Ball — On Stars ivith a Large Annual Parallax. 225 



The corresponding correction to be applied to the position angle o£ 

 the star which has no parallax, measured from the star which has 

 parallax, is 



- m'ttR cosec B cos (0 - M'), 



where I) is the distance of the two stars. The four quantities, m, m', 

 M, M', have been computed from the well-known formulae. 



It is also sometimes useful as a check to calculate the eiiect of par- 

 allax on the distance and position by the other method, which is, 

 indeed, much shorter than the general method just referred to, when 

 only one or two observations have to be reduced. 



Let a, S', be the right ascension and declination of the sun at the 

 time of the observation, then the following formulae are computed : — 



p cos cr = sin S', X cos /x. = cos 8' sin (a' - a), 



p sin o- - cos (a' - a), \ sin />(. = p COS (cr + S). 



The correction to the distance is 



+ ttRX sin {p + p.). 



The correction to the position is 



+ ttRX cosec B sin {p + p). 



