- 4.2 - 7.2 - 10.5
Pollux - 0.7 + 1.9 + 0.4
g Leonis - 39.9 - 36.5 - 40.5
Arcturus - 6.4 - 7.0 - 8.3

These results have been corrected for the earth's orbital motion round
the sun, but not for the sun's motion through space, as the amount of
the latter is practically unknown, or at least very uncertain; so that
the above figures really represent the velocity per second of the
various stars relative to the sun. We may add that the direction and
velocity of the sun's motion may eventually be ascertained from
spectroscopic measures of a great number of stars, and it seems likely
that the sun's velocity will be much more accurately found in this way
than by the older method of combining proper motions of stars with
speculations as to the average distances of the various classes of
stars. This has already been attempted by Dr. Kempf, who from the
Potsdam spectrographic observations found the sun's velocity to be 18.6
kilometres, or 11.5 miles per second, a result which is probably not far
from the truth.

But the spectra of the fixed stars can also tell us something about
orbital motion in these extremely distant systems. If one star revolved
round another in a plane passing through the sun, it must on one side of
the orbit move straight towards us and on the other side move straight
away from us, while it will not alter its distance from us while it is
passing in front of, or behind, the central body. If we therefore find
from the spectroscopic observations that a star is alternately moving
towards and away from the earth in a certain period, there can be no
doubt that this star is travelling round some unseen body (or, rather,
round the centre of gravity of both) in the period indicated by the
shifting of the spectral lines. In Chapter XIX. we mentioned the
variable star Algol in the constellation Perseus, which is one of a
class of variable stars distinguished by the fact that for the greater
part of the period they remain of unaltered brightness, while for a very
short time they become considerably fainter. That this was caused by
some sort of an eclipse--or, in other words, by the periodic passage of
a dark body in front of the star, hiding more or less of the latter from
us--was the simplest possible hypothesis, and it had already years ago
been generally accepted. But it was not possible to prove that this was
the true explanation of the periodicity o