YET MORE ABOUT THE MOON.
There are two ways of thinking about the moon. One way is to consider
her as merely the earth’s attendant satellite. The other way is to
consider her as our sister-planet, traveling with us round the central
sun.
The first is the more common view; but the second is just as true as
the first.
For the sun does actually pull the moon towards himself, with a very
much stronger pulling than that of the earth. The attraction of the sun
for the moon is more than double the attraction of the earth for the
moon. If it were not that he pulls the earth quite as hard as he pulls
the moon, he would soon overpower the earth’s attraction, and drag the
moon away from us altogether.
People are often puzzled about the orbit or pathway of the moon through
the heavens. For in one sense they have to think of her as traveling
round and round in a fixed orbit, with the earth in the center. In
another sense they have to think of her as always journeying onwards
with the earth in her journey round the sun, and thus never returning
to the same point.
There are two ways of meeting this difficulty. First of all, remember
that the one movement does not interfere with the other. Just as in the
case of the earth traveling round the sun, and also traveling onward
with him through space; just as in the case of a boy walking round and
round a mast, and also being borne onwards by the moving vessel,--so it
is here. The two movements are quite separate and independent of each
other. As regards the earth alone, the moon journeys round and round
perpetually, not in a circle, but in a pathway which comes near being
an ellipse. As regards the actual _line_ which the moon’s movements may
be supposed to draw in space, it has nothing elliptical about it, since
no one point of it is ever reached a second time by the moon.
But according to this last view of the question, nobody ever can or
will walk in a circle or an oval. Take a walk round your grass-plot,
measuring your distance carefully at all points from the center. Is
that a circle? All the while you moved, the surface of the earth was
rushing along and bearing you with it, and the whole earth was hurrying
round the sun, and was being also carried by him in a third direction.
Whatever point in space you occupied when you started, you can _never
fill that particular part of space again_. The two ends of your
so-called circle can never be joined.
But then you may come back to the same point _on the grass_, as
that from which you started. And this is all that really signifies.
Practically you have walked in a circle. Though not a circle as regards
space generally, it is a circle as regards the earth. So also the moon
comes back to the same point _in her orbit round the earth_. Letting
alone the question of space, and considering only the earth, the moon
has--roughly speaking--journeyed in an ellipse. You may, however, look
at this matter in quite another light. Forget about the moon being the
earth’s satellite, and think of earth and moon as two sister-planets
going round the sun in company.
The earth, it is true, attracts the moon. So, also, the moon attracts
the earth; though the far greater weight of the earth makes her
attraction to be far greater. If earth and moon were of the same size,
they would pull each other with equal force.
But though the pull of the earth upon the moon is strong, the pull of
the sun upon the moon is more than twice as strong. And greatly as the
earth influences the moon, yet the actual center of the moon’s orbit is
the sun, and not the earth. Just as the earth travels round the sun, so
also the moon travels round the sun.
The earth travels steadily in her path, being only a little swayed and
disturbed by the attraction of the moon. The moon on the contrary,
while traveling in her orbit, is very much swayed and disturbed indeed
by the earth’s attraction. In fact, instead of being able to journey
straight onwards like the earth, her orbit is made up of a succession
of delicate curves or scallops, passing alternately backwards and
forwards over the orbit of the earth. Now she is behind the earth; now
in front of the earth; now between earth and sun; now outside the earth
away from the sun. The order of positions is not as here given, but
each is occupied by her in turn. Sometimes she moves quickly, sometimes
she moves slowly, just according to whether the earth is pulling her
on or holding her back. Two hundred and forty thousand miles sounds
a good deal. That is the distance between earth and moon. But it is,
after all, a mere nothing, compared with the ninety-three millions of
miles which separate the sun from the earth and moon.
If we made a small model, with the sun in the center, and the earth and
moon traveling a few inches off, only one slender piece of wire would
be needed to represent the path of earth and moon together. For not
only would the earth and the moon be so small as to be quite invisible,
but the whole of the moon’s orbit would have disappeared into the
thickness of the single wire. This question of the moon’s motions is
in its nature intricate, and in its details quite beyond the grasp of
any beginner in astronomy. But so much at least may be understood,
that though the earth’s attraction powerfully affects the moon, and
causes in her motions _perturbations_, such as have been already spoken
about as taking place among the planets, yet that in reality the great
controlling power over the moon is the attraction of the sun.
The tides of the ocean are chiefly brought about by the moon’s
attraction. The sun has something to do with the matter, but the moon
is the chief agent. This action of the moon can best be seen in the
southern hemisphere, where there is less land. As the moon travels
slowly round the earth, her attraction draws up the yielding waters
of the ocean in a vast wave which travels slowly along with her. The
same pulling which thus lifts a wave on the side of the earth towards
the moon, also pulls the earth gently _away from_ the water on the
opposite side, and causes a second wave there. The parts of the ocean
between these two huge waves are depressed, or lower in level. These
two waves on the opposite sides of the earth sweep steadily onwards,
following the moon’s movements,--not real, but seeming movements,
caused by the turning of the earth upon her axis.
Once in every twenty-four hours these wide waves sweep round the whole
earth in the southern ocean. They can not do the same in the north, on
account of the large continents, but offshoots from the south waves
travel northwards, bringing high-tide into every sea and ocean inlet.
If there were only one wave, there would be only one tide in each
twenty-four hours. As there are two waves, there are two tides, one
twelve hours after the other. In the space between these two high-tides
we have low-tide.
Twice every month we have very high and very low tides. Twice every
month we have tides not so high or so low. The highest are called
“spring-tides,” and the lowest “neap-tides.” When the moon is between
us and the sun, or when she is “new moon,” there are spring-tides;
for the pull or attraction of sun and moon upon the ocean act exactly
together. It is the same at full moon, when once more the moon is in a
straight line with earth and sun. But at the first and last quarters,
when the moon has her _sideways_ position, and when the sun pulls in
one direction and the moon pulls in another, each undoes a little of
the other’s work. Then we only have neap-tides; for the wave raised is
smaller, and the water does not flow so high upon our shores.
In speaking of the surface of the moon, we are able only to speak
about one side. The other is entirely hidden from us. This is caused
by the curious fact that the moon turns on her axis and travels round
the earth in exactly the same length of time. One-half of the moon is
thus always turned towards us, though of that half we can only see so
much as is receiving the light of the sun. But the half turned in our
direction is always the same half. One part of the moon--not quite so
much as half, though always the same portion--is turned away from us. A
small border on each side of that part becomes now and then visible to
us, owing to certain movements of the earth and the moon.
What sort of a landscape may lie in the unknown district, it is idle to
imagine. Many guesses have been made. Some have supposed it possible
that air _might_ be found there; that water _might_ exist there; that
something like earthly animals _might_ live there. It is difficult to
say what may not be, in a place about which we know nothing whatever.
But judging from our earthly experience, nothing seems more unlikely
than that air, water, clouds, should be entirely banished from over
one-half of a globe, and collected together in the space remaining.
We are on safer ground when we speak about that part of the moon
which is turned towards us. For we can say with confidence that if
any atmosphere exist there, it must be in thickness less than the
two-thousandth part of our earthly atmosphere. It seems equally clear
that water also must be entirely wanting. The tremendous heat of the
long lunar day would raise clouds of vapor, which could not fail to
be visible. But no such mistiness ever disturbs the sharply-defined
outline of the moon, and no signs of water action are seen in the
craggy mountains and deep craters.
[Illustration: THE LUNAR CRATER COPERNICUS.]
The craters which honeycomb the surface of the moon are various in
size. Many of the larger ones are from fifty to a hundred miles in
diameter. These huge craters--or, as we may call them, deep circular
plains--are surrounded by mighty mountain ramparts, rising to the
height of thousands of feet. Usually they have in their center a
sugar-loaf or cone-shape mountain, or even two or more such mountains,
somewhat lower in height than the surrounding range. The sunset-lights
upon certain of these distant mountain-peaks were first watched by
Galileo through his telescope, and have since been seen by many an
observer--intense brightness contrasting with intense blackness of
shadow.
In addition to her great craters, the moon seems to be thickly covered
with little ones, many of them being as small as can be seen at all
through a telescope. Whether these are all volcano-craters remains to
be discovered. It is not supposed that any of them are now active. From
time to time, signs of faint changes on the moon’s surface have been
noticed, which it was thought might be owing to volcanic outbursts.
Such an outburst as the worst eruptions of Mount Vesuvius would be
invisible at this distance. But the said changes may be quite as well
accounted for by the startling fortnightly variations of climate which
the moon has to endure. The general belief now inclines to the idea
that the moon-volcanoes are extinct, though no doubt there was in the
past great volcanic activity there.
A description has been given earlier of the rain of meteorites
constantly falling to our earth, and only prevented by the atmosphere
from becoming serious. But the moon has no such protecting atmosphere,
and the amount of cannonading which she has to endure must be by no
means small. Perhaps in past times, when her slowly-cooling crust
was yet soft, these celestial missiles showering upon her may have
occasionally made deep round holes in her surface.
This is another guess, which time may prove to be true. Guesses at
possible explanations of mysteries do no harm, so long as we do not
accept them for truth without ample reason.
[Illustration: LUNAR ERUPTION--BRISK ACTION.]
The origin of the lunar craters must be referred to some ancient epoch
in the moon’s history. How ancient that epoch is we have no means of
knowing; but in all probability the antiquity of the lunar craters is
enormously great. At the time when the moon was sufficiently heated
to have these vast volcanic eruptions, of which the mighty craters
are the survivals, the earth must have been very much hotter than it
is at present. It is not, indeed, at all unreasonable to believe that
when the moon was hot enough for its volcanoes to be active, the earth
was so hot that life was impossible on its surface. This supposition
would point to an antiquity for the moon’s craters far too great to
be estimated by the centuries and the thousands of years which are
adequate for the lapse of time as recognized by the history of human
events. 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.
[Illustration: LUNAR ERUPTION--FEEBLE ACTION.]
It will now be possible for us to attempt to account for the formation
of the lunar craters. The most probable views on the subject are
certainly those adopted by Mr. Nasmyth, as represented in the cuts,
though it must be admitted that they are by no means free from
difficulty. We can explain the way in which the rampart around the
lunar crater is formed, and the great mountain which so often adorns
the center of the plain. The first of these cuts contains an imaginary
sketch of a volcanic vent on the moon in the days when the craters were
active. The eruption is here in the full flush of its energy, when the
internal forces are hurling forth a fountain of ashes or stones, which
fall at a considerable distance from the vent; and these accumulations
constitute the rampart surrounding the crater. The second cut depicts
the crater in a later stage of its history. The prodigious explosive
power has now been exhausted, and perhaps has been intermitted for some
time. A feeble jet issues from the vent, and deposits the materials
close around the orifice, and thus gradually raises a mountain in the
center.
Besides the craters and their surrounding barriers, there are ranges of
mountains on the moon, and flat plains which were once named “seas,”
before it was found that water did not exist there. Astronomers also
see bright ridges, or lines, or cracks of light, hard to explain.
One of the chief craters is called “Ptolemy,” and in size it is roughly
calculated to be no less than one hundred and fourteen miles across.
Another, “Copernicus,” is about fifty-six miles; and another, “Tycho,”
about fifty-four miles. The central cone-mountain of Tycho is five
thousand feet high. The crater of “Schickard” is supposed to be as much
as one hundred and thirty-three miles in diameter.
Astronomers have agreed to name these craters after the great
discoverers who enlarged our knowledge of the solar and sidereal
systems. It is fitting that these great names are suggested every time
the moon is seen through a telescope. To Ptolemy we are indebted for
what is known as “The Ptolemaic System of the Universe,” which makes
the earth the center around which the sun, moon, planets, and stars all
revolve, and explains the apparently erratic movements of the planets
by supposing their orbits to be epicycles; that is, curves returning
upon themselves and forming loops. Tycho Brahe was willing to allow,
with Copernicus, that the planets all revolved around the sun; but he
taught that both sun and planets turned around the earth.
The system known as “The Copernican” is now known to be the only true
one, and it is universally accepted. It is so called after Nicolaus
Copernicus, who was born in Thorn, Prussia, February 19, 1473. He was
educated for the Church, and studied medicine, but devoted himself
especially to astronomy.
Without being precisely a great genius, this quiet and thoughtful monk
seems to have been wise far beyond the age in which he lived, and
remarkable for his independence of mind. He had a “profound sagacity,”
and a wide general grasp of scientific subjects.
The extremely complicated and cumbrous nature of the Ptolemaic
system appeared to his judgment hardly compatible with the harmony
and simplicity elsewhere characteristic of nature. Moreover, he was
impressed and perplexed by the very marked changes in the brilliancy
of the planets at different seasons. These changes _now_ are no
difficulty at all. Venus, Mars, Jupiter, when on the same side of the
sun as ourselves, are comparatively near to us, and naturally look much
more bright in consequence of that nearness than when they are on
the opposite side of the sun from ourselves. But under the Ptolemaic
system, each planet was supposed to revolve round our earth, and to
be always at about the same distance from us; therefore, why such
variations in their brilliancy?
[Illustration: NICOLAUS COPERNICUS.]
During thirty-six long years he patiently worked out this theory, and
during part of those thirty-six years he wrote the one great book of
his lifetime, explaining the newer view of the Solar System which
had taken hold of his reason and imagination. This book, named _De
Revolutionibus Orbium Cœlestium_, or, “Concerning the Revolutions of
the Celestial Spheres,” which came out only a few hours before his
death, was dedicated to the Bishop of Rome--a little touch of worldly
wisdom which doubtless staved off for a while the opposition of the
Vatican.
Copernicus was not the first who thought of interpreting the
celestial motions by the theory of the earth’s motion. That immortal
astronomer has taken care to give, with rare sincerity, the passages
in the ancient writers from which he derived the first idea of the
probability of this motion--especially Cicero, who attributed this
opinion to Nicetas of Syracuse; Plutarch, who puts forward the names
of Philolaus, Heraclides of Pontus, and Ecphantus the Pythagorean;
Martianus Capella, who adopted, with the Egyptians, the motion of
Mercury and Venus around the sun, etc. Even a hundred years before the
publication of the work of Copernicus, Cardinal Nicolas of Cusa, in
1444, in his great theological and scientific encyclopedia, had also
spoken in favor of the idea of the earth’s motion and the plurality
of worlds. From ancient times to the age of Copernicus, the system of
the earth’s immobility had been doubted by clear-sighted minds, and
that of the earth’s motion was proposed under different forms. But all
these attempts still leave to Copernicus the glory of establishing it
definitely.
Not content with merely admitting the idea of the earth’s motion as a
simple arbitrary hypothesis, which several astronomers had done before
him, he wished--and this is his glory--to demonstrate it to himself
by acquiring the conviction by study, and wrote his book to prove it.
The true prophet of a creed, the apostle of a doctrine, the author of
a theory, is the man who, by his works, demonstrates the theory, makes
the creed believed in, and spreads the doctrine. He is not the creator.
“There is nothing new under the sun,” says an ancient proverb. We may
rather say, Nothing which succeeds is entirely new. The newborn is
unformed and incapable. The greatest things are born from a state of
germ, so to say, and increase unperceived. Ideas fertilize each other.
The sciences help each other; progress marches. Men often feel a truth,
sympathize with an opinion, touch a discovery, without knowing it. The
day arrives when a synthetical mind feels in some way an idea, almost
ripe, becoming incarnate in his brain. He becomes enamored of it, he
fondles it, he contemplates it. It grows as he regards it. He sees,
grouping round it, a multitude of elements which help to support it.
To him the idea becomes a doctrine. Then, like the apostles of good
tidings, he becomes an evangelist, announces the truth, proves it by
his works, and all recognize in him the author of the new contemplation
of nature, although all know perfectly well that he has not invented
the idea, and that many others before him have foreseen its grandeur.
Such is the position of Copernicus in the history of astronomy. The
hypothesis of the earth’s motion had been suggested long before his
birth on this planet. This theory counted partisans in his time.
But he--he did his work. He examined it with the patience of an
astronomer, the rigor of a mathematician, the sincerity of a sage, and
the mind of a philosopher. He demonstrated it in his works. Then he
died without seeing it understood, and it was not till a century after
his death that astronomy adopted it, and popularized it by teaching
it. However, Copernicus is really the author of the true system of the
world, and his name will remain respected to the end of time.
The so-called “seas” on the moon are those large dark spots to be seen
on its surface, in the shape of “eyes, nose, and mouth,” or of the
famous old man with his bundle of sticks. The brighter parts are the
more mountainous parts.
The chief ranges of lunar mountains have been named by astronomers
after mountains on earth, such as the Apennines, the Alps, the
Caucasian range, the Carpathian and the Altai Mountains.