, and Saturn. Beyond these again revolved
the background of the heaven, upon which it was believed that the stars
were fixed--

"Stellis ardentibus aptum,"

as Virgil puts it (see Fig. 1).

[Illustration: FIG. 1.--The Ptolemaic idea of the Universe.]

The Ptolemaic system persisted unshaken for about fourteen hundred years
after the death of its author. Clearly men were flattered by the notion
that their earth was the most important body in nature, that it stood
still at the centre of the universe, and was the pivot upon which all
things revolved.

CHAPTER II

THE MODERN VIEW

It is still well under four hundred years since the modern, or
Copernican, theory of the universe supplanted the Ptolemaic, which had
held sway during so many centuries. In this new theory, propounded
towards the middle of the sixteenth century by Nicholas Copernicus
(1473-1543), a Prussian astronomer, the earth was dethroned from its
central position and considered merely as one of a number of planetary
bodies which revolve around the sun. As it is not a part of our purpose
to follow in detail the history of the science, it seems advisable to
begin by stating in a broad fashion the conception of the universe as
accepted and believed in to-day.

The Sun, the most important of the celestial bodies so far as we are
concerned, occupies the central position; not, however, in the whole
universe, but only in that limited portion which is known as the Solar
System. Around it, in the following order outwards, circle the planets
Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
(see Fig. 2, p. 21). At an immense distance beyond the solar system, and
scattered irregularly through the depth of space, lie the stars. The two
first-mentioned members of the solar system, Mercury and Venus, are
known as the Inferior Planets; and in their courses about the sun, they
always keep well inside the path along which our earth moves. The
remaining members (exclusive of the earth) are called Superior Planets,
and their paths lie all outside that of the earth.

[Illustration: FIG. 2.--The Copernican theory of the Solar System.]

The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been
known from all antiquity. Nothing then can bring home to us more
strongly the immense advance which has taken place in astronomy during
modern times than the fact that it is only 127 years since observation
of the skies first added a