one of a wholly
different character from that of the sun's distance, which we have
already discussed in these pages. The observations for the determination
of stellar parallax are founded on the familiar truth that the earth
revolves around the sun. We may for our present purpose assume that the
earth revolves in a circular path. The centre of that path is at the
centre of the sun, and the radius of the path is 92,900,000 miles. Owing
to our position on the earth, we observe the stars from a point of view
which is constantly changing. In summer the earth is 185,800,000 miles
distant from the position which it occupied in winter. It follows that
the apparent positions of the stars, as projected on the background of
the sky, must present corresponding changes. We do not now mean that the
actual positions of the stars are really displaced. The changes are only
apparent, and while oblivious of our own motion, which produces the
displacements, we attribute the changes to the stars.

On the diagram in Fig. 93 is an ellipse with certain months--viz.,
January, April, July, October--marked upon its circumference. This
ellipse may be regarded as a miniature picture of the earth's orbit
around the sun. In January the earth is at the spot so marked; in April
it has moved a quarter of the whole journey; and so on round the whole
circle, returning to its original position in the course of one year.
When we look from the position of the earth in January, we see the star
A projected against the point of the sky marked 1. Three months later
the observer with his telescope is carried round to April; but he now
sees the star projected to the position marked 2. Thus, as the observer
moves around the whole orbit in the annual revolution of the earth, so
the star appears to move round in an ellipse on the background of the
sky. In the technical language of astronomers, we speak of this as the
parallactic ellipse, and it is by measuring the major axis of this
ellipse that we determine the distance of the star from the sun. Half of
this major axis, or, what comes to the same thing, the angle which the
radius of the earth's orbit subtends as seen from the star, is called
the star's "annual parallax."

[Illustration: Fig. 93.--The Parallactic Ellipse.]

The figure shows another star, B, more distant from the earth and the
solar system generally than the star previously considered. This star
also describes an elliptic path. We cannot, however,