74 SCHULTZ-STEINHEIL, ON THE ELEMENTS OF THE SUN's ROTATION". 



In the fig. is the centre of the sun, TI the pole of tbe 

 ecliptic, P the pole of the true sun-equator, Q. the point of 

 intersection of the true sun-equator with the ecliptic, Y is the 

 equinoctial point and OF a line in the ecliptic rectangular to 

 OY . Take OA = x to be the velocity of rotation of a point 

 on the sun-equator round the true sunaxis, this velocity calcu- 

 lated from along OP. The projection of x on the three 



c 



axes OF, OY and OlI we will denote by ?', s and t resp., and 

 we have thus: 



r ^ ,2? sin i cos Q 



s = X sin i sin £2 



t = X cos i. 

 The plane IIOD represents a plane rectangular to the line 

 of sight at the moment of observation, M the point observed, 

 MOn = p the angle which makes M with the axis OU. Round 

 an axis OG perpendicular to OM and lying in the plane DOJI, 

 the point M seems to move with a velocity v^ which is exactly 

 the observed velocity. If we project x, r, s and t on this line 

 OG, we have: 



pr. r + pr. s + pr. t = pr. x 



