Part 2

Between Mars and Jupiter, a number of small bodies are known to exist, which have sometimes been dignified by the name of “minor planets.” They have also been called “Planetoids” and “Asteroids.” Special names have been given these individual small bodies--Eros, Ceres, Pallas, Vesta, Juno, etc. Eros passes, upon occasions, very close to our earth; again passing beyond the orbit of Mars. Several hundreds of these smaller planets are now known to exist--as though they were the remnants of some shattered world. These bodies are very small: Ceres, _e.g._, being about 250 miles in diameter; Pallas, 304 miles; Vesta, 211 miles; while a number of the others are thought to be from 5 to 15 miles in diameter.

ARE THERE OTHER PLANETS?

Inasmuch as some of the planets known to us have only been discovered so lately (relatively) the question has naturally been asked: “Why may there not be _other_ planets, beyond Neptune, still undiscovered?” It is a perfectly legitimate question, and no definite answer to this query can be given. It seems rather improbable that another planet will be discovered. However, it is a conceivable possibility, and M. Flammarion has stated that, in his estimation, such a planet probably exists--gravitating at a distance 48 times as great as the distance between the earth and the sun--that is to say, 7,500 million miles, in an immense orbit which it takes at least 330 years to accomplish. Proof as to the existence of such a planet has not, however, as yet been forthcoming.

THE MOON

The Moon is the Earth’s only satellite, and by far the nearest body in space to our Earth. Many astronomers are inclined to think that the Moon at one time formed a part of the Earth, but was wrenched away from it, leaving a huge cavity, which is now occupied by the Pacific Ocean. The Moon is a cold body, emitting no light or heat of its own; all the light which it seems to shed is entirely _reflected_ light--reflecting the sun’s rays, much as a mirror might reflect them; hence its beautiful silver color.

In round numbers, the Moon is approximately 240,000 miles distant from us in space--the distance varying from 221,600 miles to 252,970 miles--causing a corresponding variation in its apparent diameter and parallax.[A] The circumference of the Moon’s orbit is a little more than a million-and-a-half miles, and it travels through space with the velocity of 2,288.6 miles per hour, or 3,357 feet per second.

[Footnote A: See page 58 for a definition of this term.]

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

During the past century, many of these “tramps of the solar system” have been discovered and their orbits computed. The “head” may range from ten thousand to a million miles, or more, while its “tail” may stream across the heavens for millions of miles. These comets’ tails always point away from the sun; and for long the reason for this was not known. It is now believed that this is due to light-pressure; the energy of the sun’s rays press this delicate matter outwards into space. (This theory has been elaborated at considerable length by the Swedish astronomer and chemist, Arrhenius.)

Many readers of this little book will remember the excitement caused by Halley’s comet, which came relatively close to the earth in 1910, so that many persons thought there would be a collision, and were terrified accordingly! As a matter of fact, the tails of comets are usually of almost inconceivable tenuosity. Halley first observed this comet, computed its orbit and predicted the date of its return.

Some comets have tails: others do not. Not much is known concerning the origin and destination of comets; where they originated, or how. They travel at tremendous speed over many millions of miles of space, returning after a few years, or after a lapse of several centuries. They are very striking looking, even when observed by the naked eye. A number of comets have been noted. The following are a few of the more remarkable comets which were observed during the past century:

The Comet of 1811. This was visible for nearly a year and a half, and was carefully studied by William Herschel. Its tail was said to be nearly a hundred million miles long, and fifteen million miles broad.

Encke’s Comet (1819). This comet is of extreme interest because of its change of volume. Moulton says: “On October 28, 1828, it was 135,000,000 miles from the sun, and had a diameter of 312,000 miles. On December 24, its distance was 50,000,000 miles and its diameter was 14,000 miles; while at its perihelion passage of December 17, 1838, at a distance of 32,000,000 miles, its diameter was only 3,000 miles.”

Beila’s Comet (1826). This comet has a most interesting history. In 1846 it was again seen; and a month later it had divided into two parts. They traveled along parallel orbits, some 160,000 miles apart. In 1852, they were seen to be 1,500,000 miles apart. Since then they have never been seen. They have, apparently, vanished from the face of creation!

Donati’s Comet (1858). This comet was visible for more than nine months. Its tail was estimated as 54,000,000 miles long. Its period of revolution was more than 2,000 years.

The Great Comets of 1880 and 1882. The latter of these passed through some hundreds of thousands of miles of the sun’s corona. Its orbit was not appreciably changed, but, after emerging, it was seen to possess at least five nuclei--showing the effect upon the comet of the disruptive forces through which it had passed.

NEBULAE

These are of especial interest, for the reason that they have played so large a part in forming cosmic theories--the Laplacian, the Planetestimal, etc. Nebulæ are of various kinds--“Annular Nebulæ,” resembling a flat, oval, solid ring, having a dark hole in the center. Then there are “Elliptic Nebulæ,” of varying degrees of eccentricity; the Great Nebula in Andromeda being a good example. (Numbers of isolated stars may be found within its limits.) “Spiral Nebulæ” are, perhaps, the best known of all, and their name accurately describes their appearance. There are also the so-called “Planetary Nebulæ,” as well as Nebulous Stars, Irregular Nebulæ, etc. Of late years, much interest has been centered upon the so-called “Dark Nebulæ.” Herschel had long before described various “holes in the heavens,” wherein no stars could be discerned. It is now believed that such spots do not represent “holes,” as much as dark masses of matter, which seem to blot out the bright stars behind them. The interested reader may refer to Hale’s “The Depths of the Universe” for additional information upon this topic, which is relatively new to astronomy. It is also interesting to note that the spectra of Nebulæ contain the bright lines in the green of a substance called “nebulium,” because it is not found except in nebulæ.

THE MILKY WAY

This is, in a sense, one vast nebula running right round the heavens in the form of a belt, or ring; its familiar resemblance to spilt milk being the origin of its popular name. To the naked eye, it appears merely a hazy band of light, but the telescope shows that it is made up of an enormous number of stars, millions of miles apart, but which can only be distinguished from one another by telescopic aid. It constitutes the so-called “Galaxy.” It seems to be spread out in the form of a vast disk, whose diameter is many times its thickness. Our solar system appears to be near the center of this vast system, and, as we penetrate further and further into space, it becomes apparent that fewer and fewer stars, and fewer and fewer nebulæ, seem to exist. Hence the limitation of the material Universe. The Milky Way is made up of thousands of millions of suns; yet their enormous distances make them appear to constitute one vast, luminous belt encircling our globe!

THE NUMBER OF STARS

When the heavens are viewed with the naked eye, a few hundred stars may perhaps be seen--some bright, some faint. Viewed through opera glasses, many more stars may been seen; while their number is again greatly increased by the use of a telescope. The larger and more powerful the telescope employed, the greater the number of stars thus discovered in the depths of space. The interesting question thus arises: What is the total number of stars in the entire firmament? Can they be estimated? And if so, what would their approximate number be?

What we call “stars” are, of course, in practically all cases _suns_--often vastly larger and hotter than our own. These stars differ from one another in order of brilliance; some are brighter than others. They are accordingly classified according to their order of brilliance, and known as stars of the “First Magnitude,” of the “Second Magnitude,” etc., up to about the Seventeenth Magnitude. Any star of a given magnitude is, roughly, about two-and-a-half times as brilliant as one of the next lower order, and this variation holds throughout--each magnitude being that much greater in brilliance.

The “magnitude” of the stars varies according to their light-giving power, and also their distance from us. One of the methods adapted to measure the magnitude is to compare its brightness with an artificial star, gradually cutting-off its light by means of neutral, tinted glass until the two are equal. The color of the star must be taken into account, in such measurements, the eye being more sensitive to some colors than to others.

Now, it is an interesting and significant fact that the number of the stars decreases as their magnitude decreases; that is to say, the greatest number of stars are found of the first magnitude; a lesser number of the second; still less of the third, and so on (broadly speaking). After reaching the ninth magnitude, the number very rapidly diminishes. It has been calculated that there are about 120,000,000 stars in the first 16 or 17 magnitudes. If the proportion were maintained throughout, however, there would be more than ten times that number. Some authorities have asserted that there are, roughly, half a billion stars of varying magnitudes in the heavens.

THE POSITION OF OUR SOLAR SYSTEM

It has been maintained that our solar system is at, or very near, the center of the whole Universe. Certain it is that the further we proceed into space, the less the number of stars encountered, which has given rise to the suspicion that their number is actually _limited_, and that the whole Universe consists of a sort of sphere, in which is enclosed all the stars that exist, and that, beyond this sphere, no stars whatever remain. No matter exists beyond this point! Such a view fits in rather well with Einstein’s conception of “curved space,” and a finite universe of infinite proportions! Of course, it is conceivable that, outside this vast system, another similar system may exist, and another and still another; but of such systems we know nothing, and it seems improbable that proof of their existence could ever be obtained by man. So far as we can tell, the universe is _One_, and the matter and energy of that one are limited.

THE MOVEMENT OF OUR SOLAR SYSTEM