The proper motion of the star 61
Cygni, for instance, is so considerable, that it has amounted to a whole
degree in the course of 700 years.
The amount or quantity of these alterations in the fixed stars (that is to
say, the changes in the relative position of self-luminous stars toward each
other), can be determined with a greater degree of certainty than we are
able to attach to the genetic explanation of the phenomenon. After taking
into consideration what is due to the precession of the equinoxes, and the
nutation of the earth's axis produced by the action of the Sun and Moon on
the spheroidal figure of our globe, and what may be ascribed to the
transmission of light, that is to say, to its aberration, and to the
parallax formed by the diametrically opposite position of the Earth in its
course round the Sun, we still find that there is a residual portion
p 146
of the annual motion of the fixed stars due to the translation of the whole
solar system in universal space, and to the true proper motion of the stars.
The difficult problem of numerically separating these two elements, the
true and the apparent motion, has been effected by the careful study of the
direction of the motion of certain individual stars, and by the
consideration of the fact that, if all the stars were in a state of absolute
rest, they would appear perspectively to recede from the point in space
toward which the Sun was directing its course.
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