest is accentuated by the fact that the moon is a sphere where
practically every detail is new and strange.
If the moon be crescent, or near one or other of her quarters at the
time of observation, the eye will at once be caught by a multitude
of circular, or nearly circular depressions, more clearly marked the
nearer they are to the line of division between the illuminated
and unilluminated portions of the disc. (This line is known as the
Terminator, the circular outline, fully illuminated, being called the
Limb). The margins of some of these depressions will be seen actually
to project like rings of light into the darkness, while their
interiors are filled with black shadow (Plates XI., XIII., XV., and
XVI.). At one or two points long bright ridges will be seen, extending
for many miles across the surface, and marking the line of one or
other of the prominent ranges of lunar mountains (Plates XI., XIII.,
XVI., XVII.); while the whole disc is mottled over with patches of
varied colour, ranging from dark grey up to a brilliant yellow which,
in some instances, nearly approaches to white.
If observation be conducted at or near the full, the conditions will
be found to have entirely changed. There are now very few ruggednesses
visible on the edge of the disc, which now presents an almost smooth
circular outline, nor are there any shadows traceable on the surface.
The circular depressions, formerly so conspicuous, have now almost
entirely vanished, though the positions and outlines of a few of them
may still be traced by their contrast in colour with the surrounding
regions. The observer's attention is now claimed by the extraordinary
brilliance and variety of the tones which diversify the sphere, and
particularly by the curious systems of bright streaks radiating from
certain well-marked centres, one of which, the system originating
near Tycho, a prominent crater not very far from the South Pole, is
so conspicuous as to give the full moon very much the appearance of a
badly-peeled orange (Plate XII.).
[Illustration:
PLATE XII.
The Moon, November 13, 1902. Paris Observatory.]
As soon as the moon has passed the full, the ruggedness of its margin
begins once more to become apparent, but this time on the opposite
side; and the observer, if he have the patience to work late at night
or early in the morning, has the opportunity of seeing again all
the features which he saw on the waxing moon, but this time with the
shadows thrown the reverse way--under evening instead of under morning
illumination. In fact the character of any formation cannot be truly
appreciated until it has been carefully studied under the setting as
well as under the rising and meridian sun.
We must now turn our attention to the various types of formation which
are to be found upon the moon. These may be roughly summarized as
follows: (1) The great grey plains, commonly known as Maria, or seas;
(2) the circular or approximately circular formations, known generally
as the lunar craters, but divided by astronomers into a number of
classes to which reference will be made later; (3) the mountain
ranges, corresponding with more or less closeness to similar features
on our own globe; (4) the clefts or rills; (5) the systems of bright
rays, to which allusion has already been made.
1. THE GREAT GREY PLAINS.--These are, of course, the most conspicuous
features of the lunar surface. A number of them can be easily seen
with the naked eye; and, so viewed, they unite with the brighter
portions to form that resemblance to a human face--'the man in
the moon'--with which everyone is familiar. A field-glass or small
telescope brings out their boundaries with distinctness, and suggests
a likeness to our own terrestrial oceans and seas. Hence the name
Maria, which was applied to them by the earlier astronomers, whose
telescopes were not of sufficient power to reveal more than their
broader outlines. But a comparatively small aperture is sufficient
to dispel the idea that these plains have any right to the title of
'seas.' The smoothness which at first suggests water proves to be only
relative. They are smooth compared with the brighter regions of the
moon, which are rugged beyond all terrestrial precedent; but they
would probably be considered no smoother than the average of our own
non-mountainous land surfaces. A 2 or 2-1/2-inch telescope will reveal
the fact that they are dotted over with numerous irregularities, some
of them very considerable. It is indeed not common to find a crater of
the largest size associated with them; but, at the same time, craters
which on our earth would be considered huge are by no means uncommon
upon their surface, and every increase of telescopic power reveals a
corresponding increase in the number of these objects (Plates XIII.,
XV., XVII.).
[Illustration:
PLATE XIII.
The Moon, September 12, 1903. Paris Observatory.]
Further, the grey plains are characterized by features of which
instances may be seen with a very small instrument, though the more
delicate specimens require considerable power--namely, the long
winding ridges which either run concentrically with the margins of the
plains, or cross their surface from side to side. Of these the
most notable is the great serpentine ridge which traverses the Mare
Serenitatis in the north-west quadrant of the moon. As it runs,
approximately, in a north and south direction, it is well placed
for observation, and even a low power will bring out a good deal of
remarkable detail in connection with it. It rises in some places to a
height of 700 or 800 feet (Neison), and is well shown on many of the
fine lunar photographs now so common. Another point of interest in
connection with the Maria is the existence on their borders of a
number of large crater formations which present the appearance of
having had their walls breached and ruined on the side next the mare
by the action of some obscure agency. From consideration of these
ruined craters, and of the 'ghost craters,' not uncommon on the
plains, which present merely a faint outline, as though almost
entirely submerged, it has been suggested, by Elger and others, that
the Maria, as we see them represent, not the beds of ancient seas,
but the consolidated crust of some fluid or viscous substance such as
lava, which has welled forth from vents connected with the interior
of the moon, overflowing many of the smaller formations, and partially
destroying the walls of these larger craters. Notable instances of
these half-ruined formations will be found in Fracastorius (Plate
XIX., No. 78, and Plate XI.), and Pitatus (Plate XIX., No. 63,
and Plate XV.). The grey plains vary in size from the vast Oceanus
Procellarum, nearly 2,000,000 square miles in area, down to the Mare
Humboldtianum, whose area of 42,000 square miles is less than that of
England.
2. THE CIRCULAR, OR APPROXIMATELY CIRCULAR FORMATIONS.--These, the
great distinguishing feature of lunar scenery, have been classified
according to the characteristics, more or less marked, which
distinguish them from one another, as walled-plains, mountain-rings,
ring-plains, craters, crater-cones, craterlets, crater-pits, and
depressions. For general purposes we may content ourselves with the
single title craters, using the more specific titles in outstanding
instances.
[Illustration:
PLATE XIV.
Region of Maginus: Overlapping Craters. Paris Observatory.]
To these strange formations we have scarcely the faintest analogy
on earth. Their multitude will at once strike even the most casual
observer. Galileo compared them to the 'eyes' in a peacock's tail, and
the comparison is not inapt, especially when the moon is viewed with
a small telescope and low powers. In the Southern Hemisphere
particularly, they simply swarm to such an extent that the district
near the terminator presents much the appearance of a honeycomb with
very irregular cells, or a piece of very porous pumice (Plate XIV.).
Their vast size is not less remarkable than their number. One of the
most conspicuous, for example, is the great walled-plain Ptolemäus,
which is well-placed for observation near the centre of the visible
hemisphere. It measures 115 miles from side to side of its great
rampart, which, in at least one peak, towers more than 9,000 feet
above the floor of the plain within. The area of this enormous
enclosure is about equal to the combined areas of Yorkshire,
Lancashire, and Westmorland--an extent so vast that an observer
stationed at its centre would see no trace of the mountain-wall which
bounds it, save at one point towards the West, where the upper part of
the great 9,000-feet peak already referred to would break the line of
the horizon (Plate XIX., No. 111; Plate XIII.).
Nor is Ptolemäus by any means the largest of these objects. Clavius,
lying towards the South Pole, measures no less than 142 miles from
wall to wall, and includes within its tremendous rampart an area of at
least 16,000 square miles. The great wall which encloses this space,
itself no mean range of mountains, stands some 12,000 feet above the
surface of the plain within, while in one peak it rises to a height
of 17,000 feet. Clavius is remarkable also for the number of smaller
craters associated with it. There are two conspicuous ones, one on the
north, one on the south side of its wall, each about twenty-five
miles in diameter, while the floor is broken by a chain of four large
craters and a considerable number of smaller ones.
Though unfavourably placed for observation, there is no lunar feature
which can compare in grandeur with Clavius when viewed either at
sunrise or sunset. At sunrise the great plain appears first as a huge
bay of black shadow, so large as distinctly to blunt the southern horn
of the moon to the naked eye. As the sun climbs higher, a few bright
points appear within this bay of darkness--the summits of the walls of
the larger craters--these bright islands gradually forming fine rings
of light in the shadow which still covers the floor of the great
plain. In the East some star-like points mark where the peaks of the
eastern wall are beginning to catch the dawn. Then delicate streaks
of light begin to stream across the floor, and the dark mass of shadow
divides itself into long pointed shafts, which stretch across the
plain like the spires of some great cathedral. The whole spectacle
is so magnificent and strange that no words can do justice to it; and
once seen it will not readily be forgotten. Even a small telescope
will enable the student to detect and draw the more important features
of this great formation; and for those whose instruments are more
powerful there is practically no limit to the work that may be done
on Clavius, which has never been studied with the minuteness that so
great and interesting an object deserves. (Clavius is No. 13, Plate
XIX. See also Plates XIII. and XV., and Fig. 22, the latter a rough
sketch with a 2-5/8-inch refractor.)
From such gigantic forms as these, the craters range downwards in an
unbroken sequence through striking objects such as Tycho and the grand
Copernicus, both distinguished for their systems of bright rays, as
well as for their massive and regular ramparts, to tiny pits of black
shadow, a few hundred feet across, and with no visible walls, which
tax the powers of the very finest instruments. Schmidt's great map
lays down nearly 33,000 craters, and it is quite certain that these
are not nearly all which can be seen even with a moderate-sized
telescope.
[Illustration:
PLATE XV.
Clavius, Tycho, and Mare Nubium. Yerkes Observatory.]
As to the cause which has resulted in this multitude of circular
forms, there is no definite consensus of opinion. Volcanic action is
the agency generally invoked; but, even allowing for the diminished
force of gravity upon the moon, it is difficult to conceive of
volcanic action of such intensity as to have produced some of the
great walled-plains. Indeed, Neison remarks that such formations are
much more akin to the smaller Maria, and bear but little resemblance
to true products of volcanic action. But it seems difficult to tell
where a division is to be made, with any pretence to accuracy, between
such forms as might certainly be thus produced and those next above
them in size. The various classes of formation shade one into the
other by almost imperceptible degrees.
[Illustration: FIG. 22.
CLAVIUS, June 7, 1889, 10 p.m., 2-5/8 inch.]
3. THE MOUNTAIN RANGES.--These are comparatively few in number, and
are never of such magnitude as to put them, like the craters, beyond
terrestrial standards of comparison. The most conspicuous range is
that known as the Lunar Apennines, which runs in a north-west and
south-east direction for a distance of upwards of 400 miles along
the border of the Mare Imbrium, from which its mass rises in a steep
escarpment, towering in one instance (Mount Huygens) to a height of
more than 18,000 feet. On the western side the range slopes gradually
away in a gentle declivity. The spectacle presented by the Apennines
about first quarter is one of indescribable grandeur. The shadows of
the great peaks are cast for many miles over the surface of the Mare
Imbrium, magnificently contrasting with the wild tract of hill-country
behind, in which rugged summits and winding valleys are mingled in a
scene of confusion which baffles all attempt at delineation. Two other
important ranges--the Caucasus and the Alps--lie in close proximity
to the Apennines; the latter of the two notable for the curious Alpine
Valley which runs through it in a straight line for upwards of eighty
miles. This wonderful chasm varies in breadth from about two miles,
at its narrowest neck, to about six at its widest point. It is closely
bordered, for a considerable portion of its length, by almost vertical
cliffs thousands of feet in height, and under low magnifying powers
appears so regular as to suggest nothing so much as the mark of
a gigantic chisel, driven by main force through the midst of the
mountain mass. The Alpine Valley is an easy object, and a power of
50 on a 2-inch telescope will show its main outlines quite clearly.
Indeed, the whole neighbourhood is one which will well repay the
student, some of the finest of the lunar craters, such as Plato,
Archimedes, Autolycus, and Aristillus, lying in the immediate vicinity
(Plates XIII. and XVII.).
[Illustration:
PLATE XVI.
Region of Theophilus and Altai Mountains. Yerkes Observatory.]
Among the other mountain-ranges may be mentioned the Altai Mountains,
in the south-west quadrant (Plate XVI.), the Carpathians, close to the
great crater Copernicus, and the beautiful semicircle of hills which
borders the Sinus Iridum, or Bay of Rainbows, to the east of the
Alpine range. This bay forms one of the loveliest of lunar landscapes,
and under certain conditions of illumination its eastern cape, the
Heraclides Promontory, presents a curious resemblance, which I have
only seen once or twice, to the head of a girl with long floating
hair--'the moon-maiden.' The Leibnitz and Doerfel Mountains, with
other ranges whose summits appear on the edge of the moon, are
seldom to be seen to great advantage, though they are sometimes
very noticeably projected upon the bright disc of the sun during the
progress of an eclipse.[*] They embrace some of the loftiest lunar
peaks reaching 26,000 feet in one of or two instances, according to
Schröter and Mädler.
[Illustration: FIG. 23.
ARISTARCHUS and HERODOTUS, February 20, 1891, 6.15 p.m., 3-7/8 inch.]
4. THE CLEFTS OR RILLS.--In these, and in the ray-systems, we again
meet with features to which a terrestrial parallel is absolutely
lacking. Schröter of Lilienthal was the first observer to detect the
existence of these strange chasms, and since his time the number known
has been constantly increasing, till at present it runs to upwards of
a thousand. These objects range from comparatively coarse features,
such as the Herodotus Valley (Fig. 23), and the well-known Ariadæus
and Hyginus clefts, down to the most delicate threads, only to be seen
under very favourable conditions, and taxing the powers of the finest
instruments. They present all the appearance of cracks in a shrinking
surface, and this is the explanation of their existence which at
present seems to find most favour. In some cases, such as that of
the great Sirsalis cleft, they extend to a length of 300 miles; their
breadth varies from half a mile, or less, to two miles; their depth is
very variously estimated, Nasmyth putting it at ten miles, while Elger
only allows 100 to 400 yards. In a number of instances they appear
either to originate from a small crater, or to pass through one or
more craters in their course. The student will quickly find out for
himself that they frequently affect the neighbourhood of one or other
of the mountain ranges (as, for example, under the eastern face of the
Apennines, Plate XVII.), or of some great crater, such as Archimedes.
They are also frequently found traversing the floor of a great
walled-plain, and at least forty have been detected in the interior
of Gassendi (Plate XIX., No. 90). Smaller instruments are, of course,
incompetent to reveal more than a few of the larger and coarser of
these strange features. The Serpentine Valley of Herodotus, the cleft
crossing the floor of Petavius, and the Ariadæus and Hyginus rills
are among the most conspicuous, and may all be seen with a 2-1/2-inch
telescope and a power of 100.
[Illustration:
PLATE XVII.
Apennines, Alps, and Caucasus. Paris Observatory.]
5. THE SYSTEMS OF BRIGHT RAYS, radiating from certain craters, remain
the most enigmatic of the features of lunar scenery. Many of these
systems have been traced and mapped, but we need only mention the
three principal--those connected with Tycho, Copernicus, and
Kepler, all shown on Plate XII. The Tycho system is by far the most
noteworthy, and at once attracts the eye when even the smallest
telescope is directed towards the full moon. The rays, which are of
great brilliancy, appear to start, not exactly from the crater itself,
but from a greyish area surrounding it, and they radiate in all
directions over the surface, passing over, and almost completely
masking in their course some of the largest of the lunar craters.
Clavius, for example, and Maginus (Plate XIV.), become at full almost
unidentifiable from this cause, though Neison's statement that 'not
the slightest trace of these great walled-plains, with their extremely
lofty and massive walls, can be detected in full,' is certainly
exaggerated. The rays are not well seen save under a high sun--_i.e._,
at or near full, though some of them can still be faintly traced under
oblique illumination.
In ordinary telescopes, and to most eyes, the Tycho rays appear to
run on uninterruptedly for enormous distances, one of them traversing
almost the whole breadth of the moon in a north-westerly direction,
and crossing the Mare Serenitatis, on whose dark background it is
conspicuous. Professor W. H. Pickering, who has made a special study
of the subject under very favourable conditions, maintains, however,
that this appearance of great length is an illusion, and that the
Tycho rays proper extend only for a short distance, being reinforced
at intervals by fresh rays issuing from small craters on their track.
The whole subject is one which requires careful study with the best
optical means.
None of the other ray-systems are at all comparable with that of
Tycho, though those in connection with Copernicus and Kepler are
very striking. As to the origin and nature of these strange features,
little is known. There are almost as many theories as there are
systems; but it cannot be said that any particular view has commanded
anything like general acceptance. Nasmyth's well-known theory was
that they represented cracks in the lunar surface, caused by internal
pressure, through which lava had welled forth and spread to a
considerable distance on either side of the original chasm. Pickering
suggests that they may be caused by a deposit of white powder, pumice,
perhaps, emitted by the craters from which the rays originate. Both
ideas are ingenious, but both present grave difficulties, and neither
has commended itself to any very great extent to observers, a remark
which applies to all other attempts at explanation.
Such are the main objects of interest upon the visible hemisphere of
our satellite. In observing them, the beginner will do well, after the
inevitable preliminary debauch of moon-gazing, during which he may
be permitted to range over the whole surface and observe anything
and everything, not to attempt an attack on too wide a field. Let
him rather confine his energies to the detailed study of one or two
particular formations, and to the delineation of all their features
within reach of his instrument under all aspects and illuminations.
By so doing he will learn more of the actual condition of the lunar
surface than by any amount of general and haphazard observation; and
may, indeed, render valuable service to the study of the moon.
Neither let him think that observations made with a small telescope
are now of no account, in view of the number of large instruments
employed, and of the great photographic atlases which are at present
being constructed. It has to be remembered that the famous map of
Beer and Mädler was the result of observations made with a 3-3/4-inch
telescope, and that Lohrmann used an instrument of only 4-4/5 inches,
and sometimes one of 3-1/4. Anyone who has seen the maps of these
observers will not fail to have a profound respect for the work that
can be done with very moderate means. Nor have even the beautiful
photographs of the Paris, Lick, and Yerkes Observatories superseded
as yet the work of the human eye and hand. The best of the Yerkes
photographs, taken with a 40-inch refractor, are said to show detail
'sufficiently minute to tax the powers of a 6-inch telescope.' But
this can be said only of a very few photographs; and, generally
speaking, a good 3-inch glass will show more detail than can be seen
on any but a few exceptionally good negatives.
In conducting his observations, the student should be careful to
outline his drawing on such a scale as will permit of the easy
inclusion of all the details which he can see, otherwise the sketch
will speedily become so crowded as to be indistinct and valueless. A
scale of 1 inch to about 20 miles, corresponding roughly to 100 inches
to the moon's diameter, will be found none too large in the case of
formations where much detail has to be inserted--that is to say, in
the case of the vast majority of lunar objects. Further, only such a
moderate amount of surface should be selected for representation as
can be carefully and accurately sketched in a period of not much over
an hour at most; for, though the lunar day is so much longer than our
own, yet the changes in aspect of the various formations due to the
increasing or diminishing height of the sun become very apparent if
observation be prolonged unduly; and thus different portions of the
sketch represent different angles of illumination, and the finished
drawing, though true in each separate detail, will be untrue as a
whole.
Above all, care must be taken to set down only what is seen with
certainty, _and nothing more_. The drawing may be good or bad, but it
must be true. A coarse or clumsy sketch which is truthful to the
facts seen is worth fifty beautiful works of art where the artist has
employed imagination or recollection to eke out the meagre results of
observation. The astronomer's primary object is to record facts, not
to make pictures. If he is skilful in recording what he sees, his
sketch will be so much the more truthful; but the facts must come
first. Such practical falsehoods as the insertion of uncertain
details, or the practice of drawing upon one's recollection of the
work of other observers, or of altering portions of a sketch which
do not please the eye, are to be studiously avoided. The observer's
record of what he has seen should be above suspicion. It may
be imperfect; it should never be false. Such cautions may seem
superfluous, but a small acquaintance with the subject of astronomical
drawing will show that they are not.
The want of a good lunar chart will speedily make itself felt.
Fortunately in these days it can be easily supplied. The great
photographic atlases now appearing are, of course, for the luxurious;
and the elaborate maps of Beer and Mädler or Schmidt are equally
out of the question for beginners. The smaller chart of the former
observers is, however, inexpensive and good, though a little crowded.
For a start there is still nothing much better than Webb's reduction
of Beer and Mädler's large chart, published in 'Celestial Objects for
Common Telescopes.' It can also be obtained separately; but requires
to be backed before use. Mellor's chart is also useful, and is
published in a handy form, mounted on mill-board. Those who wish
charts between these and the more elaborate ones will find their
wants met by such books as those of Neison or Elger. Neison's volume
contains a chart in twenty-two sections on a scale of 2 feet to
the moon's diameter. It includes a great amount of detail, and
is accompanied by an elaborate description of all the features
delineated. Its chief drawbacks are the fact that it was published
thirty years ago, and that it is an extremely awkward and clumsy
volume to handle, especially in the dim light of an observatory.
Elger's volume is, perhaps, for English students, the handiest general
guide to the moon. Its chart is on a scale of 18 inches to the moon's
diameter, and is accompanied by a full description. With either this
or Webb's chart, the beginner will find himself amply provided with
material for many a long and delightful evenings work.
[Illustration:
PLATE XVIII.
Chart of the Moon. Nasmyth and Carpenter.]
[Illustration:
PLATE XIX.
Key to Chart of Moon. Nasmyth and Carpenter.]
The small chart which accompanies this chapter, and which, with its
key-map, I owe to the courtesy of Mr. John Murray, the publisher of
Messrs. Nasmyth and Carpenter's volume on the moon, is not in any
sense meant as a substitute for those already mentioned, but merely
as an introduction to some of the more prominent features of lunar
scenery. The list of 229 named and numbered formations will be
sufficient to occupy the student for some time; and the essential
particulars with regard to a few of the more important formations are
added in as brief a form as possible (Appendix I.).
Before we leave our satellite, something must be said as to the
conditions prevailing on her surface. The early astronomers who
devoted attention to lunar study were drawn on in their labours
largely by the hope of detecting resemblances to our own earth, or
even traces of human habitation. Schröter and Gruithuisen imagined
that they had discovered not only indications of a lunar atmosphere,
but also evidence of change upon the surface, and traces of the
handiwork of lunarian inhabitants. Gruithuisen, in particular, was
confident that in due time it would become possible to trace the
cities and the works of the Lunarians. Gradually these hopes have
receded into the distance. The existence of a lunar atmosphere is,
indeed, no longer positively denied now, as it was a few years ago;
but it is certain that such atmosphere as may exist is of extreme
rarity, quite inadequate to support animal life as we understand such
a thing. Certain delicate changes of colour which take place within
some of the craters--Plato for instance--have been referred to
vegetation; and Professor Pickering has intimated his observation
of something which he considers to be the forming and melting of
hoar-frost within certain areas, Messier and a small crater near
Herodotus among others. But the observations at best are very delicate
and the inferences uncertain. It cannot be denied that the moon may
have an atmosphere; but positive traces of its existence are so faint
that, even if their reality be admitted, very little can be built upon
them.
At the same time when the affirmation is made that the moon is 'a
world where there is no weather, and where nothing ever happens,' the
most careful modern students of lunar matters would be the first
to question such a statement. Even supposing it to be true that no
concrete evidence of change upon the lunar surface can be had, this
would not necessarily mean that no change takes place. The moon has
certainly never been studied to advantage with any power exceeding
1,000, and the average powers employed have been much less. Nasmyth
puts 300 as about the profitable limit, and 500 would be almost an
outside estimate for anything like regular work. But even assuming the
use of a power of 1,000, that means that the moon is seen as large as
though she were only 240 miles distant from us. The reader can judge
how entirely all but the very largest features of our world would
be lost to sight at such a distance, and how changes involving the
destruction of large areas might take place and the observer be none
the wiser. When it is remembered that even at this long range we are
viewing our object through a sea of troubled air of which every tremor
is magnified in proportion to the telescopic power employed, until the
finer details are necessarily blurred and indistinct, it will be seen
that the case has been understated. Indeed it may be questioned if the
moon has ever been as well seen as though it had been situated at a
distance of 500 miles from the earth. At such a distance nothing short
of the vastest cataclysms would be visible; and it is therefore going
quite beyond the mark to assume that nothing ever happens on the moon
simply because we do not see it happening. Moreover, the balance of
evidence does appear to be inclining, slightly perhaps, but still
almost unquestionably, towards the view that change does occur upon
the moon. Some of the observations which seem to imply change may
be explained on other grounds; but there is a certain residuum which
appears to defy explanation, and it is very noteworthy that while
those who at once dismiss the idea of lunar change are, generally
speaking, those who have made no special study of the moon's surface,
the contrary opinion is most strongly maintained by eminent observers
who have devoted much time to our satellite with the best modern
instruments to aid them in their work.
The admission of the possibility of change does not, however, imply
anything like fitness for human habitation. The moon, to use Beer
and Mädler's oft-quoted phrase, is 'no copy of the earth'; and the
conditions of her surface differ widely from anything that we are
acquainted with. The extreme rarity of her atmosphere must render her,
were other conditions equally favourable, an ideal situation for an
observatory. From her surface the stars, which are hidden from us
in the daytime by the diffused light in our air, would be visible at
broad noonday; while multitudes of the smaller magnitudes which here
require telescopic power would there be plain to the unaided eye. The
lunar night would be lit by our own earth, a gigantic moon, presenting
a surface more than thirteen times as large as that which the full
moon offers us, and hanging almost stationary in the heavens, while
exhibiting all the effects of rapid rotation upon its own axis. Those
appendages of the sun, which only the spectroscope or the fleeting
total eclipse can reveal to us, the corona, the chromosphere, and the
prominences, would there be constantly visible.
Our astronomers who are painfully wrestling with atmospheric
disturbance, and are gradually being driven from the plains to the
summits of higher and higher hills in search of suitable sites for
the giant telescopes of to-day, may well long for a world where
atmospheric disturbance must be unknown, or at least a negligible
quantity.
[Footnote *: See drawings by Colonel Markwick with 2-3/4-inch
refractor, of the eclipse of August 30, 1905, 'The Total Solar
Eclipse, 1905,' British Astronomical Association, pp. 59, 60.]