The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

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.