h really far apart. Those, when observed from different
points of the Earth's orbit, would appear to change their positions
relatively to each other. The smaller and more distant star would remain
unaltered, whilst the larger and nearer star would have changed its
position with respect to the other. By continuing to observe the larger
star during the time that the Earth accomplished a revolution of her
orbit, Galileo believed that its parallax might be successfully
determined. Though he did not himself put this method into practice, it
has been tried by others with successful results.

In 1669, Hooke made the first attempt to ascertain the parallax of a
fixed star, and selected for this purpose Gamma Draconis, a bright star
in the Head of the Dragon. This constellation passed near the zenith of
London at the time that he made his observations, and was favourably
situated, so as to avoid the effects of refraction. Hooke made four
observations in the months of July, August, and October, and believed
that he determined the parallax of the star; but it was afterwards
discovered that he was in error, and that the apparent displacement of
the star was mainly due to the aberration of light--a phenomenon which
was not discovered at that time.

A few years later, Picard, a French astronomer, attempted to find the
parallax of Alpha Lyrae, but was unsuccessful. In 1692-93, Roemer, a
Danish astronomer, observed irregularities in the declinations of the
stars which could neither be ascribed to parallax or refraction, and
which he imagined resulted from a changing position of the Earth's axis.

One of the principal causes which baffled astronomers in their
endeavours to determine the parallax of the fixed stars was a phenomenon
called the 'Aberration of Light,' which was discovered and explained by
Bradley in 1727. The peculiar effect of aberration was perceived by him
when endeavouring to obtain the parallax of Gamma Draconis.

Owing to the progressive transmission of light, conjointly with the
motion of the Earth in her orbit, there results an apparent slight
displacement of a star from its true position. The extent of the
displacement depends upon the ratio of the velocity of light as compared
with the speed of the Earth in her orbit, which is as 10,000 to 1. As a
consequence of this, each star describes a small ellipse in the course
of a year, the central point of which would indicate the place occupied
by the star if the Eart