to the other. But we have already remarked that our
solar system, with the earth as one of its bodies, has been journeying
straightforward through space during all historic times. It follows,
therefore, that we are continually changing the position from which we
view the stars, and that, if the latter were at rest, we could, by
measuring the apparent speed with which they are moving in the opposite
direction from that of the earth, determine their distance. But since
every star has its own motion, it is impossible, in any one case, to
determine how much of the apparent motion is due to the star itself,
and how much to the motion of the solar system through space. Yet, by
taking general averages among groups of stars, most of which are
probably near each other, it is possible to estimate the average
distance by this method. When an attempt is made to apply it, so as to
obtain a definite result, the astronomer finds that the data now
available for the purpose are very deficient. 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. Speaking roughly, we have
reason, from the data so far available, to believe that the stars of
the Milky Way are situated at a distance between 100,000,000 and
200,000,000 times the distance of the sun. At distances less than this
it seems likely that the stars