ercury, Saturn
about twenty-five times, Uranus about forty-nine times, and Neptune
about seventy-seven. (See Fig. 2, p. 21.)

It will thus be seen how enormously the solar system was enlarged in
extent by the discovery of the outermost planets. The finding of Uranus
plainly doubled its breadth; the finding of Neptune made it more than
half as broad again. Nothing indeed can better show the import of these
great discoveries than to take a pair of compasses and roughly set out
the above relative paths in a series of concentric circles upon a large
sheet of paper, and then to consider that the path of Saturn was the
supposed boundary of our solar system prior to the year 1781.

We have seen that the usual shape of celestial bodies themselves is
spherical. Of what form then are their paths, or _orbits_, as these are
called? One might be inclined at a venture to answer "circular," but
this is not the case. The orbits of the planets cannot be regarded as
true circles. They are ovals, or, to speak in technical language,
"ellipses." Their ovalness or "ellipticity" is, however, in each case
not by any means of the same degree. Some orbits--for instance, that of
the earth--differ only slightly from circles; while others--those of
Mars or Mercury, for example--are markedly elliptic. The orbit of the
tiny planet Eros is, however, far and away the most elliptic of all, as
we shall see when we come to deal with that little planet in detail.

It has been stated that the sun and planets are always rotating. The
various rates at which they do so will, however, be best appreciated by
a comparison with the rate at which the earth itself rotates.

But first of all, let us see what ground we have, if any, for asserting
that the earth rotates at all?

If we carefully watch the heavens we notice that the background of the
sky, with all the brilliant objects which sparkle in it, appears to turn
once round us in about twenty-four hours; and that the pivot upon which
this movement takes place is situated somewhere near what is known to us
as the _Pole Star_. This was one of the earliest facts noted with regard
to the sky; and to the men of old it therefore seems as if the heavens
and all therein were always revolving around the earth. It was natural
enough for them to take this view, for they had not the slightest idea
of the immense distance of the celestial bodies, and in the absence of
any knowledge of the kind they were inclined to imagi