All the variations we have hitherto considered take place with such
rapidity that they can be observed by comparisons embracing but a short
interval of time--a few days or months at the outside. A somewhat
different question of great importance is still left open. May not
individual stars be subject to a secular variation of brilliancy,
meaning by this term a change which would not be sensible in the course
of only one generation of men, but admitting of being brought out by
a comparison of the brightness of the stars at widely distant epochs?
Is it certain that, in the case of stars which we do not recognize
as variable, no change has taken place since the time of Hipparchus
and Ptolemy? This question has been investigated by C. S. Pierce and
others. The conclusion reached is that no real evidence of any change
can be gathered. The discrepancies are no greater than might arise from
errors of estimates.
There is, however, an analogous question which is of great interest and
has been much discussed in recent times. In several ancient writings
the color of Sirius is described as red. This fact would, at first
sight, appear to afford very strong evidence that, within historic
times, the color of the brightest star in the heavens has actually
changed from red to a bluish white.
Two recent writers have examined the evidence on this subject most
exhaustively and reached opposite conclusions. The first of these was
Dr. T. J. J. See, who collated a great number of cases in which Sirius
was mentioned by ancient writers as red or fiery, and thus concluded
that the evidence was in favor of a red color in former times. Shortly
afterwards, Schiaparelli examined the evidence with equal care and
thoroughness and reached an opposite conclusion, showing that the
terms used by the ancient authors, which might have indicated redness
of color, were susceptible of other interpretations; they might mean
fiery, blazing, etc., as well as red in color, and were therefore
probably suggested by the extraordinary brightness of Sirius and the
strangeness with which it twinkled when near the horizon. In this
position a star not only twinkles, but changes its color rapidly. This
change is not sensible in the case of a faint star, but if one watches
Sirius when on the horizon, it will be seen that it not only changes in
appearance, but seems to blaze forth in different colors.
It seems to the writer that this conclusion of Schiaparelli is the
more likely of the two. From what we know of the constitution of
the stars, a change in the color of one of these bodies in so short
a period of time as that embraced by history is so improbable as
to require much stronger proofs than any that can be adduced from
ancient writers. In addition to the possible vagueness or errors of
the original writers, we have to bear in mind the possible mistakes or
misinterpretations of the copyists who reproduced the manuscripts.