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Page 4
The next question to be considered is in what direction we may expect
the greatest advance in astronomy will be made. Fortunate indeed would
be the astronomer who could answer this question correctly. When Ptolemy
made the first catalogue of the stars, he little expected that his
observations would have any value nearly two thousand years later. The
alchemists had no reason to doubt that their results were as important
as those of the chemists. The astrologers were respected as much as the
astronomers. Although there is a certain amount of fashion in astronomy,
yet perhaps the best test is the judgment of those who have devoted
their lives to that science. Thirty years ago the field was narrow. It
was the era of big telescopes. Every astronomer wanted a larger
telescope than his neighbors, with which to measure double stars. If he
could not get such an instrument, he measured the positions of the stars
with a transit circle. Then came astrophysics, including photography,
spectroscopy and photometry. The study of the motion of the stars along
the line of sight, by means of photographs of their spectra, is now the
favorite investigation at nearly all the great observatories of the
world. The study of the surfaces of the planets, while the favorite
subject with the public, next to the destruction of the earth by a
comet, does not seem to appeal to astronomers. Undoubtedly, the only way
to advance our knowledge in this direction is by the most powerful
instruments, mounted in the best possible locations. Great astronomers
are very conservative, and any sensational story in the newspapers is
likely to have but little support from them. Instead of aiding, it
greatly injures real progress in science.
There is no doubt that, during the next half century, much time and
energy will be devoted to the study of the fixed stars. The study of
their motions as indicated by their change in position was pursued with
great care by the older astronomers. The apparent motions were so small
that a long series of years was required and, in general, for want of
early observations of the precise positions of the faint stars, this
work was confined mainly to the bright stars. Photography is yearly
adding a vast amount of material available for this study, but the
minuteness of the quantities to be measured renders an accurate
determination of their laws very difficult. Moreover, we can thus only
determine the motions at right angles to the line of sight, the motion
towards us or from us being entirely insensible in this way. Then came
the discovery of the change in the spectrum when a body was in motion,
but still this change was so small that visual observations of it proved
of but little value. Attaching a carefully constructed spectroscope to
one of the great telescopes of the world, photographing the spectrum of
a star, and measuring it with the greatest care, provided a tool of
wonderful efficiency. The motion, which sometimes amounts to several
hundreds of miles a second could thus be measured to within a fraction
of a mile. The discovery that the motion was variable, owing to the
star's revolving around a great dark planet sometimes larger than the
star, added greatly not only to the interest of these researches, but
also to the labor involved. Instead of a single measure for each star,
in the case of the so-called spectroscopic binaries, we must make enough
measures to determine the dimensions of the orbit, its form and the
period of revolution.
What has been said of the motions of the stars applies also, in general,
to the determination of their distances. A vast amount of labor has been
expended on this problem. When at length the distance of a single star
was finally determined, the quantity to be measured was so small as to
be nearly concealed by the unavoidable errors of measurement. The
parallax, or one half of the change in the apparent position of the
stars as the earth moves around the sun, has its largest value for the
nearest stars. No case has yet been found in which this quantity is as
large as a foot rule seen at a distance of fifty miles, and for
comparatively few stars is it certainly appreciable. An extraordinary
degree of precision has been attained in recent measures of this
quantity, but for a really satisfactory solution of this problem, we
must probably devise some new method, like the use of the spectroscope
for determining motions. Two or three illustrations of the kind of
methods which might be used to solve this problem may be of interest.
There are certain indications of the presence of a selective absorbing
medium in space. That is, a medium like red glass, for instance, which
would cut off the blue light more than the red light. Such a medium
would render the blue end of the spectrum of a distant star much
fainter, as compared with the red end, than in the case of a near star.
A measure of the relative intensity of the two rays would servo to
measure the distance, or thickness of the absorbing medium. The effect
would be the same for all stars of the same class of spectrum. It could
be tested by the stars forming a cluster, like the Pleiades, which are
doubtless all at nearly the same distance from us. The spectra of stars
of the tenth magnitude, or fainter, can be photographed well enough to
be measured in this way, so that the relative distances of nearly a
million stars could be thus determined.
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