When and how these conceptions vanished from the mind of man, it
would be as hard to say as when and how Santa Claus gets
transformed in the mind of the child. They are not treated as
realities by any astronomical writer from Ptolemy down; yet, the
impressions and forms of thought to which they gave rise are well
marked in Copernicus and faintly evident in Kepler. The latter was
perhaps the first to suggest that the sun might be one of the
stars; yet, from defective knowledge of the relative brightness of
the latter, he was led to the conclusion that their distances from
each other were less than the distance which separated them from
the sun. The latter he supposed to stand in the centre of a vast
vacant region within the system of stars.
For us the great collection of millions of stars which are made
known to us by the telescope, together with all the invisible
bodies which may be contained within the limits of the system,
form the universe. Here the term "universe" is perhaps
objectionable because there may be other systems than the one with
which we are acquainted. The term stellar system is, therefore, a
better one by which to designate the collection of stars in
question.
It is remarkable that the first known propounder of that theory of
the form and arrangement of the system which has been most
generally accepted seems to have been a writer otherwise unknown
in science--Thomas Wright, of Durham, England.
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