CHAPTER XXII

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."