Astronomical and Nautical Collections. 



343 



iii. Rules for compelling an observed Occultation. 

 Nautical Alma^iac for 1827. 



JVb'm the 



Dr. Young's Method. 

 I. Observe, if possible, tlie difference of apparent altitudes at 

 the time of immersion or emersion ; or at least the altitude of the 

 moon, the altitude of the star being computed from the true latitude 

 of the place. 



Example : — Supposing the immersion of S i^ to be observed at 

 Greenwich, the 5th of Jan. 1824, at 3'' 46"" 50', the moon's alti- 

 titude, corrected for refraction, being 29° 15' 37" : we have for 

 the declination of the star 8° 39' 17", that of the moon being 

 7° 47' 12" at the observed time, and the difference of declination, 

 from the elements in the Nautical Almanac, 55' 31" : the moon's 

 right ascension, 22'' 7" 2\ that of the star being 22'' 7"" 32= : the 

 sun's right ascension, 19'' 2"" 19'; the star's less the sun's, 

 3" 5-" 13^ which, deducted from 3" 46" 50% gives 41'" 37^ for 

 the star's horary angle. With these elements we proceed to com- 

 pute the altitude of the star. 



■^'s Log. rising .... 41' 37" . . . 3.21594 



Log. cos. declination . . 8° 39' 17" . . . 9.99503 



COS. Lat. ... 51 28 40 . . . 9.79436 



II. Having found the difference of the true altitudes from the 

 difference of the apparent altitudes combined with the parallax of 

 the moon, add together the squares of the semidiameter, properly- 

 augmented, and of the difference of the true altitudes, and subtract 



