THE VOLCANOES OF THE MOON
There can be no doubt that the moon was once the seat of very great
volcanic activity. It was formerly believed that the very many
volcanic craters which can be seen on its surface when it is examined
by a comparatively small telescope, were all extinct. While this
is nearly true, yet recent investigations have shown that in all
probability a feeble volcanic activity still exists in a few of these
craters.
The distinctness with which the surface of the moon is seen does not
depend so much on the size of the telescope employed, as it does
on the steadiness of the atmosphere when the telescope is being
used. When one wishes to examine a very distant body like a star,
it is necessary to use a powerful telescope, but in the case of a
comparatively near body, like one of the planets or the moon, a big
telescope is not necessary. It is, however, necessary to make the
observations at some time of the year, or in some part of the world,
when the air is apt to be free from winds.
A person on the earth's surface looking at the heavenly bodies through
a telescope is practically in the position in which he would be were
he at the bottom of the water in a large lake looking up through the
water at some body in the heavens. He would have no difficulty in
seeing such a body distinctly as long as the upper surface of the
water remained quiet, and unruffled by waves. As soon, however, as
waves were set up, the images seen in the telescope are so distorted
as to become practically worthless. It is for this reason that it is
customary to build great astronomical observatories in parts of the
world where there are apt to be many days in the year when the air is
almost entirely free from wind.
Since the atmosphere is apt to be disturbed by winds in both the
temperate and the polar latitudes, these parts of the world are
not very satisfactory as sites for astronomical observatories. The
conditions are more favorable near the equator, since, although at
certain seasons of the year there are very severe storms in these
regions, yet there are quite long periods when the air is almost
entirely free from winds.
It is for this reason that Harvard University has erected an
astronomical observatory at Arequipa, Peru, at an elevation of 8,000
feet above the level of the Pacific Ocean. Here, with a comparatively
small object glass, of about twelve inches aperture, magnificent
photographs have been obtained not only of the moon but also of the
planet Mars.
According to Professor Pickering, from whose magnificent work,
entitled, "The Moon," much of the information in this chapter has been
obtained, the moon, which is generally spoken of as a satellite of the
earth, ought rather to be called the earth's twin planet. Although the
moon appears to revolve in a small elliptical orbit around the earth
it should properly be said to revolve around the sun; for, together
with the earth, it revolves around the sun once every year. As seen
from any of the planets that lie near the earth the earth and moon
would appear as a very beautiful double star.
In order the more readily to understand what will be said shortly
concerning the origin of the moon, it may be mentioned that the
moon's diameter is 2,163 miles, or a little more than one-fourth the
diameter of our earth.
You will, most probably, be surprised to learn that the origin of the
moon is believed to be very different from the origin of the moons
or satellites of Jupiter, Saturn, and the other planets. As we have
already seen, according to the nebular hypothesis, all the planets
except the earth probably had their moons formed from the rings that
were left surrounding them when they shrunk on cooling to their
present dimensions. Such a ring is still to be seen surrounding Saturn.
Now it is believed that our moon was formed in a different manner.
It was not thrown off from the earth while the latter was in a
highly fluid or gaseous condition, but after the earth had shrunken
to nearly its present size and, most probably, after a solid crust
had been formed on its surface. In order that our earth should be
able to violently throw off a large portion of its mass, it is only
necessary that at the time this separation occurred, its motion of
rotation on its axis was sufficiently great to enable it to make one
complete revolution in rather less than three hours instead of in
the twenty-four hours it now requires. At this velocity of rotation,
objects would fly off the earth in the neighborhood of the equator,
under the influence of the high centrifugal force. Let us, then,
endeavor to see if it was at all probable that the earth ever did turn
so rapidly on its axis.
You all probably know that it is principally the attraction of the
moon that produces the earth's tides. Of course, the sun also produces
tides on the earth, but it is so far off from the earth that not
withstanding its greater mass the tides it forms are much smaller
than those produced by the moon. You also know that the moon produces
at the same time two tides in every twenty-four hours, on directly
opposite sides of the earth; one on the side immediately under the
moon, and the other on the side furthest from the moon. As the earth
rotates between these two tides, they act as a break which serves to
impede its motion. Every high tide, therefore, tends to make the earth
rotate more slowly, and thus to slowly increase the length of the day.
For this reason to-day is a trifle longer than yesterday, and still
longer than a day a hundred years ago.
You must not suppose for a moment that this increase in the length of
the day is large. On the contrary, it is so small that since the year
A. D. 1, up to the present time, the day is only a very small
fraction of a second longer.
But it was very different in the earth's geological past, when the
inside of the earth was in a molten condition; for then great tides
were set up in the melted interior of the earth that not only greatly
changed the shape of the earth, but decreased the rate of rotation
much more rapidly than it does when the earth's tides are limited as
they are now to the waters on the earth's surfaces.
There was, however, at the same time, something going on that tended
greatly to make the earth turn more rapidly on its axis. While the
originally melted earth was cooling and shrinking, the rate of
its rotation was necessarily increasing. As you know, the time of
vibration of a pendulum, that is, the time it requires to make one
complete to-and-fro motion, is shorter the shorter the length of the
pendulum. A pendulum two feet long moves to and fro more slowly than
a pendulum one foot in length. In the same way a rotating sphere will
make one complete rotation in a shorter time when its radius, which
corresponds to the length of a pendulum, is shorter. Therefore, as the
earth shrunk, it rotated more and more rapidly, and at last reached a
rapidity of motion at which an immense quantity of matter flew off
its surface nearest the equator and went out into space, never again
to return. It was this mass that constituted the earth's moon.
Necessarily such a tremendous catastrophe was attended by an
earthquake as well as by the most fearful volcanic phenomena that
the earth has ever witnessed. The terrible catastrophe produced by
the explosive eruption of Krakatoa was but as a small drop of rain
falling on the earth, when compared with the catastrophe produced when
the "five thousand million cubic miles of material left the earth's
surface, never again to return to it."
It is not known whether this matter was torn off the earth at a single
time or during successive times, but quoting the beautiful language of
Professor Pickering:
"We may try in vain to imagine the awful uproar and fearful
volcanic phenomena exhibited when a planet was cleft in
twain, and a new planet was born into the solar system."
This terrible catastrophe took place at a time not when the earth was
a gaseous mass, but when it had condensed into a comparatively small
mass not much larger than it is at its present time, and possibly
even after it had hardened sufficiently to form a solid crust on its
outside.
If you look at a map of the earth on a Mercator's projection, such,
for example, as that employed in illustrating the distribution of the
world's volcanoes in Fig. 24, you can see, even without any very close
examination, that the great water area of the Atlantic Ocean has its
eastern and western shores almost parallel to each other, so that
if you conceive the Eastern and Western Continents as being pushed
together, they would, except at the south, almost completely fit
together, and the same thing is true, if Greenland is pushed towards
the northeastern coast of North America. Of course, some portions of
the coast would not fit exactly, but then these portions might either
have been worn away, or, as is more probable, have been changed in
shape by the deposit of immense beds of sedimentary rocks spread over
the borders of the Atlantic by the great rivers that empty into it.
This is so remarkable a fact that it will be well worth your while to
turn to the map mentioned and convince yourself of the proof of what
I have just said. As you will see, Europe and Africa would almost
exactly fit against South America and North America, while Greenland
would even more closely fit against the northeastern coast of North
America.
Now, while we do not say that it was so, it has been suggested as just
possible that the great depression of the Pacific Ocean represents the
spot that was once filled by the moon. That the Eastern and Western
Continents, then torn asunder by the great force of the convulsion,
were left floating on the surface of a sea of molten matter, a greatly
widened crack marking positions they assumed at the end of this
cataclysm.
Of course, you must understand that all this is a mere supposition,
and that we do not know whether the earth was actually cooled on the
outside when this occurred, since it might have still been in a liquid
condition throughout. It would seem, however, to have occurred rather
recently, since it could not have occurred until the earth shrunk so
much that it became so small in radius as to acquire a very rapid rate
of motion on its axis.
It is an interesting fact that we are, perhaps, better acquainted with
that side of the moon which is turned towards us than we are with the
surface of the earth on which we live. Of course, I do not mean in the
small details of the moon's surface, but with such portions as can be
seen through a good telescope when the air is quiet. While there are
no parts of the moon's surface that have not been carefully examined
in detail probably thousands of times by acute astronomers, there are
still comparatively large areas of the earth that have never been once
trodden by civilized man.
When I speak of all parts of the moon's surface, I only mean those
parts that are turned towards us. You may possibly be ignorant of the
fact that the moon always turns exactly the same face towards the
earth. Not only has no man ever seen the opposite side of the moon,
but he never can hope to see it while he remains on the earth. This is
because the moon turns or rotates on its axis in exactly the same time
that it revolves in its orbit.
When I say that the time of rotation is the same as the time of
revolution of the moon, I do not mean that it is almost the same, but
that it is exactly the same. If it differed even but a small fraction
of a second, a time would come when we would be able to see the other
side of the moon. Now, since astronomers have made careful pictures
of the moon, many, many years ago, we can see by comparing them
with photographs taken at the present time there has been no change
whatever in that face of the moon which is turned towards us, and
this, of course, proves beyond question, that the time of the moon's
rotation during this great period has remained exactly the same as the
time of its revolution.
It may possibly seem to you that it cannot be a matter of great
importance in a book like this on the Wonders of Volcanoes and
Earthquakes, whether or not the moon always turns its face towards
the earth; on the contrary, it is a matter of the greatest importance
since by it we can prove positively that the moon was at one time at
least in a partly fluid condition. It was the presence of this partly
fluid interior that resulted in the time of the moon's rotation
agreeing exactly with the time of its revolution. The tides of the
earth set up in the moon's molten interior, tides, that instead of
reaching twice every day the height of a few feet only, were set up
in the molten mass in the moon's interior, probably reaching miles in
height, rapidly decreased the time of the moon's rotation until the
moon rotated once only during every complete revolution.
Even now that the moon is probably solid throughout, the time of its
rotation and revolution exactly agree because, while in a molten
condition, the action of the earth changed its shape from that of an
exact sphere to a spheroid, with its longest axis in the direction of
the earth. Even, therefore, if the moon at any time began to rotate
faster than the earth, the earth acting on its projecting surface
retarded it until the time of its rotation agreed exactly with the
time of its revolution.
It was at one time believed that the moon had no atmosphere. It is
now known, however, that it has an atmosphere. It is true this is a
rare atmosphere, probably not greater in density than the one-ten
thousandth of the earth's atmosphere. This important question was
settled once for all on August 12th, 1892, at the Harvard Observatory
at Arequipa, Peru, when a photograph was taken of an object on the
moon. It could be readily seen on examining this photograph that the
light coming from the moon experienced a bending, known as refraction,
in passing from the space outside the moon to its atmosphere on to its
surface.
Of course, when the moon was thrown off from the earth by reason of
its great centrifugal force, it carried along with it a portion of the
earth's atmosphere. But since the quantity of matter in the moon is
only about one-eightieth of that of the earth, the force of gravity
on the moon is much smaller than that on the earth, being almost
exactly one-sixth that of the earth's gravity. In other words, if you
could succeed in reaching the moon's surface, you would only weigh
one-sixth of what you weigh on the earth, but then you could carry a
weight six times heavier with no greater effort, and, as for running,
jumping, and other athletic exercises, the surface of the moon would,
indeed, be a great place on which to break records, since one could
readily jump six times higher, put the shot six times further, than
on the earth, or go through most other athletic exercises with a
corresponding increase.
Without going any further into this question it will be sufficient
to say that the moon's present atmosphere is believed to consist of
carbonic acid gas, and that while on the general surface of the moon
this atmosphere must be very rare, yet, at the bottom of the great
fissures that cross the moon's surface, it may possess a fairly great
density, especially if the moon still possesses feeble volcanic
activity; that carbonic acid gas is still being given off from the
inside of the moon as we know it is being given off from inside the
earth.
Under the best conditions of atmosphere and telescope, we can see the
moon's surface as it would appear at a distance varying from 800 miles
to 300 miles from the earth. With a fifteen-inch telescope, under
perfect conditions of vision, objects can be seen as if they were at
a distance of 800 miles from the earth, and with the most powerful
glasses, and the best conditions of atmosphere this distance can be
reduced to about 300 miles. This would enable us to clearly see large
objects like rivers, lakes, seas, or forests, if they existed, but
would not be sufficient to enable us to see houses, buildings, or
roads.
When we come to examine the surface of the moon under the most
favorable conditions, we find that it is extremely irregular. There
are plenty of high mountains. These mountains are not collected
in ranges as they are on the earth's surface, but are completely
separated from each other, and are scattered in great numbers over the
moon's surface.
You may form some idea of the number of volcanoes that have been
observed on the moon when I tell you that as many as 32,000 have been
seen on that side of the moon that is turned towards the earth.
Now it is an interesting fact that almost all these mountains possess
great craters that are not unlike some of the volcanic craters we see
on the earth. The volcanic craters of the moon, however, are of very
much greater size than those on the earth, many being from fifty to
sixty miles in diameter, while some of them are more than 100 miles
in diameter. Smaller craters, say from twenty to twenty-five miles in
diameter, can be counted by the hundreds.
Like most of the moon's craters, the largest crater more closely
resembles one of the pit-craters or calderas on the island of Hawaii.
This volcanic crater consists of a huge circular ring with a small
irregular peak that rises inside the ring. This peak, by the way,
might at first appear to resemble the crater of Vesuvius, which after
a long period of inactivity of the mountain during the eruption that
destroyed Pompeii and Herculaneum was thrown up inside of what had
been left standing of the old crater of Somma. But it has no crater
at its summit, and, therefore, resembles rather the irregular pile or
rock that rises from the surface of a lava lake in the craters of Mt.
Loa or Mt. Kilauea in Hawaii.
Besides the numerous craters to be seen on the moon's surface there
are many lines of deep, crooked valleys, known as _rills_, that may
at one time have been the beds of rivers. Besides the rills, there are
many straight clefts about half a mile in width, that extend down into
the surface of the moon for unknown depths. These clefts can be seen
passing directly through mountains and valleys. They are believed to
be cracks or fissures in the moon's surface.
On the moon is a great crater called Tycho. It is situated near the
moon's south pole. The great crater of Tycho is by far the most
prominent object on the moon's surface. It has a system of rays that
extend for great distances around its craters.
You will also see if you examine the moon's surface by a powerful
glass that there are immense plains called _oceans_ or _seas_. By an
appropriate custom the names of the different craters on the moon are
the same as the names of the great astronomers and philosophers that
have long since passed from their labors, such as Tycho, Copernicus,
Kepler, Plato, etc.
Various explanations have been given as to the origin of the craters
on the moon's surface, but without going into a discussion it may be
said that they are now generally regarded as having been formed in the
main just as were the craters of the earth's volcanoes.
The tremendous size of the moon's craters is of course due to the
great decrease in the force of gravity. This would make the craters,
approximately, six times as great as the craters on the earth.
Professor Pickering points out that while the moon's craters resemble
more closely those of Hawaii than those of any other of the earth's
volcanoes, yet there is this difference in them: that while the
earth's crater floors are generally considerably higher than the level
of the sea, the moon's crater floors are generally below the level of
the surrounding country. Still, taking them all in all, the craters
of the moon's volcanoes resemble those of the island of Hawaii, or
again quoting from Pickering:--
"There seems, indeed, to be no feature found upon the moon
which is not presented by these Hawaiian volcanoes, there
is no feature of the volcanoes that does not also have its
counterpart in the moon."