of the earth's change of place during the interval
upon the positions of the other bodies of our system. But when we desire
to sound the depths of space beyond, and to reach out to measure the
distance of the nearest star, we find ourselves at once thrown upon the
greatest change of place which we can possibly hope for; and this we get
during the long journey of many millions of miles which our earth
performs around the sun during the course of each year. But even this
last change of place, great as it seems in comparison with terrestrial
measurements, is insufficient to show anything more than the tiniest
displacements in a paltry forty-three out of the entire host of the
stars.

We can thus realise at what a disadvantage the ancients were. The
measuring instruments at their command were utterly inadequate to detect
such small displacements. It was reserved for the telescope to reveal
them; and even then it required the great telescopes of recent times to
show the slight changes in the position of the nearer stars, which were
caused by the earth's being at one time at one end of its orbit, and
some six months later at the other end--stations separated from each
other by a gulf of about one hundred and eighty-six millions of miles.

The actual distances of certain celestial bodies being thus
ascertainable, it becomes a matter of no great difficulty to determine
the actual sizes of the measurable ones. It is a matter of everyday
experience that the size which any object appears to have, depends
exactly upon the distance it is from us. The farther off it is the
smaller it looks; the nearer it is the bigger. If, then, an object which
lies at a known distance from us looks such and such a size, we can of
course ascertain its real dimensions. Take the moon, for instance. As we
have already shown, we are able to ascertain its distance. We observe
also that it looks a certain size. It is therefore only a matter of
calculation to find what its actual dimensions should be, in order that
it may look that size at that distance away. Similarly we can ascertain
the real dimensions of the sun. The planets, appearing to us as points
of light, seem at first to offer a difficulty; but, by means of the
telescope, we can bring them, as it were, so much nearer to us, that
their broad expanses may be seen. We fail, however, signally with regard
to the stars; for they are so very distant, and therefore such tiny
points of light, that our m