Besides its more direct use in the chemical analysis of the heavenly
bodies, the spectroscope had given to us a great and unexpected power
of advance along the lines of the older astronomy. In the future a
higher value might, indeed, be placed upon this indirect use of the
spectroscope than upon its chemical revelations. By no direct
astronomical methods could motions of approach or of recession of the
stars be even detected, much less could they be measured. A body
coming directly toward us or going directly from us appeared to stand
still. In the case of the stars we could receive no assistance from
change of size or of brightness. The stars showed no true disks in our
instruments, and the nearest of them was so far off that if it were
approaching us at the rate of a hundred miles in a second of time, a
whole century of such rapid approach would not do more than increase
its brightness by the one-fortieth part. Still it was formerly only
too clear that, so long as we were unable to ascertain directly those
components of the stars' motions which lay in the line of sight, the
speed and direction of the solar motion in space, and many of the
great problems of the constitution of the heavens must have remained
more or less imperfectly known. Now the spectroscope had placed in our
hands this power, which, though so essential, had previously appeared
almost in the nature of things to lie forever beyond our grasp; it
enabled us to measure directly, and, under favorable circumstances, to
within a mile per second, or even less, the speed of approach or of
recession of a heavenly body. This method of observation had the great
advantage for the astronomer of being independent of the distance of
the moving body, and was, therefore, as applicable and as certain in
the case of a body on the extreme confines of the visible universe, so
long as it was bright enough, as in the case of a neighboring planet.