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.