Slow and
halting, indeed, is that star which does not make more than a mile
a second. With two or three exceptions, where the attraction of a
companion comes in, the motion of every star, so far as yet
determined, takes place in a straight line. In its outward motion
the flying body deviates neither to the right nor left. It is safe
to say that, if any deviation is to take place, thousands of years
will be required for our terrestrial observers to recognize it.
Rapid as the course of these objects is, the distances which we
have described are such that, in the great majority of cases, all
the observations yet made on the positions of the stars fail to
show any well-established motion. It is only in the case of the
nearer of these objects that we can expect any motion to be
perceptible during the period, in no case exceeding one hundred
and fifty years, through which accurate observations extend. The
efforts of all the observatories which engage in such work are, up
to the present time, unequal to the task of grappling with the
motions of all the stars that can be seen with the instruments,
and reaching a decision as to the proper motion in each particular
case. As the question now stands, the aim of the astronomer is to
determine what stars have proper motions large enough to be well
established.
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