If we are
going, from merely counting the stars, observing their magnitudes
and determining their proper motions, to draw conclusions as to
the structure of the universe in space, the question may arise how
we can form any estimate whatever of the possible distance of the
stars, a conclusion as to which must be the very first step we
take. We can hardly say that the parallaxes of more than one
hundred stars have been measured with any approach to certainty.
The individuals of this one hundred are situated at very different
distances from. us. We hope, by long and repeated observations, to
make a fairly approximate determination of the parallaxes of all
the stars whose distance is less than twenty times that of a
Centauri. But how can we know anything about the distance of stars
outside this sphere? What can we say against the view of Kepler
that the space around our sun is very much thinner in stars than
it is at a greater distance; in fact, that, the great mass of the
stars may be situated between the surfaces of two concentrated
spheres not very different in radius. May not this universe of
stars be somewhat in the nature of a hollow sphere?
This objection requires very careful consideration on the part of
all who draw conclusions as to the distribution of stars in space
and as to the extent of the visible universe.
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