The proper
motion of a star can be determined only by comparing its observed
position in the heavens at two widely separate epochs.
Observations of sufficient precision for this purpose were
commenced about 1750 at the Greenwich Observatory, by Bradley,
then Astronomer Royal of England. But out of 3000 stars which he
determined, only a few are available for the purpose. Even since
his time, the determinations made by each generation of
astronomers have not been sufficiently complete and systematic to
furnish the material for anything like a precise determination of
the proper motions of stars. To determine a single position of any
one star involves a good deal of computation, and if we reflect
that, in order to attack the problem in question in a satisfactory
way, we should have observations of 1,000,000 of these bodies made
at intervals of at least a considerable fraction of a century, we
see what an enormous task the astronomers dealing with this
problem have before them, and how imperfect must be any
determination of the distance of the stars based on our motion
through space. So far as an estimate can be made, it seems to
agree fairly well with the results obtained by the other methods.
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