To make our statement on this subject clear, it must
be understood that by this term the astronomer does not mean the
speed of a star in space, but its angular motion as he observes it
on the celestial sphere. A star moving forward with a given speed
will have a greater proper motion according as it is nearer to us.
To avoid all ambiguity, we shall use the term "speed" to express
the velocity in miles per second with which such a body moves
through space, and the term "proper motion" to express the
apparent angular motion which the astronomer measures upon the
celestial sphere.
Up to the present time, two stars have been found whose proper
motions are so large that, if continued, the bodies would make a
complete circuit of the heavens in less than 200,000 years. One of
these would require about 160,000; the other about 180,000 years
for the circuit. Of other stars having a rapid motion only about
one hundred would complete their course in less than a million of
years.
Quite recently a system of observations upon stars to the ninth
magnitude has been nearly carried through by an international
combination of observatories. The most important conclusion from
these observations relates to the distribution of the stars with
reference to the Milky Way, which we have already described.
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