47] THE OBSERVATORY OF CAMBRIDGE. 381 



Apparent N.P.D. from the observation, given in the seventeenth column, 

 is obtained. Accordingly, when a circumpolar star is observed below the 

 pole, in which case S.P. is appended to the name of the star in the 

 second column, this apparent N.P.D. is affected with the negative sign. 



Occultations of Fixed Stars by the Moon. 



The following are the formulae employed in obtaining the Equations 

 of Condition given in this volume. 



Let T=mean local time of observation. 



1 = assumed longitude of place of observation, + when West. 

 T+ 1 = ( = approximate time on first meridian, 

 a, 8, the Moon's Right Ascension and Declination. 



TT, cr, the horizontal equatorial parallax and semi-diameter, all 

 calculated from the Ephemeris for the time t. 



Up to the end of 18G1 the quantities given in the Nautical Almanac are 



sin IT -, sin cr 



sin 1" 



sin 



... j 



subsequently the quantities given are TT and -7, 



sin J. 





Hansen gives <r = [4-750519]sin7r = [9-436094] jjj p- 



p = radius vector of place of observation, taking the Earth's equatorial 

 radius to be unity. 



<' = geocentric latitude. 

 = sidereal time corresponding to time T. 

 a', 8', the Right Ascension and Declination of the star occulted. 



sin (a a') -, 



Find x = > , cos 8, 



sin 1 



sin (8 8') . ~., , . i A 



y = ; -,T/ + * sin 8' tan (a - a ), 



sin i 



.. Sin 7T ;///) A 



c = T. . p cos <p sm ( t> a ), 



