the stage of their development.

A fable related by the Persian astronomer, Al Sufi (tenth century, A.D.)
shows well the changes in the face of the sky which proper motions are
bound to produce after great lapses of time. According to this fable the
stars Sirius and Procyon were the sisters of the star Canopus. Canopus
married Rigel (another star,) but, having murdered her, he fled towards
the South Pole, fearing the anger of his sisters. The fable goes on to
relate, among other things, that Sirius followed him across the Milky
Way. Mr. J. E. Gore, in commenting on the story, thinks that it may be
based upon a tradition of Sirius having been seen by the men of the
Stone Age on the opposite side of the Milky Way to that on which it now
is.

Sirius is in that portion of the heavens _from_ which the sun is
advancing. Its proper motion is such that it is gaining upon the earth
at the rate of about ten miles per second, and so it must overtake the
sun after the lapse of great ages. Vega, on the other hand, is coming
towards us from that part of the sky _towards_ which the sun is
travelling. It should be about half a million years before the sun and
Vega pass by one another. Those who have specially investigated this
question say that, as regards the probability of a near approach, it is
much more likely that Vega will be then so far to one side of the sun,
that her brightness will not be much greater than it is at this moment.

Considerations like these call up the chances of stellar collisions.
Such possibilities need not, however, give rise to alarm; for the stars,
as a rule, are at such great distances from each other, that the
probability of relatively near approaches is slight.

We thus see that the constellations do not in effect exist, and that
there is in truth no real background to the sky. We find further that
the stars are strewn through space at immense distances from each other,
and are moving in various directions hither and thither. The sun, which
is merely one of them, is moving also in a certain direction, carrying
the solar system along with it. It seems, therefore, but natural to
suppose that many a star may be surrounded by some planetary system in a
way similar to ours, which accompanies it through space in the course of
its celestial journeyings.

[28] Vega, for instance, shines one hundred times more brightly than the
sun would do, were it to be removed to the distance at which that star
is fro