glance at the planetary system as a whole. To do this we may throw off, in imagination, the dominance of the sun, and suppose ourselves to be in the midst of open space, far removed both from the sun and the other stars. In this situation it is only by chance, or through foreknowledge, that we can distinguish our sun at all, for it is lost among the stars; and when we discover it we find that it is only one of the smaller and less conspicuous members of the sparkling host.

We rapidly approach, and when we have arrived within a distance comparable with that of its planets, we see that the sun has increased in apparent magnitude, until now it enormously outshines all the other stars, and its rays begin to produce the effect of daylight upon the orbs that they reach. But we are in no danger of mistaking its apparent superiority to its fellow stars for a real one, because we clearly perceive that our nearness alone makes it seem so great and overpowering.

And now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor. It is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space. Although they vary in relative size, yet we observe a striking similarity among them. They are all globular bodies, they all turn upon their axes, they all travel about the sun in the same direction, and their paths all lie very nearly in one plane. Some of them have one or more moons, or satellites, circling about them in imitation of their own revolution about the sun. Their family relationship to one another and to the sun is so evident that it colors our judgment about them as individuals; and when we happen to find, upon closer approach, that one of them, the earth, is covered with vegetation and water and filled with thousands of species of animated creatures, we are disposed to believe, without further examination, that they are all alike in this respect, just as they are all alike in receiving light and heat from the sun.

This preliminary judgment, arising from the evident unity of the planetary system, can only be varied by an examination of its members in detail.

One striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. The solar system occupies a disk-shaped, or flat circular, expanse, about 5,580,000,000 miles across and relatively very thin, the sun being in the center. From the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. But the vast majority of the stars are probably a hundred times yet more remote. In other words, if the Solar system be represented by a circular flower-bed ten feet across, the nearest star must be placed at a distance of nine and a half miles, and the great multitude of the stars at a distance of nine hundred miles!

Or, to put it in another way, let us suppose the sun and his planets to be represented by a fleet of ships at sea, all included within a space about half a mile across; then, in order that there might be no shore relatively nearer than the nearest fixed star is to the sun, we should have to place our fleet in the middle of the Pacific Ocean, while the distance of the main shore of the starry universe would be so immense that the whole surface of the earth would be far too small to hold the expanse of ocean needed to represent it!

From these general considerations we next proceed to recall some of the details of the system of worlds amid which we dwell. Besides the earth, the sun has seven other principal planets in attendance. These eight planets fall into two classes—the terrestrial planets and the major, or jovian, planets. The former class comprises Mercury, Venus, the earth, and Mars, and the latter Jupiter, Saturn, Uranus, and Neptune. I have named them all in the order of their distance from the sun, beginning with the nearest.

The terrestrial planets, taking their class name from terra, the earth, are relatively close to the sun and comparatively small. The major planets—or the jovian planets, if we give them a common title based upon the name of their chief, Jupiter or Jove—are relatively distant from the sun and are characterized both by great comparative size and slight mean density. The terrestrial planets are all included within a circle, having the sun for a center, about 140,000,000 miles in radius. The space, or gap, between the outermost of them, Mars, and the innermost of the jovian planets, Jupiter, is nearly two and a half times as broad as the entire radius of the circle within which they are included. And not only is the jovian group of planets widely separated from the terrestrial group, but the distances between the orbits of its four members are likewise very great and progressively increasing. Between Jupiter and Saturn is a gap 400,000,000 miles across, and this becomes 900,000,000 miles between Saturn and Uranus, and more than 1,000,000,000 miles between Uranus and Neptune. All of these distances are given in round numbers.

Finally, we come to some very extraordinary worlds—if we can call them worlds at all—the asteroids. They form a third group, characterized by the extreme smallness of its individual members, their astonishing number, and the unusual eccentricities and inclinations of their orbits. They are situated in the gap between the terrestrial and the jovian planets, and about 500 of them have been discovered, while there is reason to think that their real number may be many thousands. The largest of them is less than 500 miles in diameter, and many of those recently discovered may be not more than ten or twenty miles in diameter. What marvelous places of abode such little planets would be if it were possible to believe them inhabited, we shall see more clearly when we come to consider them in their turn. But without regard to the question of habitability, the asteroids will be found extremely interesting.

In the next chapter we proceed to take up the planets for study as individuals, beginning with Mercury, the one nearest the sun.

CHAPTER II

MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS

Mercury, the first of the other worlds that we are going to consider, fascinates by its grotesqueness, like a piece of Chinese ivory carving, so small is it for its kind and so finished in its eccentric details. In a little while we shall see how singular Mercury is in many of the particulars of planetary existence, but first of all let us endeavor to obtain a clear idea of the actual size and mass of this strange little planet. Compared with the earth it is so diminutive that it looks as if it had been cut out on the pattern of a satellite rather than that of an independent planet. Its diameter, 3,000 miles, only exceeds the moon's by less than one half, while both Jupiter and Saturn, among their remarkable collections of moons, have each at least one that is considerably larger than the planet Mercury. But, insignificant though it be in size, it holds the place of honor, nearest to the sun.

It was formerly thought that Mercury possessed a mass greatly in excess of that which its size would seem to imply, and some