fail to notice that
the parallactic ellipse belonging to B is much smaller than that of A.
The difference in the sizes of the ellipses arises from the different
distances of the stars from the earth. The nearer the star is to the
earth the greater is the ellipse, so that the nearest star in the
heavens will describe the largest ellipse, while the most distant star
will describe the smallest ellipse. We thus see that the distance of the
star is inversely proportional to the size of the ellipse, and if we
measure the angular value of the major axis of the ellipse, then, by an
exceedingly simple mathematical manipulation, the distance of the star
can be expressed as a multiple of a radius of the earth's orbit.
Assuming that radius to be 92,900,000 miles, the distance of the star is
obtained by simple arithmetic. The difficulty in the process arises from
the fact that these ellipses are so small that our micrometers often
fail to detect them.

How shall we adequately describe the extreme minuteness of the
parallactic ellipses in the case of even the nearest stars? In the
technical language of astronomers, we may state that the longest
diameter of the ellipse never subtends an angle of more than one and a
half seconds. In a somewhat more popular manner, we would say that one
thousand times the major axis of the very largest parallactic ellipse
would not be as great as the diameter of the full moon. For a still more
simple illustration, let us endeavour to think of a penny-piece placed
at a distance of two miles. If looked at edgeways it will be linear, if
tilted a little it would be elliptic; but the ellipse would, even at
that distance, be greater than the greatest parallactic ellipse of any
star in the sky. Suppose a sphere described around an observer, with a
radius of two miles. If a penny-piece were placed on this sphere, in
front of each of the stars, every parallactic ellipse would be totally
concealed.

The star in the Swan known as 61 Cygni is not remarkable either for its
size or for its brightness. It is barely visible to the unaided eye, and
there are some thousands of stars which are apparently larger and
brighter. It is, however, a very interesting example of that remarkable
class of objects known as double stars. It consists of two nearly equal
stars close together, and evidently connected by a bond of mutual
attraction. The attention of astronomers is also specially directed
towards the star by its large p