266 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
or 4’’ per century. Second, the motion in the line of sight, or radial 
velocity, measured by the spectroscope, which again varies between 
0 and about 250 kilometers per second, the velocity of a faint star 
in the Southern Hemisphere determined last year. It is evident that 
in order to get the true direction and velocity of a star we must 
know, in addition to its radial motion, its velocity in kilometers per 
second at right angles to the line of sight. If its proper motion is 
known this can be readily computed when we know its distance, 
and hence we can obtain the direction and magnitude of its motion. 
In determining these motions we have to remember that we are 
on a moving body, the earth, which has a velocity of revolution 
around the sun of about 20 miles per second, and we must also 
remember that the sun, which is one of the stars, is also in motion. 
That this is the case has long been recognised, and the direction of 
this motion was determined from the proper motions of the stars by 
Sir William Herschel over 100 years ago. The method of doing this 
can be readily understood, for if we imagine the stars to be moving 
in all directions at random, it is, nevertheless, evident that in the 
portion of the sky which we are approaching, the general tendency 
will be for them to open out, while they will tend to close in in the 
opposite direction, and to drift backward at the sides. Hence, if the 
motion of the stars is at random, it is only a question of mathematics 
to determine the direction and magnitude of the sun’s motion in 
space. 
Over 20 different determinations, based upon the proper mo- 
tions of different numbers of stars, have been worked out, which 
all agree reasonably well in showing the sun to be moving toward the 
dividing line between Lyra and Hercules just a little south and east 
of the bright star Vega. This point has shifted around considerably 
between Hercules and Lyra, but the last determination, from Boss’s 
Preliminary General Catalogue, issued only last year, places it where 
T have just stated (R. A. 270.5°, Dec. +34.3°). 
If we consider, on the other hand, a determination of the apex of 
the sun’s way, as this point is called, derived from the radial velocities 
of stars, we find it to be in a somewhat different position. We have 
had three or four determinations of the solar apex from radial velocity 
measures; but none of these need be considered here except that 
obtained last year by Prof. Campbell, director of the Lick Observa- 
tory, the pioneer and foremost exponent of accurate radial velocity 
determinations, whose methods have been universally followed and 
their accuracy never excelled. I place the results of his 14 years’ 
work as the most important astronomical result announced during 
the year. In determining the velocity and direction of the sun’s 
motion, the radial velocities of 1,073 stars brighter than the fifth 
magnitude, well distributed over the sky, were used, 1,020 of which 
