ne them
comparatively near. It was indeed only after the lapse of many
centuries, when men had at last realised the enormous gulf which
separated them from even the nearest object in the sky, that a more
reasonable opinion began to prevail. It was then seen that this
revolution of the heavens about the earth could be more easily and more
satisfactorily explained by supposing a mere rotation of the solid earth
about a fixed axis, pointed in the direction of the polar star. The
probability of such a rotation on the part of the earth itself was
further strengthened by the observations made with the telescope. When
the surfaces of the sun and planets were carefully studied these bodies
were seen to be rotating. This being the case, there could not surely
be much hesitation in granting that the earth rotated also; particularly
when it so simply explained the daily movement of the sky, and saved men
from the almost inconceivable notion that the whole stupendous vaulted
heaven was turning about them once in every twenty-four hours.

If the sun be regularly observed through a telescope, it will gradually
be gathered from the slow displacement of sunspots across its face,
their disappearance at one edge and their reappearance again at the
other edge, that it is rotating on an axis in a period of about
twenty-six days. The movement, too, of various well-known markings on
the surfaces of the planets Mars, Jupiter, and Saturn proves to us that
these bodies are rotating in periods, which are about twenty-four hours
for the first, and about ten hours for each of the other two. With
regard, however, to Uranus and Neptune there is much more uncertainty,
as these planets are at such great distances that even our best
telescopes give but a confused view of the markings which they display;
still a period of rotation of from ten to twelve hours appears to be
accepted for them. On the other hand the constant blaze of sunlight in
the neighbourhood of Mercury and Venus equally hampers astronomers in
this quest. The older telescopic observers considered that the rotation
periods of these two planets were about the same as that of the earth;
but of recent years the opinion has been gaining ground that they turn
round on their axes in exactly the same time as they revolve about the
sun. This question is, however, a very doubtful one, and will be again
referred to later on; but, putting it on one side, it will be seen from
what we have said above