THE MOON.
The Moon and the Tides--The Use of the Moon in Navigation--The
Changes of the Moon--The Moon and the Poets--Whence the Light of
the Moon?--Sizes of the Earth and the Moon--Weight of the
Moon--Changes in Apparent Size--Variations in its
Distance--Influence of the Earth on the Moon--The Path of the
Moon--Explanation of the Moon's Phases--Lunar Eclipses--Eclipses of
the Sun, how produced--Visibility of the Moon in a Total
Eclipse--How Eclipses are Predicted--Uses of the Moon in finding
Longitude--The Moon not connected with the Weather--Topography of
the Moon--Nasmyth's Drawing of Triesnecker--Volcanoes on the
Moon--Normal Lunar Crater--Plato--The Shadows of Lunar
Mountains--The Micrometer--Lunar Heights--Former Activity on the
Moon--Nasmyth's View of the Formation of Craters--Gravitation on
the Moon--Varied Sizes of the Lunar Craters--Other Features of the
Moon--Is there Life on the Moon?--Absence of Water and of Air--Dr.
Stoney's Theory--Explanation of the Rugged Character of Lunar
Scenery--Possibility of Life on Distant Bodies in Space.
If the moon were suddenly struck out of existence, we should be
immediately apprised of the fact by a wail from every seaport in the
kingdom. From London and from Liverpool we should hear the same
story--the rise and fall of the tide had almost ceased. The ships in
dock could not get out; the ships outside could not get in; and the
maritime commerce of the world would be thrown into dire confusion.
The moon is the principal agent in causing the daily ebb and flow of the
tide, and this is the most important work which our satellite has to do.
The fleets of fishing boats around the coasts time their daily movements
by the tide, and are largely indebted to the moon for bringing them in
and out of harbour. Experienced sailors assure us that the tides are of
the utmost service to navigation. The question as to how the moon causes
the tides is postponed to a future chapter, in which we shall also
sketch the marvellous part which the tides seem to have played in the
early history of our earth.
Who is there that has not watched, with admiration, the beautiful series
of changes through which the moon passes every month? We first see her
as an exquisite crescent of pale light in the western sky after sunset.
If the night is fine, the rest of the moon is visible inside the
crescent, being faintly illumined by light reflected from our own earth.
Night after night she moves further and further to the east, until she
becomes full, and rises about the same time that the sun sets. From the
time of the full the disc of light begins to diminish until the last
quarter is reached. Then it is that the moon is seen high in the heavens
in the morning. As the days pass by, the crescent shape is again
assumed. The crescent wanes thinner and thinner as the satellite draws
closer to the sun. Finally she becomes lost in the overpowering light of
the sun, again to emerge as the new moon, and again to go through the
same cycle of changes.
The brilliance of the moon arises solely from the light of the sun,
which falls on the not self-luminous substance of the moon. Out of the
vast flood of light which the sun pours forth with such prodigality into
space the dark body of the moon intercepts a little, and of that little
it reflects a small fraction to illuminate the earth. The moon sheds so
much light, and seems so bright, that it is often difficult at night to
remember that the moon has no light except what falls on it from the
sun. Nevertheless, the actual surface of the brightest full moon is
perhaps not much brighter than the streets of London on a clear sunshiny
day. A very simple observation will suffice to show that the moon's
light is only sunlight. Look some morning at the moon in daylight, and
compare the moon with the clouds. The brightness of the moon and of the
clouds are directly comparable, and then it can be readily comprehended
how the sun which illuminates the clouds has also illumined the moon. An
attempt has been made to form a comparative estimate of the brightness
of the sun and the full moon. If 600,000 full moons were shining at
once, their collective brilliancy would equal that of the sun.
The beautiful crescent moon has furnished a theme for many a poet.
Indeed, if we may venture to say so, it would seem that some poets have
forgotten that the moon is not to be seen every night. A poetical
description of evening is almost certain to be associated with the
appearance of the moon in some phase or other. We may cite one notable
instance in which a poet, describing an historical event, has enshrined
in exquisite verse a statement which cannot be correct. Every child who
speaks our language has been taught that the burial of Sir John Moore
took place
"By the struggling moonbeams' misty light."
There is an appearance of detail in this statement which wears the garb
of truth. We are not inclined to doubt that the night was misty, nor as
to whether the moonbeams had to struggle into visibility; the question
at issue is a much more fundamental one. We do not know who was the
first to raise the point as to whether any moon shone on that memorable
event at all or not; but the question having been raised, the Nautical
Almanac immediately supplies an answer. From it we learn in language,
whose truthfulness constitutes its only claim to be poetry, that the
moon was new at one o'clock in the morning of the day of the battle of
Corunna (16th January, 1809). The ballad evidently implies that the
funeral took place on the night following the battle. We are therefore
assured that the moon can hardly have been a day old when the hero was
consigned to his grave. But the moon in such a case is practically
invisible, and yields no appreciable moonbeams at all, misty or
otherwise. Indeed, if the funeral took place at the "dead of night," as
the poet asserts, then the moon must have been far below the horizon at
the time.[6]
In alluding to this and similar instances, Mr. Nasmyth gives a word of
advice to authors or to artists who desire to bring the moon on a scene
without knowing as a matter of fact that our satellite was actually
present. He recommends them to follow the example of Bottom in _A
Midsummer's Night's Dream_, and consult "a calendar, a calendar! Look in
the almanac; find out moonshine, find out moonshine!"
[Illustration: Fig. 23.--Comparative Sizes of the Earth and the Moon.]
Among the countless host of celestial bodies--the sun, the moon, the
planets, and the stars--our satellite enjoys one special claim on our
attention. The moon is our nearest permanent neighbour. It is just
possible that a comet may occasionally approach the earth more closely
than the moon but with this exception the other celestial bodies are all
many hundreds or thousands, or even many millions, of times further from
us than the moon.
It is also to be observed that the moon is one of the smallest visible
objects which the heavens contain. Every one of the thousands of stars
that can be seen with the unaided eye is enormously larger than our
satellite. The brilliance and apparent vast proportions of the moon
arise from the fact that it is only 240,000 miles away, which is a
distance almost immeasurably small when compared with the distances
between the earth and the stars.
Fig. 23 exhibits the relative sizes of the earth and its attendant. The
small globe shows the moon, while the larger globe represents the earth.
When we measure the actual diameters of the two globes, we find that of
the earth to be 7,918 miles and of the moon 2,160 miles, so that the
diameter of the earth is nearly four times greater than the diameter of
the moon. If the earth were cut into fifty pieces, all equally large,
then one of these pieces rolled into a globe would equal the size of the
moon. The superficial extent of the moon is equal to about one
thirteenth part of the surface of the earth. The hemisphere our
neighbour turns towards us exhibits an area equal to about one
twenty-seventh part of the area of the earth. This, to speak
approximately, is about double the actual extent of the continent of
Europe. The average materials of the earth are, however, much heavier
than those contained in the moon. It would take more than eighty globes,
each as ponderous as the moon, to weigh down the earth.
Amid the changes which the moon presents to us, one obvious fact stands
prominently forth. Whether our satellite be new or full, at first
quarter or at last, whether it be high in the heavens or low near the
horizon, whether it be in process of eclipse by the sun, or whether the
sun himself is being eclipsed by the moon, the apparent size of the
latter is nearly constant. We can express the matter numerically. A
globe one foot in diameter, at a distance of 111 feet from the observer,
would under ordinary circumstances be just sufficient to hide the disc
of the moon; occasionally, however, the globe would have to be brought
in to a distance of only 103 feet, or occasionally it might have to be
moved out to so much as 118 feet, if the moon is to be exactly hidden.
It is unusual for the moon to approach either of its extreme limits of
position, so that the distance from the eye at which the globe must be
situated so as to exactly cover the moon is usually more than 105 feet,
and less than 117 feet. These fluctuations in the apparent size of our
satellite are contained within such narrow limits that in the first
glance at the subject they may be overlooked. It will be easily seen
that the apparent size of the moon must be connected with its real
distance from the earth. Suppose, for the sake of illustration, that the
moon were to recede into space, its size would seem to dwindle, and long
ere it had reached the distance of even the very nearest of the other
celestial bodies it would have shrunk into insignificance. On the other
hand, if the moon were to come nearer to the earth, its apparent size
would gradually increase until, when close to our globe, it would seem
like a mighty continent stretching over the sky. We find that the
apparent size of the moon is nearly constant, and hence we infer that
the average distance of the same body is also nearly constant. The
average value of that distance is 239,000 miles. In rare circumstances
it may approach to a distance but little more than 221,000 miles, or
recede to a distance hardly less than 253,000 miles, but the ordinary
fluctuations do not exceed more than about 13,000 miles on either side
of its mean value.
From the moon's incessant changes we perceive that she is in constant
motion, and we now further see that whatever these movements may be, the
earth and the moon must at present remain at _nearly_ the same distance
apart. If we further add that the path pursued by the moon around the
heavens lies nearly in a plane, then we are forced to the conclusion
that our satellite must be revolving in a nearly circular path around
the earth at the centre. It can, indeed, be shown that the constant
distance of the two bodies involves as a necessary condition the
revolution of the moon around the earth. The attraction between the moon
and the earth tends to bring the two bodies together. The only way by
which such a catastrophe can be permanently avoided is by making the
satellite move as we actually find it to do. The attraction between the
earth and the moon still exists, but its effect is not then shown in
bringing the moon in towards the earth. The attraction has now to exert
its whole power in restraining the moon in its circular path; were the
attraction to cease, the moon would start off in a straight line, and
recede never to return.
[Illustration: Fig. 24.--The Moon's Path around the Sun.]
The fact of the moon's revolution around the earth is easily
demonstrated by observations of the stars. The rising and setting of our
satellite is, of course, due to the rotation of the earth, and this
apparent diurnal movement the moon possesses in common with the sun and
with the stars. It will, however, be noticed that the moon is
continually changing its place among the stars. Even in the course of a
single night the displacement will be conspicuous to a careful observer
without the aid of a telescope. The moon completes each revolution
around the earth in a period of 27.3 days.
[Illustration: Fig. 25.--The Phases of the Moon.]
In Fig. 24 we have a view of the relative positions of the earth, the
sun, and the moon, but it is to be observed that, for the convenience of
illustration, we have been obliged to represent the orbit of the moon on
a much larger scale than it ought to be in comparison with the distance
of the sun. That half of the moon which is turned towards the sun is
brilliantly illuminated, and, according as we see more or less of that
brilliant half, we say that the moon is more or less full, the several
"phases" being visible in the succession shown by the numbers in Fig.
25. A beginner sometimes finds considerable difficulty in understanding
how the light on the full moon at night can have been derived from the
sun. "Is not," he will say, "the earth in the way? and must it not
intercept the sunlight from every object on the other side of the earth
to the sun?" A study of Fig. 24 will explain the difficulty. The plane
in which the moon revolves does not coincide with the plane in which the
earth revolves around the sun. The line in which the plane of the
earth's motion is intersected by that of the moon divides the moon's
path into two semicircles. We must imagine the moon's path to be tilted
a little, so that the upper semicircle is somewhat above the plane of
the paper, and the other semicircle below. It thus follows that when the
moon is in the position marked full, under the circumstances shown in
the figure, the moon will be just above the line joining the earth and
the sun; the sunlight will thus pass over the earth to the moon, and the
moon will be illuminated. At new moon, the moon will be under the line
joining the earth and the sun.
As the relative positions of the earth and the sun are changing, it
happens twice in each revolution that the sun comes into the position of
the line of intersection of the two planes. If this occurs at the time
of full moon, the earth lies directly between the moon and the sun; the
moon is thus plunged into the shadow of the earth, the light from the
sun is intercepted, and we say that the moon is eclipsed. The moon
sometimes only partially enters the earth's shadow, in which case the
eclipse is a partial one. When, on the other hand, the sun is situated
on the line of intersection at the time of new moon, the moon lies
directly between the earth and the sun, and the dark body of the moon
will then cut off the sunlight from the earth, producing a solar
eclipse. Usually only a part of the sun is thus obscured, forming the
well-known partial eclipse; if, however, the moon pass centrally over
the sun, then we must have one or other of two very remarkable kinds of
eclipse. Sometimes the moon entirely blots out the sun, and thus is
produced the sublime spectacle of a total eclipse, which tells us so
much as to the nature of the sun, and to which we have already referred
in the last chapter. Even when the moon is placed centrally over the
sun, a thin rim of sunlight is occasionally seen round the margin of the
moon. We then have what is known as an annular eclipse.
It is remarkable that the moon is sometimes able to hide the sun
completely, while on other occasions it fails to do so. It happens that
the average apparent size of the moon is nearly equal to the average
apparent size of the sun, but, owing to the fluctuations in their
distances, the actual apparent sizes of both bodies undergo certain
changes. On certain occasions the apparent size of the moon is greater
than that of the sun. In this case a central passage produces a total
eclipse; but it may also happen that the apparent size of the sun
exceeds that of the moon, in which case a central passage can only
produce an annular eclipse.
[Illustration: Fig. 26.--Form of the Earth's Shadow, showing the
Penumbra, or partially shaded region. Within the Penumbra, the Moon is
visible; in the Shadow it is nearly invisible.]
There are hardly any more interesting celestial phenomena than the
different descriptions of eclipses. The almanac will always give timely
notice of the occurrence, and the more striking features can be observed
without a telescope. In an eclipse of the moon (Fig. 26) it is
interesting to note the moment when the black shadow is first detected,
to watch its gradual encroachment over the bright surface of the moon,
to follow it, in case the eclipse is total, until there is only a thin
crescent of moonlight left, and to watch the final extinction of that
crescent when the whole moon is plunged into the shadow. But now a
spectacle of great interest and beauty is often manifested; for though
the moon is so hidden behind the earth that not a single direct ray of
the sunlight could reach its surface, yet we often find that the moon
remains visible, and, indeed, actually glows with a copper-coloured hue
bright enough to permit several of the markings on the surface to be
discerned.
This illumination of the moon even in the depth of a total eclipse is
due to the sunbeams which have just grazed the edge of the earth. In
doing so they have become bent by the refraction of the atmosphere, and
have thus been turned inwards into the shadow. Such beams have passed
through a prodigious thickness of the earth's atmosphere, and in this
long journey through hundreds of miles of air they have become tinged
with a ruddy or copper-like hue. Nor is this property of our atmosphere
an unfamiliar one. The sun both at sunrise and at sunset glows with a
light which is much more ruddy than the beams it dispenses at noonday.
But at sunset or at sunrise the rays which reach our eyes have much more
of our atmosphere to penetrate than they have at noon, and accordingly
the atmosphere imparts to them that ruddy colour so characteristic and
often so lovely. If the spectrum of the sun when close to the horizon is
examined it is seen to be filled with numerous dark lines and bands
situated chiefly towards the blue and violet end. These are caused by
the increased absorption which the light suffers in the atmosphere, and
give rise to the preponderating red light on the sun under such
conditions. In the case of the eclipsed moon, the sunbeams have to take
an atmospheric journey more than double as long as that at sunrise or
sunset, and hence the ruddy glow of the eclipsed moon may be accounted
for.
The almanacs give the full particulars of each eclipse that happens in
the corresponding year. These predictions are reliable, because
astronomers have been carefully observing the moon for ages, and have
learned from these observations not only how the moon moves at present,
but also how it will move for ages to come. The actual calculations are
so complicated that we cannot here discuss them. There is, however, one
leading principle about eclipses which is so simple that we must refer
to it. The eclipses occurring this year have no very obvious relation
to the eclipses that occurred last year, or to those that will occur
next year. Yet, when we take a more extended view of the sequence of
these phenomena, a very definite principle becomes manifest. If we
observe all the eclipses in a period of eighteen years, or nineteen
years, then we can predict, with at least an approximation to the truth,
all the future eclipses for many years. It is only necessary to
recollect that in 6,585-1/3 days after one eclipse a nearly similar
eclipse follows. For instance, a beautiful eclipse of the moon occurred
on the 5th of December, 1881. If we count back 6,585 days from that
date, or, that is, eighteen years and eleven days, we come to November
24th, 1863, and a similar eclipse of the moon took place then. Again,
there were four eclipses in the year 1881. If we add 6,585-1/3 days to
the date of each eclipse, it will give the dates of all the four
eclipses in the year 1899. It was this rule which enabled the ancient
astronomers to predict the recurrence of eclipses, at a time when the
motions of the moon were not understood nearly so well as they now are.
During a long voyage, and perhaps in critical circumstances, the moon
will often render invaluable information to the sailor. To navigate a
ship, suppose from Liverpool to China, the captain must frequently
determine the precise position which his ship then occupies. If he could
not do this, he would never find his way across the trackless ocean.
Observations of the sun give him his latitude and tell him his local
time, but the captain further requires to know the Greenwich time before
he can place his finger at a point of the chart and say, "My ship is
here." To ascertain the Greenwich time the ship carries a chronometer
which has been carefully rated before starting, and, as a precaution,
two or three chronometers are usually provided to guard against the risk
of error. An unknown error of a minute in the chronometer might perhaps
lead the vessel fifteen miles from its proper course.
[Illustration: PLATE VI.
CHART OF THE MOON'S SURFACE.]
[Illustration: Fig. 27.--Key to Chart of the Moon (Plate VI.).]
It is important to have the means of testing the chronometers during the
progress of the voyage; and it would be a great convenience if every
captain, when he wished, could actually consult some infallible standard
of Greenwich time. We want, in fact, a Greenwich clock which may be
visible over the whole globe. There is such a clock; and, like any other
clock, it has a face on which certain marks are made, and a hand which
travels round that face. The great clock at Westminster shrinks into
insignificance when compared with the mighty clock which the captain
uses for setting his chronometer. The face of this stupendous dial is
the face of the heavens. The numbers engraved on the face of a clock are
replaced by the twinkling stars; while the hand which moves over the
dial is the beautiful moon herself. When the captain desires to test
his chronometer, he measures the distance of the moon from a
neighbouring star. For example, he may see that the moon is three
degrees from the star Regulus. In the Nautical Almanac he finds the
Greenwich time at which the moon was three degrees from Regulus.
Comparing this with the indications of the chronometer, he finds the
required correction.
There is one widely-credited myth about the moon which must be regarded
as devoid of foundation. The idea that our satellite and the weather
bear some relation has no doubt been entertained by high authority, and
appears to be an article in the belief of many an excellent mariner.
Careful comparison between the state of the weather and the phases of
the moon has, however, quite discredited the notion that any connection
of the kind does really exist.
We often notice large blank spaces on maps of Africa and of Australia
which indicate our ignorance of parts of the interior of those great
continents. We can find no such blank spaces in the map of the moon.
Astronomers know the surface of the moon better than geographers know
the interior of Africa. Every spot on the face of the moon which is as
large as an English parish has been mapped, and all the more important
objects have been named.
A general map of the moon is shown in Plate VI. It has been based upon
drawings made with small telescopes, and it gives an entire view of that
side of our satellite which is presented towards us. The moon is shown
as it appears in an astronomical telescope, which inverts everything, so
that the south is at the top and the north at the bottom (to show
objects upright a telescope requires an additional pair of lenses in the
eye-piece, and as this diminishes the amount of light reaching the eye
they are dispensed with in astronomical telescopes). We can see on the
map some of the characteristic features of lunar scenery. Those dark
regions so conspicuous in the ordinary full moon are easily recognised
on the map. They were thought to be seas by astronomers before the days
of telescopes, and indeed the name "Mare" is still retained, though it
is obvious that they contain no water at present. The map also shows
certain ridges or elevated portions, and when we apply measurement to
these objects we learn that they must be mighty mountain ranges. But the
most striking features on the moon are those ring-like objects which are
scattered over the surface in profusion. These are known as the lunar
craters.
To facilitate reference to the chief points of interest we have arranged
an index map (Fig. 27) which will give a clue to the names of the
several objects depicted upon the plate. The so-called seas are
represented by capital letters; so that A is the Mare Crisium, and H the
Oceanus Procellarum. The ranges of mountains are indicated by small
letters; thus a on the index is the site of the so-called Caucasus
mountains, and similarly the Apennines are denoted by _c_. The numerous
craters are distinguished by numbers; for example, the feature on the
map corresponding to 20 on the index is the crater designated Ptolemy.
A. Mare Crisium.
B. Mare Foecunditatis.
C. Mare Tranquillitatis.
D. Mare Serenitatis.
E. Mare Imbrium.
F. Sinus Iridum.
G. Mare Vaporum.
H. Oceanus Procellarum.
I. Mare Humorum.
J. Mare Nubium.
K. Mare Nectaris.
_a._ Caucasus.
_b._ Alps.
_c._ Apennines.
_d._ Carpathians.
_f._ Cordilleras & D'Alembert mountains.
_g._ Rook mountains.
_h._ Doerfel mountains.
_i._ Leibnitz mountains.
1. Posidonius.
2. Linne.
3. Aristotle.
4. Great Valley of the Alps.
5. Aristillus.
6. Autolycus.
7. Archimedes.
8. Plato.
9. Eratosthenes.
10. Copernicus.
11. Kepler.
12. Aristarchus.
13. Grimaldi.
14. Gassendi.
15. Schickard.
16. Wargentin.
17. Clavius.
18. Tycho.
19. Alphonsus.
20. Ptolemy.
21. Catharina.
22. Cyrillus.
23. Theophilus.
24. Petavius.
25. Hyginus.
26. Triesnecker.
In every geographical atlas there is a map showing the two hemispheres
of the earth, the eastern and the western. In the case of the moon we
can only give a map of one hemisphere, for the simple reason that the
moon always turns the same side towards us, and accordingly we never get
a view of the other side. This is caused by the interesting circumstance
that the moon takes exactly the same time to turn once round its own
axis as it takes to go once round the earth. The rotation is, however,
performed with uniform speed, while the moon does not move in its orbit
with a perfectly uniform velocity (_see_ Chapter IV.). The consequence
is that we now get a slight glimpse round the east limb, and now a
similar glimpse round the west limb, as if the moon were shaking its
head very gently at us. But it is only an insignificant margin of the
far side of the moon which this _libration_ permits us to examine.
Lunar objects are well suited for observation when the sunlight falls
upon them in such a manner as to exhibit strongly contrasted lights and
shadows. It is impossible to observe the moon satisfactorily when it is
full, for then no conspicuous shadows are cast. The most opportune
moment for seeing any particular lunar object is when it lies just at
the illuminated side of the boundary between light and shade, for then
the features are brought out with exquisite distinctness.
Plate VII.[7] gives an illustration of lunar scenery, the object
represented being known to astronomers by the name of Triesnecker. The
district included is only a very small fraction of the entire surface of
the moon, yet the actual area is very considerable, embracing as it does
many hundreds of square miles. We see in it various ranges of lunar
mountains, while the central object in the picture is one of those
remarkable lunar craters which we meet with so frequently in every lunar
landscape. This crater is about twenty miles in diameter, and it has a
lofty mountain in the centre, the peak of which is just illuminated by
the rising sun in that phase of our satellite which is represented in
the picture.
A typical view of a lunar crater is shown in Plate VIII. This is, no
doubt, a somewhat imaginary sketch. The point of view from which the
artist is supposed to have taken the picture is one quite unattainable
by terrestrial astronomers, yet there can be little doubt that it is a
fair representation of objects on the moon. We should, however,
recollect the scale on which it is drawn. The vast crater must be many
miles across, and the mountain at its centre must be thousands of feet
high. The telescope will, even at its best, only show the moon as well
as we could see it with the unaided eye if it were 250 miles away
instead of being 240,000. We must not, therefore, expect to see any
details on the moon even with the finest telescopes, unless they were
coarse enough to be visible at a distance of 250 miles. England from
such a point of view would only show London as a coloured spot, in
contrast with the general surface of the country.
We return, however, from a somewhat fancy sketch to a more prosaic
examination of what the telescope does actually reveal. Plate IX.
represents the large crater Plato, so well known to everyone who uses a
telescope. The floor of this remarkable object is nearly flat, and the
central mountain, so often seen in other craters, is entirely wanting.
We describe it more fully in the general list of lunar objects.
The mountain peaks on the moon throw long, well-defined shadows,
characterised by a sharpness which we do not find in the shadows of
terrestrial objects. The difference between the two cases arises from
the absence of air from the moon. Our atmosphere diffuses a certain
amount of light, which mitigates the blackness of terrestrial shadows
and tends to soften their outline. No such influences are at work on the
moon, and the sharpness of the shadows is taken advantage of in our
attempts to measure the heights of the lunar mountains.
It is often easy to compute the altitude of a church steeple, a lofty
chimney, or any similar object, from the length of its shadow. The
simplest and the most accurate process is to measure at noon the number
of feet from the base of the object to the end of the shadow. The
elevation of the sun at noon on the day in question can be obtained from
the almanac, and then the height of the object follows by a simple
calculation. Indeed, if the observations can be made either on the 6th
of April or the 6th of September, at or near the latitude of London,
then calculations would be unnecessary. The noonday length of the shadow
on either of the dates named is equal to the altitude of the object. In
summer the length of the noontide shadow is less than the altitude; in
winter the length of the shadow exceeds the altitude. At sunrise or
sunset the shadows are, of course, much longer than at noon, and it is
shadows of this kind that we observe on the moon. The necessary
measurements are made by that indispensable adjunct to the equatorial
telescope known as the _micrometer_.
This word denotes an instrument for measuring _small_ distances. In one
sense the term is not a happy one. The objects to which the astronomer
applies the micrometer are usually anything but small. They are
generally of the most transcendent dimensions, far exceeding the moon or
the sun, or even our whole system. Still, the name is not altogether
inappropriate, for, vast though the objects may be, they generally seem
minute, even in the telescope, on account of their great distance.
We require for such measurements an instrument capable of the greatest
nicety. Here, again, we invoke the aid of the spider, to whose
assistance in another department we have already referred. In the filar
micrometer two spider lines are parallel, and one intersects them at
right angles. One or both of the parallel lines can be moved by means of
screws, the threads of which have been shaped by consummate workmanship.
The distance through which the line has been moved is accurately
indicated by noting the number of revolutions and parts of a revolution
of the screw. Suppose the two lines be first brought into coincidence,
and then separated until the apparent length of the shadow of the
mountain on the moon is equal to the distance between the lines: we then
know the number of revolutions of the micrometer screw which is
equivalent to the length of the shadow. The number of miles on the moon
which correspond to one revolution of the screw has been previously
ascertained by other observations, and hence the length of the shadow
can be determined. The elevation of the sun, as it would have appeared
to an observer at this point of the moon, at the moment when the
measures were being made, is also obtainable, and hence the actual
elevation of the mountain can be calculated. By measurements of this
kind the altitudes of other lunar objects, such, for example, as the
height of the rampart surrounding a circular-walled plane, can be
determined.
The beauty and interest of the moon as a telescopic object induces us to
give to the student a somewhat detailed account of the more remarkable
features which it presents. Most of the objects we are to describe can
be effectively exhibited with very moderate telescopic power. It is,
however, to be remembered that all of them cannot be well seen at one
time. The region most distinctly shown is the boundary between light and
darkness. The student will, therefore, select for observation such
objects as may happen to lie near that boundary at the time when he is
observing.
1. _Posidonius._--The diameter of this large crater is nearly 60 miles.
Although its surrounding wall is comparatively slender, it is so
distinctly marked as to make the object very conspicuous. As so
frequently happens in lunar volcanoes, the bottom of the crater is below
the level of the surrounding plain, in the present instance to the
extent of nearly 2,500 feet.
2. _Linne._--This small crater lies in the Mare Serenitatis. About sixty
years ago it was described as being about 6-1/2 miles in diameter, and
seems to have been sufficiently conspicuous. In 1866 Schmidt, of Athens,
announced that the crater had disappeared. Since then an exceedingly
small shallow depression has been visible, but the whole object is now
very inconsiderable. This seems to be the most clearly attested case of
change in a lunar object. Apparently the walls of the crater have
tumbled into the interior and partly filled it up, but many astronomers
doubt that a change has really taken place, as Schroeter, a Hanoverian
observer at the end of the eighteenth century, appears not to have seen
any conspicuous crater in the place, though it must be admitted that
his observations are rather incomplete. To give some idea of Schmidt's
amazing industry in lunar researches, it may be mentioned that in six
years he made nearly 57,000 individual settings of his micrometer in the
measurement of lunar altitudes. His great chart of the mountains in the
moon is based on no less than 2,731 drawings and sketches, if those are
counted twice that may have been used for two divisions of the map.
3. _Aristotle._--This great philosopher's name has been attached to a
grand crater 50 miles in diameter, the interior of which, although very
hilly, shows no decidedly marked central cone. But the lofty wall of the
crater, exceeding 10,500 feet in height, overshadows the floor so
continuously that its features are never seen to advantage.
4. _The Great Valley of the Alps._--A wonderfully straight valley, with
a width ranging from 3-1/2 to 6 miles, runs right through the lunar
Alps. It is, according to Maedler, at least 11,500 feet deep, and over 80
miles in length. A few low ridges which are parallel to the sides of the
valley may possibly be the result of landslips.
5. _Aristillus._--Under favourable conditions Lord Rosse's great
telescope has shown the exterior of this magnificent crater to be scored
with deep gullies radiating from its centre. Aristillus is about 34
miles wide and 10,000 feet in depth.
6. _Autolycus_ is somewhat smaller than the foregoing, to which it forms
a companion in accordance with what Maedler thought a well-defined
relation amongst lunar craters, by which they frequently occurred in
pairs, with the smaller one more usually to the south. Towards the edge
this arrangement is generally rather apparent than real, and is merely a
result of foreshortening.
7. _Archimedes._--This large plain, about 50 miles in diameter, has its
vast smooth interior divided by unequally bright streaks into seven
distinct zones, running east and west. There is no central mountain or
other obvious internal sign of former activity, but its irregular wall
rises into abrupt towers, and is marked outside by decided terraces.
[Illustration: PLATE B.
PORTION OF THE MOON.
(ALPS, ARCHIMEDES, APENNINES.)
_Messrs. Loewy & Puiseux_.]
8. _Plato._--We have already referred to this extensive circular plain,
which is noticeable with the smallest telescope. The average height of
the rampart is about 3,800 feet on the eastern side; the western side is
somewhat lower, but there is one peak rising to the height of nearly
7,300 feet. The plain girdled by this vast rampart is of ample
proportions. It is a somewhat irregular circle, about 60 miles in
diameter, and containing an area of 2,700 square miles. On its floor the
shadows of the western wall are shown in Plate IX., as are also three of
the small craters, of which a large number have been detected by
persevering observers. The narrow sharp line leading from the crater to
the left is one of those remarkable "clefts" which traverse the moon in
so many directions. Another may be seen further to the left. Above Plato
are several detached mountains, the loftiest of which is Pico, about
8,000 feet in height. Its long and pointed shadow would at first sight
lead one to suppose that it must be very steep; but Schmidt, who
specially studied the inclinations of the lunar slopes, is of opinion
that it cannot be nearly so steep as many of the Swiss mountains that
are frequently ascended. As many as thirty minute craters have been
carefully observed on the floor of Plato, and variations have been
thought by Mr. W.H. Pickering to be perceptible.
9. _Eratosthenes._--This profound crater, upwards of 37 miles in
diameter, lies at the end of the gigantic range of the Apennines. Not
improbably, Eratosthenes once formed the volcanic vent for the
stupendous forces that elevated the comparatively craterless peaks of
these great mountains.
10. _Copernicus._--Of all the lunar craters this is one of the grandest
and best known. The region to the west is dotted over with innumerable
minute craterlets. It has a central many-peaked mountain about 2,400
feet in height. There is good reason to believe that the terracing shown
in its interior is mainly due to the repeated alternate rise, partial
congelation, and subsequent retreat of a vast sea of lava. At full moon
the crater of Copernicus is seen to be surrounded by radiating streaks.
11. _Kepler._--Although the internal depth of this crater is scarcely
less than 10,000 feet, it has but a very low surrounding wall, which is
remarkable for being covered with the same glistening substance that
also forms a system of bright rays not unlike those surrounding the last
object.
12. _Aristarchus_ is the most brilliant of the lunar craters, being
specially vivid with a low power in a large telescope. So bright is it,
indeed, that it has often been seen on the dark side just after new
moon, and has thus given rise to marvellous stories of active lunar
volcanoes. To the south-east lies another smaller crater, Herodotus,
north of which is a narrow, deep valley, nowhere more than 2-1/2 miles
broad, which makes a remarkable zigzag. It is one of the largest of the
lunar "clefts."
13. _Grimaldi_ calls for notice as the darkest object of its size in the
moon. Under very exceptional circumstances it has been seen with the
naked eye, and as its area has been estimated at nearly 14,000 square
miles, it gives an idea of how little unaided vision can discern in the
moon; it must, however, be added that we always see Grimaldi
considerably foreshortened.
14. The great crater _Gassendi_ has been very frequently mapped on
account of its elaborate system of "clefts." At its northern end it
communicates with a smaller but much deeper crater, that is often filled
with black shadow after the whole floor of Gassendi has been
illuminated.
15. _Schickard_ is one of the largest walled plains on the moon, about
134 miles in breadth. Within its vast expanse Maedler detected 23 minor
craters. With regard to this object Chacornac pointed out that, owing to
the curvature of the surface of the moon, a spectator at the centre of
the floor "would think himself in a boundless desert," because the
surrounding wall, although in one place nearly 10,000 feet high, would
lie entirely beneath his horizon.
16. Close to the foregoing is _Wargentin_. There can be little doubt
that this is really a huge crater almost filled with congealed lava, as
there is scarcely any fall towards the interior.
17. _Clavius._--Near the 60th parallel of lunar south latitude lies
this enormous enclosure, the area of which is not less than 16,500
square miles. Both in its interior and on its walls are many peaks and
secondary craters. The telescopic view of a sunrise upon the surface of
Clavius is truly said by Maedler to be indescribably magnificent. One of
the peaks rises to a height of 24,000 feet above the bottom of one of
the included craters. Maedler even expressed the opinion that in this
wild neighbourhood there are craters so profound that no ray of sunlight
ever penetrated their lowest depths, while, as if in compensation, there
are peaks whose summits enjoy a mean day almost twice as long as their
night.
18. If the full moon be viewed through an opera-glass or any small
hand-telescope, one crater is immediately seen to be conspicuous beyond
all others, by reason of the brilliant rays or streaks that radiate from
it. This is the majestic _Tycho_, 17,000 feet in depth and 50 miles in
diameter (Plate X.). A peak 6,000 feet in height rises in the centre of
its floor, while a series of terraces diversity its interior slopes; but
it is the mysterious bright rays that chiefly surprise us. When the sun
rises on Tycho, these streaks are utterly invisible; indeed, the whole
object is then so obscure that it requires a practised eye to recognise
Tycho amidst its mountainous surroundings. But as soon as the sun has
attained a height of about 30 deg. above its horizon, the rays emerge from
their obscurity and gradually increase in brightness until the moon
becomes full, when they are the most conspicuous objects on her surface.
They vary in length, from a few hundred miles to two or, in one
instance, nearly three thousand miles. They extend indifferently across
vast plains, into the deepest craters, or over the loftiest elevations.
We know of nothing on our earth to which they can be compared. As these
rays are only seen about the time of full moon, their visibility
obviously depends on the light falling more or less closely in the line
of sight, quite regardless of the inclination of the surfaces, mountains
or valleys, on which they appear. Each small portion of the surface of
the streak must therefore be of a form which is symmetrical to the
spectator from whatever point it is seen. The sphere alone appears to
fulfil this condition, and Professor Copeland therefore suggests that
the material constituting the surface of the streak must be made up of a
large number of more or less completely spherical globules. The streaks
must represent parts of the lunar surface either pitted with minute
cavities of spherical figure, or strewn over with minute transparent
spheres.[8]
Near the centre of the moon's disc is a fine range of ring plains fully
open to our view under all illuminations. Of these, two may be
mentioned--_Alphonsus_ (19), the floor of which is strangely
characterised by two bright and several dark markings which cannot be
explained by irregularities in the surface.--_Ptolemy_ (20). Besides
several small enclosed craters, its floor is crossed by numerous low
ridges, visible when the sun is rising or setting.
21, 22, 23.--When the moon is five or six days old this beautiful group
of three craters will be favourably placed for observation. They are
named _Catharina_, _Cyrillus_, and _Theophilus_. Catharina, the most
southerly of the group, is more than 16,000 feet deep, and connected
with Cyrillus by a wide valley; but between Cyrillus and Theophilus
there is no such connection. Indeed, Cyrillus looks as if its huge
surrounding ramparts, as high as Mont Blanc, had been completely
finished before the volcanic forces commenced the formation of
Theophilus, the rampart of which encroaches considerably on its older
neighbour. Theophilus stands as a well-defined circular crater about 64
miles in diameter, with an internal depth of 14,000 to 18,000 feet, and
a beautiful central group of mountains, one-third of that height, on its
floor. Although Theophilus is the deepest crater we can see in the moon,
it has suffered little or no deformation from secondary eruptions, while
the floor and wall of Catharina show complete sequences of lesser
craters of various sizes that have broken in upon and partly destroyed
each other. In the spring of the year, when the moon is somewhat before
the first quarter, this instructive group of extinct volcanoes can be
seen to great advantage at a convenient hour in the evening.
[Illustration: PLATE VII.
TRIESNECKER.
(AFTER NASMYTH.)]
24. _Petavius_ is remarkable not only for its great size, but also for
the rare feature of having a double rampart. It is a beautiful object
soon after new moon, or just after full moon, but disappears absolutely
when the sun is more than 45 deg. above its horizon. The crater floor is
remarkably convex, culminating in a central group of hills intersected
by a deep cleft.
25. _Hyginus_ is a small crater near the centre of the moon's disc. One
of the largest of the lunar chasms passes right through it, making an
abrupt turn as it does so.
26. _Triesnecker._--This fine crater has been already described, but is
again alluded to in order to draw attention to the elaborate system of
chasms so conspicuously shown in Plate VII. That these chasms are
depressions is abundantly evident by the shadows inside. Very often
their margins are appreciably raised. They seem to be fractures in the
moon's surface.
Of the various mountains that are occasionally seen as projections on
the actual edge of the moon, those called after Leibnitz (_i_) seem to
be the highest. Schmidt found the highest peak to be upwards of 41,900
feet above a neighbouring valley. In comparing these altitudes with
those of mountains on our earth, we must for the latter add the depth of
the sea to the height of the land. Reckoned in this way, our highest
mountains are still higher than any we know of in the moon.
We must now discuss the important question as to the origin of these
remarkable features on the surface of the moon. We shall admit at the
outset that our evidence on this subject is only indirect. To establish
by unimpeachable evidence the volcanic origin of the remarkable lunar
craters, it would seem almost necessary that volcanic outbursts should
have been witnessed on the moon, and that such outbursts should have
been seen to result in the formation of the well-known ring, with or
without the mountain rising from the centre. To say that nothing of the
kind has ever been witnessed would be rather too emphatic a statement.
On certain occasions careful observers have reported the occurrence of
minute local changes on the moon. As we have already remarked, a crater
named Linne, of dimensions respectable, no doubt, to a lunar inhabitant,
but forming a very inconsiderable telescopic object, was thought to have
undergone some change. On another occasion a minute crater was thought
to have arisen near the well-known object named Hyginus. The mere
enumeration of such instances gives real emphasis to the statement that
there is at the present time no appreciable source of disturbance of the
moon's surface. Even were these trifling cases of suspected change
really established--and this is perhaps rather farther than many
astronomers would be willing to go--they are still insignificant when
compared with the mighty phenomena that gave rise to the host of great
craters which cover so large a portion of the moon's surface.
We are led inevitably to the conclusion that our satellite must have
once possessed much greater activity than it now displays. We can also
give a reasonable, or, at all events, a plausible, explanation of the
cessation of that activity in recent times. Let us glance at two other
bodies of our system, the earth and the sun, and compare them with the
moon. Of the three bodies, the sun is enormously the largest, while the
moon is much less than the earth. We have also seen that though the sun
must have a very high temperature, there can be no doubt that it is
gradually parting with its heat. The surface of the earth, formed as it
is of solid rocks and clay, or covered in great part by the vast expanse
of ocean, bears but few obvious traces of a high temperature.
Nevertheless, it is highly probable from ordinary volcanic phenomena
that the interior of the earth still possesses a temperature of
incandescence.
A large body when heated takes a longer time to cool than does a small
body raised to the same temperature. A large iron casting will take days
to cool; a small casting will become cold in a few hours. Whatever may
have been the original source of heat in our system--a question which
we are not now discussing--it seems demonstrable that the different
bodies were all originally heated, and have now for ages been gradually
cooling. The sun is so vast that he has not yet had time to cool; the
earth, of intermediate bulk, has become cold on the outside, while still
retaining vast stores of internal heat; while the moon, the smallest
body of all, has lost its heat to such an extent that changes of
importance on its surface can no longer be originated by internal fires.
We are thus led to refer the origin of the lunar craters to some ancient
epoch in the moon's history. We have no moans of knowing the remoteness
of that epoch, but it is reasonable to surmise that the antiquity of the
lunar volcanoes must be extremely great. At the time when the moon was
sufficiently heated to originate those convulsions, of which the mighty
craters are the survivals, the earth must also have been much hotter
than it is at present. When the moon possessed sufficient heat for its
volcanoes to be active, the earth was probably so hot that life was
impossible on its surface. This supposition would point to an antiquity
for the lunar craters far too great to be estimated by the centuries and
the thousands of years which are adequate for such periods as those with
which the history of human events is concerned. It seems not unlikely
that millions of years may have elapsed since the mighty craters of
Plato or of Copernicus consolidated into their present form.
We shall now attempt to account for the formation of the lunar craters.
The most probable views on the subject seem to be those which have been
set forth by Mr. Nasmyth, though it must be admitted that his doctrines
are by no means free from difficulty. According to his theory we can
explain how the rampart around the lunar crater has been formed, and how
the great mountain arose which so often adorns the centre of the plain.
The view in Fig. 28 contains an imaginary sketch of a volcanic vent on
the moon in the days when the craters were active. The eruption is here
shown in the fulness of its energy, when the internal forces are hurling
forth ashes or stones which fall at a considerable distance from the
vent. The materials thus accumulated constitute the rampart surrounding
the crater.
The second picture (Fig. 29) depicts the crater in a later stage of its
history. The prodigious explosive power has now been exhausted, and has
perhaps been intermitted for some time. Again, the volcano bursts into
activity, but this time with only a small part of its original energy. A
comparatively feeble eruption now issues from the same vent, deposits
materials close around the orifice, and raises a mountain in the centre.
Finally, when the activity has subsided, and the volcano is silent and
still, we find the evidence of the early energy testified to by the
rampart which surrounds the ancient crater, and by the mountain which
adorns the interior. The flat floor which is found in some of the
craters may not improbably have arisen from an outflow of lava which has
afterwards consolidated. Subsequent outbreaks have also occurred in many
cases.
One of the principal difficulties attending this method of accounting
for the structure of a crater arises from the great size which some of
these objects attain. There are ancient volcanoes on the moon forty or
fifty miles in diameter; indeed, there is one well-formed ring, with a
mountain rising in the centre, the diameter of which is no less than
seventy-eight miles (Petavius). It seems difficult to conceive how a
blowing cone at the centre could convey the materials to such a distance
as the thirty-nine miles between the centre of Petavius and the rampart.
The explanation is, however, facilitated when it is borne in mind that
the force of gravitation is much less on the moon than on the earth.
[Illustration: PLATE VIII.
A NORMAL LUNAR CRATER.]
[Illustration: Fig. 28.--Volcano in Activity.]
[Illustration: Fig. 29.--Subsequent Feeble Activity.]
Have we not already seen that our satellite is so much smaller than the
earth that eighty moons rolled into one would not weigh as much as the
earth? On the earth an ounce weighs an ounce and a pound weighs a pound;
but a weight of six ounces here would only weigh one ounce on the moon,
and a weight of six pounds here would only weigh one pound on the moon.
A labourer who can carry one sack of corn on the earth could, with the
same exertion, carry six sacks of corn on the moon. A cricketer who can
throw a ball 100 yards on the earth could with precisely the same
exertion throw the same ball 600 yards on the moon. Hiawatha could shoot
ten arrows into the air one after the other before the first reached
the ground; on the moon he might have emptied his whole quiver. The
volcano, which on the moon drove projectiles to the distance of
thirty-nine miles, need only possess the same explosive power as would
have been sufficient to drive the missiles six or seven miles on the
earth. A modern cannon properly elevated would easily achieve this feat.
[Illustration: Fig. 30.--Formation of the Level Floor by Lava.]
It must also be borne in mind that there are innumerable craters on the
moon of the same general type but of the most varied dimensions; from a
tiny telescopic object two or three miles in diameter, we can point out
gradually ascending stages until we reach the mighty Petavius just
considered. With regard to the smaller craters, there is obviously
little or no difficulty in attributing to them a volcanic origin, and as
the continuity from the smallest to the largest craters is unbroken, it
seems quite reasonable to suppose that even the greatest has arisen in
the same way.
It should, however, be remarked that some lunar features might be
explained by actions from without rather than from within. Mr. G.K.
Gilbert has marshalled the evidence in support of the belief that lunar
sculptures arise from the impact of bodies falling on the moon. The Mare
Imbrium, according to this view, has been the seat of a collision to
which the surrounding lunar scenery is due. Mr. Gilbert explains the
furrows as hewn out by mighty projectiles moving with such velocities as
meteors possess.
The lunar landscapes are excessively weird and rugged. They always
remind us of sterile deserts, and we cannot fail to notice the absence
of grassy plains or green forests such as we are familiar with on our
globe. In some respects the moon is not very differently circumstanced
from the earth. Like it, the moon has the pleasing alternations of day
and night, though the day in the moon is as long as twenty-nine of our
days, and the night of the moon is as long as twenty-nine of our nights.
We are warmed by the rays of the sun; so, too, is the moon; but,
whatever may be the temperature during the long day on the moon, it
seems certain that the cold of the lunar night would transcend that
known in the bleakest regions of our earth. The amount of heat radiated
to us by the moon has been investigated by Lord Rosse, and more recently
by Professor Langley. Though every point on the moon's surface is
exposed to the sunlight for a fortnight without any interruption, the
actual temperature to which the soil is raised cannot be a high one. The
moon does not, like the earth, possess a warm blanket, in the shape of
an atmosphere, which can keep in and accumulate the heat received.
Even our largest telescopes can tell nothing directly as to whether life
can exist on the moon. The mammoth trees of California might be growing
on the lunar mountains, and elephants might be walking about on the
plains, but our telescopes could not show them. The smallest object that
we can see on the moon must be about as large as a good-sized cathedral,
so that organised beings resembling in size any that we are familiar
with, if they existed, could not make themselves visible as telescopic
objects.
We are therefore compelled to resort to indirect evidence as to whether
life would be possible on the moon. We may say at once that astronomers
believe that life, as we know it, could not exist. Among the necessary
conditions of life, water is one of the first. Take every form of
vegetable life, from the lichen which grows on the rock to the giant
tree of the forest, and we find the substance of every plant contains
water, and could not exist without it. Nor is water less necessary to
the existence of animal life. Deprived of this element, all organic
life, the life of man himself, would be inconceivable.
Unless, therefore, water be present in the moon, we shall be bound to
conclude that life, as we know it, is impossible. If anyone stationed on
the moon were to look at the earth through a telescope, would he be able
to see any water here? Most undoubtedly he would. He would see the
clouds and he would notice their incessant changes, and the clouds alone
would be almost conclusive evidence of the existence of water. An
astronomer on the moon would also see our oceans as coloured surfaces,
remarkably contrasted with the land, and he would perhaps frequently see
an image of the sun, like a brilliant star, reflected from some smooth
portion of the sea. In fact, considering that much more than half of our
globe is covered with oceans, and that most of the remainder is liable
to be obscured by clouds, the lunar astronomer in looking at our earth
would often see hardly anything but water in one form or other. Very
likely he would come to the conclusion that our globe was only fitted to
be a residence for amphibious animals.
But when we look at the moon with our telescopes we see no direct
evidence of water. Close inspection shows that the so-called lunar seas
are deserts, often marked with small craters and rocks. The telescope
reveals no seas and no oceans, no lakes and no rivers. Nor is the
grandeur of the moon's scenery ever impaired by clouds over her surface.
Whenever the moon is above our horizon, and terrestrial clouds are out
of the way, we can see the features of our satellite's surface with
distinctness. There are no clouds in the moon; there are not even the
mists or the vapours which invariably arise wherever water is present,
and therefore astronomers have been led to the conclusion that the
surface of the globe which attends the earth is a sterile and a
waterless desert.
Another essential element of organic life is also absent from the moon.
Our globe is surrounded with a deep clothing of air resting on the
surface, and extending above our heads to the height of about 200 or 300
miles. We need hardly say how necessary air is to life, and therefore we
turn with interest to the question as to whether the moon can be
surrounded with an atmosphere. Let us clearly understand the problem we
are about to consider. Imagine that a traveller started from the earth
on a journey to the moon; as he proceeded, the air would gradually
become more and more rarefied, until at length, when he was a few
hundred miles above the earth's surface, he would have left the last
perceptible traces of the earth's envelope behind him. By the time he
had passed completely through the atmosphere he would have advanced only
a very small fraction of the whole journey of 240,000 miles, and there
would still remain a vast void to be traversed before the moon would be
reached. If the moon were enveloped in the same way as the earth, then,
as the traveller approached the end of his journey, and came within a
few hundred miles of the moon's surface, he would meet again with traces
of an atmosphere, which would gradually increase in density until he
arrived at the moon's surface. The traveller would thus have passed
through one stratum of air at the beginning of his journey, and through
another at the end, while the main portion of the voyage would have been
through space more void than that to be found in the exhausted receiver
of an air-pump.
Such would be the case if the moon were coated with an atmosphere like
that surrounding our earth. But what are the facts? The traveller as he
drew near the moon would seek in vain for air to breathe at all
resembling ours. It is possible that close to the surface there are
faint traces of some gaseous material surrounding the moon, but it can
only be equal to a very small fractional part of the ample clothing
which the earth now enjoys. For all purposes of respiration, as we
understand the term, we may say that there is no air on the moon, and an
inhabitant of our earth transferred thereto would be as certainly
suffocated as he would be in the middle of space.
It may, however, be asked how we learn this. Is not air transparent, and
how, therefore, could our telescopes be expected to show whether the
moon really possessed such an envelope? It is by indirect, but
thoroughly reliable, methods of observation that we learn the destitute
condition of our satellite. There are various arguments to be adduced;
but the most conclusive is that obtained on the occurrence of what is
called an "occultation." It sometimes happens that the moon comes
directly between the earth and a star, and the temporary extinction of
the latter is an "occultation." We can observe the moment when the
phenomenon takes place, and the suddenness of the disappearance of the
star is generally remarked. If the moon were enveloped in a copious
atmosphere, the interposition of this gaseous mass by the movement of
the moon would produce a gradual evanescence of the star wholly wanting
the abruptness which marks the obscuration.[9]
Let us consider how we can account for the absence of an atmosphere from
the moon. What we call a gas has been found by modern research to be a
collection of an immense number of molecules, each of which is in
exceedingly rapid motion. This motion is only pursued for a short
distance in one direction before a molecule comes into collision with
some other molecule, whereby the directions and velocities of the
individual molecules are continually changed. There is a certain average
speed for each gas which is peculiar to the molecules of that gas at a
certain temperature. When several gases are mixed, as oxygen and
nitrogen are in our atmosphere, the molecules of each gas continue to
move with their own characteristic velocities. So far as we can estimate
the temperature at the boundary of the earth's atmosphere, we may assume
that the average of the velocities of the oxygen molecules there found
is about a quarter of a mile per second. The velocities for nitrogen are
much the same, while the average speed of a molecule of hydrogen is
about one mile per second, being, in fact, by far the greatest molecular
velocity possessed by any gas.
[Illustration: PLATE IX.
PLATO.
(AFTER NASMYTH.)]
A stone thrown into the air soon regains the earth. A rifle bullet
fired vertically upwards will ascend higher and higher, until at length
its motion ceases, it begins to return, and falls to the ground. Let us
for the moment suppose that we had a rifle of infinite strength and
gunpowder of unlimited power. As we increase the charge we find that the
bullet will ascend higher and higher, and each time it will take a
longer period before it returns to the ground. The descent of the bullet
is due to the attraction of the earth. Gravitation must necessarily act
on the projectile throughout its career, and it gradually lessens the
velocity, overcomes the upward motion, and brings the bullet back. It
must be remembered that the efficiency of the attraction decreases when
the height is increased. Consequently when the body has a prodigiously
great initial velocity, in consequence of which it ascends to an
enormous height, its return is retarded by a twofold cause. In the first
place, the distance through which it has to be recalled is greatly
increased, and in the second place the efficiency of gravitation in
effecting its recall has decreased. The greater the velocity, the
feebler must be the capacity of gravitation for bringing back the body.
We can conceive the speed to be increased to that point at which the
gravitation, constantly declining as the body ascends, is never quite
able to neutralise the velocity, and hence we have the remarkable case
of a body projected away never to return.
It is possible to exhibit this reasoning in a numerical form, and to
show that a velocity of six or seven miles a second directed upwards
would suffice to convey a body entirely away from the gravitation of the
earth. This speed is far beyond the utmost limits of our artillery. It
is, indeed, at least a dozen times as swift as a cannon shot; and even
if we could produce it, the resistance of the air would present an
insuperable difficulty. Such reflections, however, do not affect the
conclusion that there is for each planet a certain specific velocity
appropriate to that body, and depending solely upon its size and mass,
with which we should have to discharge a projectile, in order to prevent
the attraction of that body from pulling the projectile back again.
It is a simple matter of calculation to determine this "critical
velocity" for any celestial body. The greater the body the greater in
general must be the initial speed which will enable the projectile to
forsake for ever the globe from which it has been discharged. As we
have already indicated, this speed is about seven miles per second on
the earth. It would be three on the planet Mercury, three and a half on
Mars, twenty-two on Saturn, and thirty-seven on Jupiter; while for a
missile to depart from the sun without prospect of return, it must leave
the brilliant surface at a speed not less than 391 miles per second.
Supposing that a quantity of free hydrogen was present in our
atmosphere, its molecules would move with an average velocity of about
one mile per second. It would occasionally happen by a combination of
circumstances that a molecule would attain a speed which exceeded seven
miles a second. If this happened on the confines of the atmosphere where
it escaped collision with other molecules, the latter object would fly
off into space, and would not be recaptured by the earth. By incessant
repetitions of this process, in the course of countless ages, all the
molecules of hydrogen gas would escape from the earth, and in this
manner we may explain the fact that there is no free hydrogen present in
the earth's atmosphere.[10]
The velocities which can be attained by the molecules of gases other
than hydrogen are far too small to permit of their escape from the
attraction of the earth. We therefore find oxygen, nitrogen, water
vapour, and carbon dioxide remaining as permanent components of our air.
On the other hand, the enormous mass of the sun makes the "critical
velocity" at the surface of that body to be so great (391 miles per
second) that not even the molecules of hydrogen can possibly emulate it.
Consequently, as we have seen, hydrogen is a most important component of
the sun's atmospheric envelope.
If we now apply this reasoning to the moon, the critical velocity is
found by calculation to be only a mile and a half per second. This seems
to be well within the maximum velocities attainable by the molecules of
oxygen, nitrogen, and other gases. It therefore follows that none of
these gases could remain permanently to form an atmosphere at the
surface of so small a body as the moon. This seems to be the reason why
there are no present traces of any distinct gaseous surroundings to our
satellite.
The absence of air and of water from the moon explains the sublime
ruggedness of the lunar scenery. We know that on the earth the action of
wind and of rain, of frost and of snow, is constantly tending to wear
down our mountains and reduce their asperities. No such agents are at
work on the moon. Volcanoes sculptured the surface into its present
condition, and, though they have ceased to operate for ages, the traces
of their handiwork seem nearly as fresh to-day as they were when the
mighty fires were extinguished.
"The cloud-capped towers, the gorgeous palaces, the solemn temples" have
but a brief career on earth. It is chiefly the incessant action of water
and of air that makes them vanish like the "baseless fabric of a
vision." On the moon these causes of disintegration and of decay are all
absent, though perhaps the changes of temperature in the transition from
lunar day to lunar night would be attended with expansions and
contractions that might compensate in some slight degree for the absence
of more potent agents of dissolution.
It seems probable that a building on the moon would remain for century
after century just as it was left by the builders. There need be no
glass in the windows, for there is no wind and no rain to keep out.
There need not be fireplaces in the rooms, for fuel cannot burn without
air. Dwellers in a lunar city would find that no dust could rise, no
odours be perceived, no sounds be heard.
Man is a creature adapted for life under circumstances which are very
narrowly limited. A few degrees of temperature more or less, a slight
variation in the composition of air, the precise suitability of food,
make all the difference between health and sickness, between life and
death. Looking beyond the moon, into the length and breadth of the
universe, we find countless celestial globes with every conceivable
variety of temperature and of constitution. Amid this vast number of
worlds with which space is tenanted, are there any inhabited by living
beings? To this great question science can make no response: we cannot
tell. Yet it is impossible to resist a conjecture. We find our earth
teeming with life in every part. We find life under the most varied
conditions that can be conceived. It is met with under the burning heat
of the tropics and in the everlasting frost at the poles. We find life
in caves where not a ray of light ever penetrates. Nor is it wanting in
the depths of the ocean, at the pressure of tons on the square inch.
Whatever may be the external circumstances, Nature generally provides
some form of life to which those circumstances are congenial.
It is not at all probable that among the million spheres of the universe
there is a single one exactly like our earth--like it in the possession
of air and of water, like it in size and in composition. It does not
seem probable that a man could live for one hour on any body in the
universe except the earth, or that an oak-tree could live in any other
sphere for a single season. Men can dwell on the earth, and oak-trees
can thrive therein, because the constitutions of the man and of the oak
are specially adapted to the particular circumstances of the earth.
Could we obtain a closer view of some of the celestial bodies, we should
probably find that they, too, teem with life, but with life specially
adapted to the environment--life in forms strange and weird; life far
stranger to us than Columbus found it to be in the New World when he
first landed there. Life, it may be, stranger than ever Dante described
or Dore sketched. Intelligence may also have a home among those spheres
no less than on the earth. There are globes greater and globes
less--atmospheres greater and atmospheres less. The truest philosophy on
this subject is crystallised in the language of Tennyson:--
"This truth within thy mind rehearse,
That in a boundless universe
Is boundless better, boundless worse.
"Think you this mould of hopes and fears
Could find no statelier than his peers
In yonder hundred million spheres?"
[Illustration: PLATE X.
TYCHO AND ITS SURROUNDINGS.
(AFTER NASMYTH.)]