h their own eyes unaided by any artificial means. In such
researches they had been very much at a disadvantage. The sun and moon,
in their opinion, were no doubt the largest bodies in the heavens, for
the mere reason that they looked so! The mighty solar disturbances,
which are now such common-places to us, were then quite undreamed of.
The moon displayed a patchy surface, and that was all; her craters and
ring-mountains were surprises as yet in store for men. Nothing of course
was known about the surfaces of the planets. These objects had indeed no
particular characteristics to distinguish them from the great host of
the stars, except that they continually changed their positions in the
sky while the rest did not. The stars themselves were considered as
fixed inalterably upon the vault of heaven. The sun, moon, and planets
apparently moved about in the intermediate space, supported in their
courses by strange and fanciful devices. The idea of satellites was as
yet unknown. Comets were regarded as celestial portents, and meteors as
small conflagrations taking place in the upper air.

In the entire absence of any knowledge with regard to the actual sizes
and distances of the various celestial bodies, men naturally considered
them as small; and, concluding that they were comparatively near,
assigned to them in consequence a permanent connection with terrestrial
affairs. Thus arose the quaint and erroneous beliefs of astrology,
according to which the events which took place upon our earth were
considered to depend upon the various positions in which the planets,
for instance, found themselves from time to time.

It must, however, be acknowledged that the study of astrology,
fallacious though its conclusions were, indirectly performed a great
service to astronomy by reason of the accurate observations and diligent
study of the stars which it entailed.

We will now inquire into the means by which the distances and sizes of
the celestial orbs have been ascertained, and see how it was that the
ancients were so entirely in the dark in this matter.

There are two distinct methods of finding out the distance at which any
object happens to be situated from us.

One method is by actual measurement.

The other is by moving oneself a little to the right or left, and
observing whether the distant object appears in any degree altered in
position by our own change of place.

One of the best illustrations of this relative change o