Vol.. 8, 1922 
ASTRONOMY: ADAMS AND JOY 
173 
in this manner indicate the existence of a libration. According to all 
indications, the motion of these planets is therefore entirely stable. The 
very extensive and accurate computations of Berberich have furnished 
very reliable sets of osculating elements of Andromache. With Berber- 
ich's elements osculating for 1877 and Miss Levy's development of the 
perturbations on the basis of the revised tables, an observation for the year 
1920 has been represented with a remarkable degree of accuracy, so that 
in this case, also, the process of computing the special perturbations from 
opposition to opposition and frequent observations may be abandoned for 
many decades to come. 
It is hoped that the results for 10 Hygiea and 175 Andromache here 
presented will prove a determining factor in the methods hereafter to be 
applied by astronomers in deriving the approximate perturbations by Jup- 
iter of planets of the Hecuba Group and of other groups with mean mo- 
tion commensurable to that of Jupiter to which Bohlin's original group 
theory is applicable with proper modifications and extensions. These 
studies should form a most fruitful field of research in theoretical astron- 
omy. 
A METHOD OF DERIVING THE DISTANCE OF 
THE A-TYPE STARS 
By W. S. Adams and A. H. Joy 
Mount Wilson Observatory, Carnegie Institution of Washington 
Read before the Academy, April 25, 1922 
Our knowledge of the distances of individual stars of the A -type of spec- 
trum is obtained almost wholly from three sources. 
1. Trigonometric parallaxes. 
2. Dynamical, sometimes called hypothetical, parallaxes of binary 
stars. 
3. Parallaxes derived from group motion. 
The third method probably furnishes the most accurate values for all 
such stars as belong to well-recognized groups. In the case of the Taurus 
Group, for example, the distances are known with a high degree of pre- 
cision. The trigonometric parallaxes are, of course, most valuable in the 
case of large values for which the probable error bears a relatively small 
ratio to the quantities measured. The dynamical parallaxes derived 
from binary stars with well-determined orbital motion are of excellent 
quality, but are affected to some extent by the uncertainty in the value 
of the mass-factor for the A -type stars which enters directly into the com- 
putation. 
