[PLASKETT-DELURY] THE SOLAR ROTATION 29 
Nearly all of Adams’ plates were made with the observed points close 
to the limb, and this final correction is in the majority of cases in- 
appreciable and only reaches in a few plates, around latitudes 45° and 
60°, 0.01 km. per second. Nevertheless, as it is always in the same di- 
rection, it should be applied. This is especially necessary in our own 
observations where the distance from the sun’s limb is frequently much 
greater and where the value of the correction may reach 0.03 km. per 
second. Two methods have been followed here in reducing the ob- 
served to the actual velocity. The first consists in applying a correc- 
tion to Adams’ method for the change in angular velocity, thus ob- 
taining the sidereal rate at the radially projected point on the limb, 
while the second determines the corrections to be applied to obtain the 
sidereal velocity at the observed points. In order to make the methods 
clearly understood it will be desirable to give a brief summary of the 
formule used. 
Let R — Radius of sun’s dise. 
r -— Distance of observed points from centre of disc. 
4 = Position angle of observed point. 
gy = Heliographic latitude of observed point. 
À — Difference of heliographic longitude between the ob- 
served point and the earth. 
D = Heliographic latitude of the earth. 
i == Inclination of sun’s equator to ecliptic — 7° 15’. 
£ — Longitude of ascending node of sun’s equator on 
ecliptic == 74° 31’.* 
© = Longitude of the sun. 
p — Angular distance of observed point from centre of 
apparent dise as viewed from sun’s centre. 
7 — Angle between direction of motion and line of sight. 
s — Sidereal correction at limb (Dunér’s Tables). 
v — Measured velocity (linear). 
V = Corrected velocity. 
£ — Daily angular sidereal velocity. 
11. First Method—Projection to Limb. 
Latitude at limb. Sin g = cos Y sin D 
LR Ss] . si LS, 2) 
Angle at limb Sine — sin 7 sin (© — 2) 
COS @ 
* At the time of writing new values obtained by the Maunders for 7 and @ have 
appeared; but these corrections would introduce only quite inappreciable changes in 
a 
our,computed values. 
