ght
variations.

[11] In employing the terms Inferior and Superior the writer bows to
astronomical custom, though he cannot help feeling that, in the
circumstances, Interior and Exterior would be much more appropriate.

[12] This question is, however, uncertain, for some very recent
spectroscopic observations of Venus seem to show a rotation period of
about twenty-four hours.

CHAPTER XV

THE EARTH

We have already seen (in Chapter I.) how, in very early times, men
naturally enough considered the earth to be a flat plane extending to a
very great distance in every direction; but that, as years went on,
certain of the Greek philosophers suspected it to be a sphere. One or
two of the latter are, indeed, said to have further believed in its
rotation about an axis, and even in its revolution around the sun; but,
as the ideas in question were founded upon fancy, rather than upon any
direct evidence, they did not generally attract attention. The small
effect, therefore, which these theories had upon astronomy, may well be
gathered from the fact that in the Ptolemaic system the earth was
considered as fixed and at the centre of things; and this belief, as we
have seen, continued unaltered down to the days of Copernicus. It was,
indeed, quite impossible to be certain of the real shape of the earth or
the reality of its motions until knowledge became more extended and
scientific instruments much greater in precision.

We will now consider in detail a few of the more obvious arguments which
can be put forward to show that our earth is a sphere.

If, for instance, the earth were a plane surface, a ship sailing away
from us over the sea would appear to grow smaller and smaller as it
receded into the distance, becoming eventually a tiny speck, and fading
gradually from our view. This, however, is not at all what actually
takes place. As we watch a vessel receding, its hull appears bit by bit
to slip gently down over the horizon, leaving the masts alone visible.
Then, in their turn, the masts are seen to slip down in the same manner,
until eventually every trace of the vessel is gone. On the other hand,
when a ship comes into view, the masts are the first portions to appear.
They gradually rise up from below the horizon, and the hull follows in
its turn, until the whole vessel is visible. Again, when one is upon a
ship at sea, a set of masts will often be seen sticking up alone above
the horizon, and these may shor