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