PROF. TROUTON AND MR. NOBLE ON THE MECHANICAL FORCES ACTING 
72 
Injiuence of the. Suns Proper Motion. 
lu the above we have only considered the drift due to the motion of the earth round 
the sun ; but it is also necessary to consider the effect when the sun’s proper motion 
is included. The magnitude of this motion has been investigated by Newcomb 
(‘Astronomical Journal,’ No. 457, pp. 4, 5), Kapteyx (‘Astronomische Nachrichten,’ 
No. 3487, p. 104), Campbell (‘ Astrophysical Journal,’ vol. 13, No. 1). The result 
of these investigations is, that there is stdl considerable doubt as to the magnitude 
and direction of this quantity; no very exact treatment is therefore possible. We 
have adopted for this calculation the rough value 15 miles/sec., in a direction whose 
right ascension is 270° and declination 38i°. This corresponds very nearly to the 
position of a Lyrm (Vega). 
In the diagram (fig. 7) let OQ be the component of the drift due to the annual 
motion. Let V lie tlie position of Vega, which Ave shall consider as having- 
right ascension = 270° = ^ and decl. = 38 V; and let OD be the resultant of both 
w 
components. 
N 
Fig. 7. 
Let V' be a point diametrically opjiosite Vega (V). Then, if B = time alter G a.m., 
PN V = (IIA Vega - RA Sun) + - B) 
= 18/l — Of + -g-TT — B = 2tt — ct — B, 
PNV' = PNV - 77 = (tt - d - «).(i.), 
NV = {hTT - 38V), NV' = (U + 381°).(ii.), 
OV' is the direction of di-ift due to the sun’s proper motion, which, as Ave have said. 
