motion and parallax must also be known.

When Huggins first applied the Doppler principle to measure velocities
in the line of sight,[7] the faintness of star spectra diminished the
accuracy; but Vogel, in 1888, overcame this to a great extent by long
exposures of photographic plates.

It has often been noticed that stars which seem to belong to a group
of nearly uniform magnitude have the same proper motion. The
spectroscope has shown that these have also often the same velocity in
the line of sight. Thus in the Great Bear, beta, gamma, delta,
epsilon, zeta, all agree as to angular proper motion. delta was too
faint for a spectroscopic measurement, but all the others have been
shown to be approaching us at a rate of twelve to twenty miles a
second. The same has been proved for proper motion, and line of sight
motion, in the case of Pleiades and other groups.

Maskelyne measured many proper motions of stars, from which W.
Herschel[8] came to the conclusion that these apparent motions are for
the most part due to a motion of the solar system in space towards a
point in the constellation Hercules, R.A. 257 degrees; N. Decl. 25
degrees. This grand discovery has been amply confirmed, and, though
opinions differ as to the exact direction, it happens that the point
first indicated by Herschel, from totally insufficient data, agrees
well with modern estimates.

Comparing the proper motions and parallaxes to get the actual velocity
of each star relative to our system, C.L. Struve found the probable
velocity of the solar system in space to be fifteen miles a second, or
five astronomical units a year.