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.