ust greatly extend or contract the apparent space between them,
since they are situated at various distances.

Independent of this, the stars themselves are all in motion, but so
vast is the distance from which we observe them that it has taken
an accumulation of centuries before they could be made measurable.
A train going forty miles an hour, seen from a distance of two
miles, almost seems to stand still. Arcturus moves through space
three times as fast as the earth, but it takes a century to appear
to move the eighth part of the diameter of the moon. There is a
star in the Hunting Dogs, known as 1830 Groombridge, which has a
velocity beyond what all the attraction of the matter of the known
universe could give it. By the year 9000 it may be in Berenice's
Hair.

Some stars have a common movement, being evidently related together.
A large proportion of the brighter stars between Aldebaran and
the Pleiades have a common motion eastward of about ten seconds
a century. All the angles marked by a, b, g, ch Orionis will be
altered in different directions; l is moving toward g. l and e
will appear as a double star. In A.D. 50,000 Procyon will be nearer
ch Orionis than Rigel now is, and Sirius will be in line with a and
ch Orionis. All the stars of the Great Dipper, except Benetnasch
and Dubhe, have a common motion somewhat in the direction [Page 228]
of Thuban (Fig. 67), while the two named have a motion nearly
opposite. In 36,000 years the end of the Dipper will have fallen out
so that it will hold no water, and the handle will be broken square
off at Mizar. "The Southern Cross," says Humboldt, "will not always
keep its characteristic form, for its four stars travel in different
directions with unequal velocities. At the present time it is not
known how many myriads of years must elapse before its entire
dislocation."

These movements are not in fortuitous or chaotic ways, but are
doubtless in accordance with some perfect plan. We have climbed
up from revolving earth and moon to revolving planets and sun,
in order to understand how two or ten suns can revolve about a
common centre. Let us now leap to the grander idea that all the
innumerable stars of a winter night not only loan, but must revolve
about some centre of gravity. Men have been looking for a central
sun of suns, and have not found it. None is needed. Two suns can
balance about a point; all suns can swing about a common centre.
That one unmoving centre may b