Part 5

Our satellite always keeps the same face turned towards the Earth, so that we only see one side of it; the other side is forever hidden from the sight of man. However, the axis of the Moon tilts, in relation to the earth, and permits us to glimpse a little more of the surface both north and south, so that about five-eighths of the surface has actually been observed.

The surface of the Moon has been subjected to intensive study, and its “geography” is now as well known as that of our own earth. Vast “seas” (i. e.) sea bottoms, mountain ranges, solitary mountain peaks, enormous craters, are readily observed, and modern telescopes have now brought the moon so close to us that it has been said that any body as high as the Woolworth Building, in New York, would cast a shadow which could be observed and noted.

The fact that the same face of the Moon is always seen by man does not mean that this body remains stationary; it revolves on its own axis, from west to east, but this revolution occupies exactly one siderial month. The result is that the days and nights on the Moon, are many times the length of our days and nights. The surface exposed to the sun’s rays must get extremely hot, and, when deprived of these rays, extremely cold. It has been estimated that the mean temperature of the moon’s surface must approximate 200° F., during the “day” time, and approach the intense cold of inter-stellar space during the “night” (perhaps -250° C.). This would render life or vegetation of any kind very unlikely. However, Professor Pickering has lately asserted that vegetation does apparently spring into being with extreme rapidity during the moon’s day time--evidently remaining latent during the intense cold of the “night.” (Some interesting material on this topic may be found in Shipley’s “Is the Moon a Dead World?” No. 557 of the present series.)

The volume of the Moon is about one-fiftieth that of the earth, but its mass is only about one-eightieth that of our planet. The Moon is practically devoid of atmosphere, which is another reason why it cannot support “life,” in our sense of the word. Two theories have been advanced as to the absence of the moon’s atmosphere: (1) that it gradually combined, chemically, with the materials on its surface; and (2) that it gradually escaped into space, because of the low gravitational pull of the moon. There is no water on our satellite, which means that there is no ice and no snow. The moon being so much smaller than our earth, the pull of gravity is of course much less also.

Yet it is well known that the tides, on the earth, are greatly influenced by the moon. Every atom composing our satellite must exert some subtle pull upon every atom of our oceans, in order thus to affect them. What is the nature of this attraction? Here we encounter the mystery of gravitation! This question must accordingly be postponed until we come to our discussion of that subject.

One of the most remarkable and distinguishing characteristics of the moon consists in the so-called “lunar craters,” which appear to be immense, extinct volcanoes. More than 30,000 of these have now been mapped, varying in size from small hills to immense basins 50, 60, 100 miles in diameter. Ptolemy is 115 miles across, while Theophilus is 64 miles in diameter and 19,000 feet deep. The curious thing about these lunar craters is that they are unlike the hilly volcanoes known to us on our earth. They are rather huge circular pits, often square miles in extent, surrounded by a circular wall, and almost invariably having a single mountainous cone in the center.

Various theories have been advanced by way of explanation of these craters. The most important of these are (1) that they represent extinct volcanoes; (2) that they indicate spots where masses of matter have dashed into the moon, from surrounding space; and (3) that they represent the surface of the moon, when it was a hot, seething mass--their resemblance to the “bubbles” formed at the surface of boiling glue, mud, etc., being pointed to as analogous. Unanimity of view does not exist even yet as to their origin.

The ever-changing “phases” of the moon have been observed by generations of lovers. Thus, the new moon, full moon, etc., are commonplace sights. These apparent changes are, of course, due entirely to the relative position of the sun at the time. If the sun illumines the whole face of the moon, as viewed from our earth, we have full moon; if only a small portion of it, we see the first quarter, etc. The whole disk of the moon may always be seen, however, by careful observation. It is hardly necessary to say that the so-called “Man in the Moon” is a mind’s eye picture, created by the configuration of the various mountains, seas, etc., upon its surface.

THE ORIGIN OF THE SOLAR SYSTEM

Men in every age have speculated as to the constitution and origin of our world, and of the Universe in general. The first really detailed and scientific attempt was made, however, little more than a hundred years ago by Laplace--and subsequently known as the Laplacian hypothesis (1796).

Concurrent with the establishment of new facts, there was a tendency, throughout the past century, to find some philosophic interpretation of the Universe and its structure; to ascertain, if possible, the “beginnings of things,” and explain them in some satisfactory manner. This has been considered as epoch-making in astronomical research as Darwin’s great theory of the Origin of Species was in biology. The history of the two theories has been similar also. Both have served a useful purpose; have helped to direct scientific thought for years; and both are now largely outgrown. Both were, however, of great value and of daring originality.

Laplace assumed the primal existence of a glowing ball of gas rapidly revolving about an imaginary axis running through its center of gravity. During the process of cooling, this mass would contract, and a disk of gas would be thrown off in this manner; and hence a number of gaseous rings be formed, which would ultimately cool down and assume a spherical form. Laplace conceived that this process might be interfered with by internal accident and by comets from without.

The first modifications of the theory were suggested by Sir Norman Lockyer, who proposed what is known as the meteoritic hypothesis in its place. The central idea of the theory was that--“All self-luminous bodies in celestial space are composed either of swarms of meteorites or of masses of meteoric vapor produced by heat.” The theory was based on spectroscopic analysis. It said that the original nebulæ were composed, not of gases, but of meteoric material and cosmic dust. This theory was never fully accepted in place of that of Laplace, however; but it paved the way for a more recent theory, which may be said to be satisfactory and more or less inclusive. This is known as the planetesimal hypothesis, and was advanced within the past few years by F. R. Moulton and T. C. Chamberlin, of the University of Chicago. At the present time, it may be said to be the accepted theory, so far as any such theories are accepted, since it accords with all the facts in a remarkable manner, and has been experimentally demonstrated. In outline, the theory is as follows.

If examination of the nebulæ in the sky be made, out of 120,000 of them, nearly every one of them is found to be in the spiral form. So common and universal is this, indeed, that it was concluded that this must represent “some prevalent process in celestial dynamics.” This process is, according to Chamberlin, the actual formation of a solar system. As this spiral revolves, it accretes to itself various smaller bodies, with their gases, atmospheres, etc., and these become consolidated with the original body. As time went on, this spiral gradually tended to decrease its speed, but at the same time, continued to accrete bodies which came into contact with it in its flight through space. Thus, we have to imagine our world, not as an expanded molten mass which has continuously cooled and contracted, but, on the contrary, as a small lump of cold and solid fragments that, moving about in accordance with its attractions, continuously fed upon its surrounding assemblage of “smaller fry,” and thus grew to its present size. About the young earth so engaged it is possible to read, on the basis of the hypothesis, something of its early history.

Thus we see that the old theory of Laplace has been reversed; and that, instead of a great central mass of moving, white-hot gas, we have a number of smaller bodies, all busily engaged in building up themselves, at the expense of the surrounding masses of still smaller matter--much as a crystal accretes to itself minute specks of crystalline matter from the solution in which it is immersed. This is the newest of the cosmological theories. According to it, all the planets might have been formed at the same time. This view of the formation of the universe opens up still wider problems, which are now the subject of keen debate.

CONSTELLATIONS

The ancients, when studying the heavens, saw all kinds of imaginary animals in the various star-groups, and named them accordingly. A constellation is really a group of stars, which seems to constitute a sort of system of its own. Thus, we find reference to the Great Bear, the Little Bear, the Bull, etc. It is difficult for the uninitiated to see the resemblances which the ancients did, in these various star-groups, and astronomical science has re-named them, as well as adding a large number of new constellations to those already known.

Stars of the first six magnitudes (roughly) are visible to the unaided eye; those of lesser magnitude must be detected by the aid of telescopes. About 5,000 are thus visible; the number is increased according to the magnifying power of the telescope used, and it is estimated that there are more than 100,000,000 within the range of visual and photographic instruments!

The names of a few of the best known constellations are as follows: Ursa Major (The Great Bear); Cassiopeia; Hercules; Scorpio (the Scorpion); Corona Borealis (The Northern Crown); Boötes (The Hunter); Leo (the Lion); Andromeda; Perseus; Auriga (The Charioteer); Taurus (the Bull); Orion; Canis Major (The Great Dog); Canis Minor (The Smaller Dog); Gemini (the Twins), etc.

In these various constellations, certain noted stars are to be found. Thus, in Gemini, its two principal stars are Castor and Pollux. In Canis Major is Sirius. In Orion may be found Aldebaran and Betelgeuse. The Pleiades and Hyades groups are in Taurus. In Perseus is Algol. In Lyra is the first-magnitude star Vega. And so on.

The “Big Dipper,” so-called, is part of the Constellation Ursa Major; and it is almost universally known that the Pole Star (Polaris) may readily be found by its means. The constellations must be traced and learned, one by one; but this the student must accomplish for himself!

METEORS: “SHOOTING STARS”

What are popularly known as “shooting stars” are not stars at all; they are really meteors which appear at altitudes of from 60 to 100 miles, as a rule, from the earth, and move over paths of 40 or 50 miles at a rate of from 10 to 50 miles per second.

The light given out by meteors is due to their being heated by friction with the atmosphere. Falling from space, they become attracted by the earth’s gravitation, and fall towards it. Here they encounter the earth’s atmosphere, and their rapid passage through it creates terrific heat, which tends to consume them before they reach the face of the earth, turning them into gases, or causing them to fall gently as dust. This sudden flash is the “shooting star” in question.

The number of such meteors is very great. It has been computed that between ten and twenty million strike the earth’s atmosphere daily. Occasionally, a large number of meteors fall together; and then we have a “meteoric shower.”

METEORITES

Occasionally, however, some of these bodies do reach our earth, despite the friction and opposition of the earth’s atmosphere. Such bodies are called meteorites, siderites, or aerolites. Only a few of these are seen to strike the earth yearly, and it is a remarkable fact that, so far as we have any record, not one of them has ever struck a town or killed an individual. The outside of the meteorite during its passage through the air is subject to intense and sudden heating, and the rapid expansion of its surface-layers often breaks it into many fragments. The surface is fused and, on striking, cools rapidly. The result is that it has a black, glossy structure, usually with many small pits where the less refractive material has been melted out. Such meteorites may be seen in most large museums.

COMETS