the rotation of any other planetary or solar body
(Art. 92). Not only has each star a rotation on its axis, but it must
also possess translational motion in an orbit, and that orbital motion
must be due to exactly a similar cause as that which produces the
orbital motion of the sun. Are there any indications given by
astronomical observations which lead us to the conclusion that stars do
possess such orbital motions? The answer is unanimously in the
affirmative; for, although all the stars and the constellations retain
apparently the same relative position to each other, yet they are all in
motion. The actual translational motion of the stars is termed proper
motion, and has been calculated with more or less success in relation to
many of the stars nearest to us. There are other motions of the stars
known as apparent motions, which are easily noted by any observer. These
apparent motions are due to the rotation of the earth on its axis, and
its orbital motion round the sun.

Nothing is more certain, however, than that careful astronomical
observations have revealed the fact that stars have actual orbital
motions of their own through space. In many cases the orbital velocity
has been approximately ascertained.

Halley discovered proper motions of certain stars as far back as 1715,
when he found out, by comparing different observations, that Sirius,
Arcturus, and Aldebaran had moved during the period which had elapsed
since the respective observations were taken.

More recent observations tend to confirm the fact that stars have indeed
proper motions, due to their actual translation through space. It has
been ascertained, for example, that Arcturus is travelling at least 54
miles per second.

The proper motion of the stars, however, only gives us an indication of
their relative motion through so-called space. It does not tell us
whether the star is apparently receding from the earth, or approaching
it.

Dr. Vogel has ascertained by a special system of photography in relation
to the spectra of stars, that Rigel has a velocity away from the earth
of nearly 39 miles per sec., Aldebaran of 30 miles per sec., and Capella
of 15 miles per sec., while the Pole star is apparently approaching the
earth at a rate of nearly 16 miles per sec.

Now if all the stars move through space with varying velocities, as
spectroscopic and telescopic observations seem to suggest, the question
naturally confronts us as to what is the partic