f4^3 -■"-'■ ,• A NEV/ METHOD OF 



must be the right ascension of the moon's limb at that instant of sidereal time, 

 and, if we know the ARn. moon's Hmb at any given instant of time, we easily 

 deduce that of her centre, for the ARn. moon's centre rr ARn. moon's limbzh 

 Semid. in ARn. at the same instant. But the ARn. moon's centre, when her 

 limb is on the meridian, is not the same as when her centre is on the meridian. 

 The difference no wise depends on the time the moon takes in passing over 

 the meridian, nor is it affected, as has been supposed, by the augmentation ; 

 for the semi-diameter in right ascension is the angle subtended by the moon's 

 radius at the centre of the earth, and measured on the equator. Nor does the 

 apparent augmentation of the semi-diameter affect the right ascension of her 

 limb on the meridian, by accelerating or retarding its approach. Now, the 

 right ascension of the moon's limb, when on the meridian, is the same as the 

 right ascension of the meridian ; in other M^ords, it is the same as the sidereal 

 time at which the moon's limb is on the meridian. Also, apparent time is the 

 diiference between the right ascension of the meridian, and the right ascension 

 of the sun's centre at the same instant; and knowing the ARn. moon's limb 

 on the meridian, we deduce the apparent time, by subtracting from it the 

 sun's A Rn. given in the Nautical Almanac. 



Such, briefly, is the process of deducing the ARn. of the moon's limb and 

 centre, and thence the apparent time of observation, from the intervals be- 

 tween her transit and that of one or more stars, whose apparent right ascen- 

 sions are known. From these data^ the longitude of the place of observation 

 is deduced in the following manner.* 



Rule 1st. — Flaving, in the manner already explained, found the right 

 ascension of the moon's limb, find also the apparent time of observation. 



* This method was first communicated to the late Colonel Blacker, by the Author, on his return 

 from the Cape of Good Hope, in May, 1825, 



