Part 19

The asteroids, or minor planets, are situated almost wholly in the vast space between Mars and Jupiter. Their orbits are very irregular, both as to shape and situation; but, so far as is known, only two of them pass beyond the orbit of Jupiter, and only one has been discovered which at any point in its journey around the sun comes nearer than the orbit of Mars.

The minor planets are called by astronomers almost indifferently asteroids or planetoids. “Asteroids” is probably the name by which they are most popularly known. But because they are in fact simply little bodies that revolve about the sun as the planets do, “planetoids” seems to be more truly descriptive of them, and it is the word I have chosen to use here.

It was early noted that, except in one instance, the planets seemed to show in their distance from the sun something like a mathematical progression. Struck by this appearance, an astronomer named Bode worked it out into a formula, known ever since as Bode’s law, though the idea seems to have originated with another astronomer. One almost always sees it mentioned in any work dealing with this phase of planetary history, and it is especially interesting because of the part it played in the discovery of the planetoids. It was as follows: Beginning with nothing for Mercury, add three for Venus, twice three, or six, for the earth, twelve for Mars, and continue thus to double the number for each planet out to and including Saturn. Then to each one of the numbers so obtained add four, and the numbers resulting will very nearly represent the relative distances of the planets from the sun. Thus:

0 3 6 12 24 48 96 192 384 4 4 4 4 4 4 4 4 4 ------------------------------------------- 4 7 10 16 28 52 100 196 388

The exception was that at the fifth number, 28, there was no planet to correspond, and Jupiter was nearly twice as far away from Mars as it should have been to conform to the law, thus leaving room for another planet to occupy the allotted position and fill out this very beautiful progression.

About nine years after this law was set forth Uranus was discovered circling out in space far beyond Saturn, and was found to conform to the law in a most satisfactory manner, its distance being approximately twice that of Saturn. With such close accord between the actual distances and the prescribed distances of the planets from the sun, and with the one exception leaving almost exactly the space allotted by Bode’s law for another planet, astronomers naturally had a very strong feeling that there must be another planet between Mars and Jupiter. They accordingly set to work to prove this, if possible, and to find what had become of this lost member of the planet family, if it ever existed.

As a result of this work, on January 1, 1801, the first planetoid was discovered, and in rapid succession many like it were found, until now many hundreds are known to astronomers. Their discovery seemed at first almost a certain confirmation of Bode’s law, and the fact that where one large planet should have been found there proved to be such a swarm of small ones could be accounted for in no other way than to suppose that something had happened in the making of the planet. At any rate, the promulgation of Bode’s law was the direct cause of the search for the missing planet which led to the discovery of the planetoids. And this is the only reason why Bode’s law has continued to be mentioned in the history of the planets. For it was no real law, it had no scientific foundation, and its conformity to the facts of the relative distances of the planets was only one of those very interesting and singular coincidences that startle one for the moment into thinking that there is some scientific significance in them. Another example of such a coincidence is in the fact that the mass of any given planet exceeds the total mass of all the planets of any less mass than itself.

In less than half a century after the discovery of the first planetoid, Neptune was discovered at a distance not at all corresponding to that indicated by Bode’s law. It was not nearly far enough away, and yet, strangely enough, it was by taking Bode’s law into consideration that the position was indicated which finally led to the discovery of the planet. So while Bode’s law has been found to be no law at all, it is, nevertheless, entitled to some mention because of its having thus stimulated research that has had such important results.

No really satisfactory and final explanation of the present state of the planetoids has ever been given. At one time it was suggested that another planet had originally existed in the space between Mars and Jupiter, and through some catastrophe had been shattered into the small bodies that now occupy that space. But this has been shown to be impossible.

It is now thought probable that in the original nebula the matter forming the planetoids might have been prevented from condensing into a planet by the powerful gravitative influence of Jupiter. This influence, however, was not sufficiently strong to bring them entirely under his control. Even yet he pulls some of them five or six degrees out of the path they otherwise would take when they venture within the limits of his domain; but he does not capture them, so they have been left to circle around the sun as mere fragments of bodies, with no force to combine and make a world, no mass to hold an atmosphere, and with nothing to prevent them from quickly condensing and from radiating all their heat into space. They are, in the main, just cold, dark, lifeless rocks and lumps of matter whirling through space in a maze of interlacing orbits, some of them almost as far from the sun as Jupiter and some almost as near as Mars--one, indeed, a little nearer than Mars at certain times--but most of them swarming more thickly about half-way between Mars and Jupiter, not far from the place that Bode’s law assigned to a planet.

After the first planetoid was discovered and had been observed for a few weeks, it was lost and had to be rediscovered by means of mathematical computation of its orbit. Where this computation showed that it ought to be, there it was found, on the very last day of the same year, 1801. Early the next year another body of the same sort was discovered, two years later another was found, and still three years later a fourth came into view. These four were the only ones known in this branch of the solar family for nearly forty years thereafter.

In 1845 another period of discovery commenced, and has ever since continued, until there are now between six and seven hundred of these little bodies that have disclosed their right to be known as members of the sun’s family. It is probable that there may be still many more of them, since a new one comes to light every now and then on a photographic plate, and there is no indication of any limit to the number that may thus appear.

It is likely that about all have been discovered that can be seen even with a telescope, for a fairly systematic and thorough search has been made of the heavens for this purpose during the last half-century. This work has resulted in a continually decreasing number of discoveries, until this method of search has finally been practically abandoned. But it not infrequently happens that in photographing the stars a little trail of light is discovered on the plate, showing that some heavenly body with sensible motion has been caught on it. And this usually proves to be a new planetoid. No matter how long a photographic plate is exposed, the fixed stars imprint themselves on it only as points of light. When the impression is a little streak of light instead of a dot, the object is shown to be in motion, and is either a planetoid, a satellite, or a comet. The fixed stars would make a trail also if the photographic apparatus were not regulated by clockwork, so as to follow the star in its apparent daily motion across the skies. The planets and other bodies in the solar system are sufficiently near to have a sensible motion in addition to the motion caused by the rotation of the earth, which is the only motion we have to take into account in dealing with the aspects of the stars.

The first planetoid discovered was called Ceres, the next one Pallas, the third Juno, and the fourth Vesta. This pretty custom of naming them after the gods and goddesses of mythology was continued, with some variations, until perhaps three hundred had been so christened. But the number of them became too prodigious; and when so many began to swarm into view, waiting to be named, the utilitarian method of designating them simply by numbers in the order of their discovery was adopted. The only distinguishing feature of so numbering them is that each number is placed in a little circle. Thus Ceres is ①, Pallas ②, and so on. Those of them that have any special claim to distinction, however, are still referred to by their own names, if they have any, in spite of this most orderly attempt to make them fit for easy reference in a list.

There are so many of the planetoids, and they are so minute, that even after they have been discovered they are frequently lost again. Hence it is sometimes uncertain when they register themselves on the photographic plates whether they are really new to us or have been known before. In such cases they are named temporarily after the letters of the alphabet, and, when the alphabet is exhausted, a second letter is added. Thus A to Z, then AB to AZ, BC to BZ, and so on in a sort of “round.” Sometimes these combinations of letters become the fixed designation of a planetoid, as a nickname sometimes clings to a person. And thus it happens that we sometimes read of one in particular of these little bodies that is conspicuous for the great eccentricity of its orbit, called “WD.” The letters are not its initials, but its nickname. It really has no name other than its number in the list; but it became famous while it was temporarily designated as “WD,” and thus it continues to be called.

The aid of a telescope is necessary in order to see the planetoids, though it is said that Vesta, under very favorable conditions, sometimes comes within the limit of visibility. She is the brightest of them all, though not the largest, and her brilliancy is the subject of much interesting speculation among astronomers, who have not yet been able to account for it. She seems from her excessive brightness to be covered with clouds; and yet it is manifestly impossible that so small a body could have held an atmosphere throughout these long ages, though clouds presuppose an atmosphere. No doubt, in time this mystery of Vesta’s brilliancy will be made plain. Bright as she is in proportion to her size, and even if she sometimes can be seen, one cannot reasonably expect anything very brilliant to our view from a body not much more than a hundred miles in diameter, shining by reflected light, nearly two hundred million miles away.

Ceres, as far as we yet know, is the largest of the planetoids, and may be something more than four hundred miles in diameter. Juno is somewhere near the same size. Pallas is about two hundred miles in diameter, and Vesta about one hundred and eighteen. No doubt, these four were the first to be discovered, because they are the largest and so the easiest to be seen. At any rate, no others yet seen exceed them in size, and some of the more lately discovered are not more than fifteen or twenty miles in diameter. Many of those discovered by photography are doubtless even smaller than these, and are, perhaps, mere meteors in size. The combined mass of all those discovered up to this time is far smaller than that of any of the large planets, or even than that of our moon. Their mass cannot, of course, really be measured, because they are too small to have any perceptible gravitative effect on other bodies, and mass can only be determined by the influence of one body on another. But we do know that their aggregate mass, if it exceeded a certain limit, would show some disturbing effect on Mars; and, since it does not do this, we know that all of them taken together would make an extremely insignificant body.

While the planetoids all revolve around the sun in the same manner and in the same direction as the planets do, yet they are very erratic in their courses, and do not all keep within the narrow limits of the zodiac through which--happily for our convenient observation--the larger bodies travel. The orbits of many of them are extremely elliptical, while some are almost circles; and their inclination to the ecliptic varies from almost nothing to nearly fifty degrees. If one could catch from one side a view of them all together, they would have much the appearance in space of a flock of swallows, the individuals darting this way and that, passing above and below one another in such intricate sweeps and sinuosities that it would be impossible to keep track of them separately. And yet time has brought these apparently tangled orbits into such nice adjustment that the little bodies can continue to cross and recross each other’s paths with no danger of interference from each other. Such collisions as there may have been occurred in the very beginning of their careers. Such of them as came into collision then traveled on together as one body until accommodation was made for all.

One of the most wide-wandering of these tiny bodies has been named Eros, after the little god of love, more commonly known as Cupid. It has a particular interest for us, because of all the heavenly bodies it at times comes nearer to us than any except the moon and an occasional comet. At its nearest it is within fourteen million miles of the earth, which is more than ten million miles nearer than the closest approach of Venus, the nearest of the large planets.

This little body was thus near us in 1894; but we did not then know this, for Eros was not discovered until 1898. After its discovery, however, it was traced back on many photographic plates, and the fact that it had been in our neighborhood was learned. For untold ages it has been making these visits to us every thirty-seven years, and we have known nothing of them. Its next near approach will be in 1931, and it will continue to come thereafter every thirty-seven years. Now that we know about them, these visits are not only pleasant to contemplate, but it is expected that when they occur the planetoid will be of great scientific value to us in helping to determine more surely and accurately the exact distance of the sun.

The planetoids, though so minute and of no value as a spectacle, have been, and still are, very useful little bodies to us in a scientific way. In addition to furnishing an easy means of measuring the distance of the sun, they promise to throw some light on various questions of physics in which the planets, too, are involved. The brilliancy of Vesta, for instance, which has been mentioned, and the unaccountable variability in the brightness of some others of them have yet to be adjusted to known physical laws. Even the extreme eccentricity of some of their orbits, and the large tilt of some of them to the ecliptic, may be suggestive in finally solving certain planetary problems, for these impish little bodies are far from conforming to the regular ways of the planets, and there is, of course, some mechanical reason for their apparent waywardness.

XVIII

CONCLUSION

The great variety of beauty that the planets present to us is sufficient to keep us always interested in them, when once we have acquired an acquaintance with them. Rarely is there an evening when some one of them does not enhance the charm of the splendid spectacle of the sky in which all the heavenly bodies save the sun have a part. Their greater brilliancy often brings them into view before the stars have begun to glow in the evening, and prolongs our sight of them after the rays of the sun have blotted out the light of the stars in the morning. Thus they are always single in their loveliness, and always hold a distinguished place in the midst of the brilliant company of the stars.

Having considered these brilliant bodies individually and in detail, as we have, we ought by this time to be able to identify any one of them that shows itself in the evening sky, and to have a pretty fair notion of the general character and peculiarities of each. But even if one does not much care for detailed information concerning them, or, before seeking that, prefers first to become familiar with their appearance, a quick and sure recognition of them may be had by noting their positions and their very striking individual aspects as set forth in the preceding chapters.

On seeing a bright object in the sky that does not seem to be a familiar star, simply stop and look at it. Does it twinkle? If it does not, it is a planet. If it is more than forty-five degrees from the sun, or if it is seen at a time when the sun has been down more than three hours, then it is neither Mercury nor Venus, and must be either Mars, Jupiter, or Saturn. Is it very bright and pinkish in tone? Then it is Jupiter. Is it very bright and quite red? It is Mars, not far from opposition. Is it not very bright, but small and rosy? Then it is Mars going toward conjunction. Is it yellow in tone and, while large and conspicuous, still not so very brilliant? It is Saturn.

If the planet we seek to name is nearer to the sun than forty-five degrees, but is still well above the horizon, it may be either of these three--Mars, Jupiter, Saturn--or it is Venus. If it is very bright and silvery, it is certainly Venus. If it is very low in the sky and very near the sun, it may be any one of the five visible planets. In such a position Mars will always be very small, and the others always larger than a first-magnitude star; and they may all twinkle a little--Mercury almost as much as a star. Their size will show them all (except Mars) as planets, but it will be somewhat more difficult to tell which is which than it is when they are higher up in the sky. The best thing to do in such circumstances is to look up their positions either in this book or in an almanac. The almanac will serve as a footman to announce them. The book, it is hoped, has so recorded their peculiarities and habits that either their appearance or their place will be sufficient to make them known.

In any event, the problem of identification in this position will not keep one long, for in a situation presenting these greater difficulties the planet will be visible for less than an hour after sundown. Besides, it is not likely at such times to attract one’s involuntary attention, but when under observation in such a situation is usually sought out by those already somewhat informed as to the planet’s habits and appearance, which will betray its identity. It is information of this sort that I have endeavored to give in these pages, and it is hoped that the reading of them will be the beginning of a long and intimate acquaintance with these charming and always interesting individuals.