51,600 |
| Combined mass of the four inner planets 5,952 |
| Combined mass of all the planets 1,341,687 |
+------------------------------------------------------------+

[Footnote 2: See Newcomb's Popular Astronomy, p. 234. Harper Brothers,
New York.]

It thus appears that the mass of all the planets is about one seven
hundredth that of the sun.

Those who wish to make a close study of celestial geography will do
well to procure the interesting set of diagrams prepared by the late
James Freeman Clarke, in which transparencies placed in a convenient
lantern show the grouping of the important stars in each
constellation. The advantage of this arrangement is that the little
maps can be consulted at night and in the open air in a very
convenient manner. After the student has learned the position of a
dozen of the constellations visible in the northern hemisphere, he can
rapidly advance his knowledge in the admirable method invented by Dr.
Clarke.

Having learned the constellations, the student may well proceed to
find the several planets, and to trace them in their apparent path
across the fixed stars. It will be well for him here to gain if he can
the conception that their apparent movement is compounded of their
motion around the sun and that of our own sphere; that it would be
very different if our earth stood still in the heavens. At this stage
he may well begin to take in mind the evidence which the planetary
motion supplies that the earth really moves round the sun, and not
the sun and planets round the earth. This discovery was one of the
great feats of the human mind; it baffled the wits of the best men for
thousands of years. Therefore the inquirer who works over the evidence
is treading one of the famous paths by which his race climbed the
steeps of science.

The student must not expect to find the evidence that the sun is the
centre of the solar system very easy to interpret; and yet any youth
of moderate curiosity, and that interest in the world about him which
is the foundation of scientific insight, can see through the matter.
He will best begin his inquiries by getting a clear notion of the fact
that the moon goes round the earth. This is the simplest case of
movements of this nature which he can see in the solar system. Noting
that the moon occupies a different place at a given hour in the
twenty-four, but is evidently at all times at a