others are increasing it. But when we project the motions on a map we find that the actual direction is very nearly the same for all five stars, and the reason why some move slightly to the north and others slightly to the south is due to the divergence of the circles of right ascension. It is worthy of remark that the community of motion is also shown by spectroscopic observations of the radial motions described below.
The five stars in question are all of the second magnitude except δ, which is of the third. It is a curious fact that no fainter stars than these five have been found to belong to the system.
From a study of these motions Höffler has concluded that the five stars lie nearly in the same plane and have an equal motion in one and the same direction. From this hypothesis he has attempted to make a determination of their relative and actual distances. The result reached in this way cannot yet, however, be regarded as conclusive.
There are three stars in Cassiopeia, β, η and μ, each having a large proper motion in so nearly the same direction that it is difficult to avoid at least a suspicion of some relation between them. The angular motions are, however, so far from equal that we cannot regard the relation as established.
In the constellation Taurus, between Aldebaran and the Pleiades, most of the stars which have been accurately determined seem to have a common motion. But these motions are not yet so well ascertained that we can base anything definite upon them. They show a phenomenon which Proctor very aptly designated as star-drift.
The systems we have just described comprise stars situated so far apart that, but for their common motion, we should not have suspected any relation between them. The community of origin which their connection indicates is of great interest and importance, but the question belongs to a later chapter.
MOTIONS IN THE LINE OF SIGHT, OR RADIAL MOTIONS.
No achievement of modern science is more remarkable than the measurement of the velocity with which stars are moving to or from us. This is effected by means of the spectroscope through a comparison of the position of the spectral lines produced by the absorption of any substance in the atmosphere of the star with the corresponding lines produced by the same substance on the earth. The principle on which the method depends may be illustrated by the analogous case of sound. It is a familiar fact that if we stand alongside a railway while a locomotive is passing us at full speed, and at the same time blowing a whistle, the pitch of the note which we hear from the whistle is higher as the engine is approaching than after it passes. The reason is that the pitch of a sound depends upon the number of sound beats per second.