lies in a
plane always parallel to the plane of the ecliptic.

We have now wrought the facts of observation into a form which enables
us to examine into the cause of a movement so systematic. Why is it that
each star should seem to describe a small circular path? Why should that
path be parallel to the ecliptic? Why should it be completed exactly in
a twelvemonth? We are at once referred to the motion of the earth around
the sun. That movement takes place in the ecliptic. It is completed in a
year. The coincidences are so obvious that we feel almost necessarily
compelled to connect in some way this apparent movement of the stars
with the annual movement of the earth around the sun. If there were no
such connection, it would be in the highest degree improbable that the
planes of the circles should be all parallel to the ecliptic, or that
the time of revolution of each star in its circle should equal that of
the revolution of the earth around the sun. As both these conditions are
fulfilled, the probability of the connection rises to a value almost
infinite.

The important question has then arisen as to why the movement of the
earth around the sun should be associated in so remarkable a manner with
this universal star movement. There is here one obvious point to be
noticed and to be dismissed. We have in a previous chapter discussed the
important question of the annual parallax of stars, and we have shown
how, in virtue of annual parallax, each star describes an ellipse. It
can further be demonstrated that these ellipses are really circles
parallel to the ecliptic; so that we might hastily assume that annual
parallax was the cause of the phenomenon discovered by Bradley. A single
circumstance will, however, dispose of this suggestion. The circle
described by a star in virtue of annual parallax has a magnitude
dependent on the distance of the star, so that the circles described by
various stars are of various dimensions, corresponding to the varied
distances of different stars. The phenomena of aberration, however,
distinctly assert that the circular path of each star is of the same
size, quite independently of what its distance may be, and hence annual
parallax will not afford an adequate explanation. It should also be
noticed that the movements of a star produced by annual parallax are
much smaller than those due to aberration. There is not any known star
whose circular path due to annual parallax has a diameter one-twen