When a star is found to be seemingly in motion, as described in the
last section, we may ascribe the phenomenon to a motion either of the
star itself or of the observer. In fact no motion can be determined
or defined except by reference to some body supposed to be at rest.
In the case of any one star, we may equally well suppose the star to
be at rest and the observer in motion, or the contrary. Or we may
suppose both to have such motions that the difference of the two shall
represent the apparent movement of the star. Hence our actual result
in the case of each separate star is a relation between the motion of
the star and the motion of the sun.
I say the motion of the sun and not of the earth, because although the
observer is actually on the earth, yet the latter never leaves the
neighborhood of the sun, and, as a matter of fact, the ultimate result
in the long run must be a motion relative to the sun itself as if we
made our observations from that body. The question then arises whether
there is any criterion for determining how much of the apparent motion
of any given star should be attributed to the star itself and how much
to a motion of the sun in the opposite direction.
If we should find that the stars, in consequence of their proper
motions, all appeared to move in the same direction, we would naturally
assume that they were at rest and the sun in motion. A conclusion of
this sort was first reached by Herschel, who observed that among the
stars having notable proper motions there was a general tendency to
move from the direction of the constellation Hercules, which is in the
northern hemisphere, towards the opposite constellation Argo, in the
southern hemisphere.
Acting on this suggestion, subsequent astronomers have adopted the
practice of considering the general average of all the stars, or a
position which we may regard as their common center of gravity, to be
at rest, and then determining the motion of the sun with respect to
this center. Here we encounter the difficulty that we cannot make any
absolute determination of the position of any such center. The latter
will vary according to what particular stars we are able to include
in our estimate. What we can do is to take all the stars which appear
to have a proper motion, and determine the general direction of that
motion. This gives us a certain point in the heavens toward which the
solar system is traveling, and which is now called the _solar apex_, or
the apex of the solar way.
The apparent motion of the stars due to this motion of the solar system
is now called their _parallactic motion_, to distinguish it from the
actual motion of the star itself.
The interest which attaches to the determination of the solar apex has
led a great number of investigators to attempt it. Owing to the rather
indefinite character of the material of investigation, the uncertainty
of the proper motions, and the additions constantly made to the number
of stars which are available for the purpose in view, different
investigators have reached different results. Until quite recently, the
general conclusion was that the solar apex was situated somewhere in
the constellation Hercules. But the general trend of recent research
has been to place it in or near the adjoining constellation Lyra. This
change has arisen mainly from including a larger number of stars, whose
motions were determined with greater accuracy.
Former investigators based their conclusions entirely on stars having
considerable proper motions, these being, in general, the nearer to us.
The fact is, however, that it is better to include stars having a small
proper motion, because the advantage of their great number more than
counterbalances the disadvantage of their distance.
The conclusions reached by some recent investigators of the position of
the solar apex will now be given. We call A the right ascension of the
apex; D its declination.