LONGITUDE OF OBSERVATORY BAY. 417 



intersection and the Pole be called 0, then, y being the astronomical co- 

 latitude, 



tan = cos hour-angle X tan Normal-centric S.P.D., 



., tan hour-angle x sin B 



tan azimuth of center = - 8 in (fl ) - 

 cot zen. dist. of center = cot (3 7) . cos azimuth. 



The approximate apparent zenith distance, column 7, has been obtained by 

 adding, to the Z.D. thus found, the approximate parallax in Z.D. taken from 

 Table 39 of Raper's Navigation. 



The azimuths are reckoned from South through West to 360. 



The azimuthal semidiameter, to be applied to the above azimuth of the 

 center to obtain that of the limb, is found by multiplying the geocentric 

 semidiameter (in seconds of arc) by the cosecant of the tabular normal- 

 centric zenith distance of the center. The small correction required to 

 reduce the geocentric semidiameter to the normal-centric diameter has 

 generally been taken into account, although its greatest value is 0"'l. 



If j, a. 2 be the two tabular azimuths of the limb, corresponding to the first 

 and second assumptions of longitude respectively, and a be the observed 

 azimuth, the longitude inferred from each observation is 



6 9X fo-"") 



4 h . 39 m . 0-0 



a l 



The details of the extensive calculations are suppressed, for the same 

 reason which applied to those of the Altazimuth Zenith Distances at 

 Rodriguez. 



ON THE LONGITUDE OF OBSERVATORY BAY. 



(1.) Longitude of Observatory Bay from the observed Bight Ascension of the 



Moon on the Meridian. 



In Table IV. are given the transits of the Moon as observed. The 

 incomplete transits have been reduced as described in Part I., page 22. In 

 Table VI. the longitude is computed from each observation, except a few 

 when the clock and instrumental errors cannot be inferred with safety. The 

 weights assigned are proportional to the square of the change of R.A. in 1 s , 

 reduced in certain cases. 



3 H 2 



