It was, however, becoming more and more a matter of question what advantage was to be gained by the continuance of the hunt, when a fresh fillip was given to interest by the discovery in 1898 of the anomalous asteroid named Eros. Hitherto no minor planet had been known to have the greater portion of its orbit within that of Mars, though several do cross the red planet's borders; but the mean distance of Eros from the sun proves to be about 135,000,000, while that of Mars is 141,000,000 miles. In addition, the orbit of the new planet is such that at intervals of sixty-seven years it comes within 15,000,000 miles of the earth, or in other words nearer to us than any other celestial body except the moon or a chance comet. It may thus come to afford a means of revising estimates of celestial distances. Eros presents another peculiarity. It has been found by E. von Oppolzer to be variable in a period of two hours thirty-eight minutes; and the theory has been put forward that the planet is double, consisting of two bodies which revolve almost in contact and mutually eclipse one another—in short, that Eros as a planet presents the same phenomenon which we shall find as a characteristic of that type of variable stars known as the Algol type. An explanation, in some respects more simple and satisfactory, is that the variation in light is caused by the different reflective power of various parts of its surface; but the question is still open.
The best results for the sizes of the four asteroids first discovered are those of Barnard, from direct measurements with the Lick telescope in 1894. He found the diameter of Ceres to be 485 miles, that of Pallas 304, those of Vesta and Juno 243 and 118 miles respectively. There appears to be as great diversity in the reflective power of these original members of the group as in their diameters. Ceres is large and dull, and, in Miss Clerke's words, 'must be composed of rugged and sombre rock, unclothed probably by any vestige of air,' while Vesta has a surface which reflects light with four times the intensity of that of Ceres, and is, in fact, almost as brilliantly white as newly fallen snow.
In the place of Olbers' discredited hypothesis of an exploded planet, has now been set the theory first suggested by Kirkwood, that instead of having in the asteroids the remnants of a world which has become defunct, we have the materials of one which was never allowed to form, the overwhelming power of Jupiter's attraction having exerted a disruptive influence over them while their formation was still only beginning.
So far as I am aware, they share with Mars the distinction of being the only celestial bodies which have been made the subjects of a testamentary disposition. In the case of Mars, readers may remember that some years ago a French lady left a large sum of money to be given to the individual who should first succeed in establishing communication with the Planet of War; in that of the asteroids, the late Professor Watson, a mighty hunter of minor planets in his day, made provision for the supervision of the twenty-two planets captured by him, lest any of them should get lost, stolen, or strayed.
Small telescopes are, of course, quite impotent to deal with such diminutive bodies as the asteroids; nor, perhaps, is it desirable that the ranks of the minor-planet hunters should be reinforced to any extent.
CHAPTER X
JUPITER
Passing outwards from the zone of the minor planets, we come to the greatest and most magnificent member of the solar system, the giant planet Jupiter. To most observers, Jupiter will probably appear not only the largest, but also the most interesting telescopic object which our system affords. Some, no doubt, will put in a claim for Mars, and some will share Sir Robert Ball's predilection for Saturn; but the interest attaching to Mars is of quite a different character from that which belongs to Jupiter, and while Saturn affords a picture of unsurpassed beauty, there is not that interest of variety and change in his exquisite system which is to be found in that of his neighbour planet. Jupiter is constantly attractive by reason of the hope, or rather the certainty, that he will always provide something fresh to observe; and the perpetual state of flux in which the details of his surface present themselves to the student offers to us the only instance which can be conveniently inspected of the process of world-formation. Jupiter is at the very opposite end of the scale from such a body, for example, as our own moon. On the latter it would appear as though all things were approaching the fixity of death; such changes as are suspected are scarcely more than suspected, and, even if established, are comparatively so small as to tax the utmost resources of observation. On the former, such a thing as fixity or stability appears to be unknown, and changes are constantly occurring on a scale so gigantic as not to be beyond the reach of small instruments, at least in their broader outlines.
The main facts relating to the planet may be briefly given before we go on to consider the physical features revealed to us by the telescope. Jupiter then travels round the sun in a period of 11 years, 314·9 days, at an average distance of almost 483,000,000 miles. According to Barnard's measures, his polar diameter is 84,570, and his equatorial diameter 90,190 miles. He is thus compressed at the poles to the extent of 1⁄16, and there is no planet which so conspicuously exhibits to the eye the actual effect of this polar flattening, though the compression of Saturn is really greater still. In volume he is equal to more than 1,300 earths, but his density is so small that only 316 of our worlds would be needed to balance him. This low density, not much greater than that of water, is quite in accordance with all the other features which are revealed by observation, and appears to be common to all the members of that group of large exterior planets of which Jupiter is at once the first and the chief.