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.
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