1063 



NAVIGATION NAUTICAL ASTRONOMY. 



[LATITCD*. 



emi-diameter, and increase it by the " Aup n 



tan in the Nautical Tables. The correction for in 1. \ 



error, dip, and semi-diameter, will reduce tho observed 



altitude of the limb to the apparent altitude of the 



oontre. 



ft To the apparent altitude of the centre add the 

 correction in altitnde, that in, the parallax in altitude 

 minus the refraction, and the true altitude of the centre 

 will be obtained. Subtract this from 90, and then, by 

 adding or subtracting tho moon's declination :it tin- tiim-, 

 u in the case of the sun, the latitude of the place will be 

 Mcertained. 



Examples. 



1. On May 27th, 1846, in longitude 49 W., the 

 meridian altitude of the moon's lower limb was found to 

 be 47 18' 30* (enith S.) ; the index correction was + 

 1' 40*, and the height of the eye 20 feet Required the 

 latitude of the ship. 



1. For the mean time at Greenwich when the observation 

 was made. 



Moon's transit at Greenwich, 



May 27th . . . . lh. 56m. Dn ? rution, 

 Cor. for long. 49 W. and 48m. 



variation . . . + 6m. 



is-7m. 



Time at ship when alt. was taken 21i. 2m. 

 Long. 49 W. in time . . 3h. 16m. 



Time at Greenwich when alt. was 



taken ..... Ch. 18m. 



2. For the moon's declination at that time. 



Declination May 27th, at 6h. . 18 56' 16" N. v r -.! n . 1 . 0m - 

 Decrease in 18m. = 2" -655X18= -48" 



Declination at 5h. 18'. 



. 18 55' 28" N. 



3. For the true altitude of the Moon's centre, and thence 

 the latitude. At page 3, of MAY, in the Nautical 

 Almanac, the moon's semi-diameter at noon on the 27th 

 is 15 1* ; and her horizontal parallax at the same time is 

 55' 16". Hence we get the latitude thus : 



Observed alt. of moon's L. L. 

 Index cor. . . + 1' 40" 

 Dip . . . . 4' 24" 

 Semi-diameter ) , , ., , r , 



+ 11* for augmen. J "* 

 App. alt. of moon's centre . 

 Parallax and refraction 



True alt. of moon's centre 



True zenith distance 

 Moon's declination 



. 47 18' 30' 



. + 12' 32" 



. 47 31' 2" 



. + 36' 10* 



48 

 90 



7' 12" 



41 52' 48 'S. 

 18 55' 28'N. 



Latitude . . . . . . 22 57' 20"S. 



It is plain that whatever be tho celestial object ob- 

 served, the error in the latitude will l>e the sum of the 

 errors in the zenith distance and declination when both 

 are of the same name, and the difference when they are 

 Mtrary names. 



The latitude at sea is seldom computed to seconds, as 

 tli- exact longitude and time of the ship, as inferred 

 from the dead-reckoning, generally deviates from the 

 truth. It is customary, therefore, to aim at deducing 

 the latitude only to the nearest minute : thus, in the 

 present example, the latitude would bo concluded to be 

 22 67' 8. 



2. On Dec. 7th, 1840, in longitude 16 W., at lOh. 

 43m. apparent time, tho meridian altitude of the moon's 

 lower limb was 83 7' (zenith S.); the index correction 

 wai - 1' 66', and the heightof the eye 16 feet. Reqnn, ,1 

 ititude to the nearest minute. Here the time at 

 the ship being girea, we shall not require to take out of 



the Kant irnl Almanac the time of the moon's meridian 

 passage at Greenwich : tin- (irn-nwii-li .Lit,-, or mean 

 tin. i- at Greenwich when tho altitude was taken, is found 

 thus : 



1. For the mean time at Greenwich when the observation 

 was made. 



Apparent time at ship, Dec. 7 . 

 Long. 16 W. in time . . . 



App. time at Greenwich . . 

 Equation of time (Nattt. Aim., p. 1) 



lOh. 43m. 

 lh. 4m. 



llh. 47m. 

 8m. 



llh. 39m. 



Mean time at Greenwich . . 



2. For the Moon's declination at that time. 



Declination, Dec. 7, at llh. . . . 24 45' N. 



(inc.) Var. iu 1<X, 8*. 

 Increase in 39m +6' 



Declination at llh. 39m. . 24 61' N. 



3. For the true altitude of the Afoon'* centre, and thence 

 the Uitititdi: 



Observed alt. of moon's L. L. 

 Index cor. . 2" 

 Dip . . - 4' 

 Serni-diain. . + 17' , 



App. alt. moon's centre 

 Parallax and refraction 



True alt. of moon's centre 



83 25' 

 90 



6 35' S. 

 24 51' N. 



. 1815'N. 



True zenith distance 

 Moon's declination 



Latitude . . 



Examples for Exercise. 



NOTE. In the following examples the latitude is to be 

 determined to the nearest minute. 



1. On the 2nd of May, 1833, the meridian altitude of 

 the sun's lower limb was 47 20' (zenith N.) ; the index 

 correction 2'; the height of the eye 20 feet, and the 

 longitude by account 32^ E. 



Also sun's dec. May 2nd, 15 23' 21* N. (inc.) Hourly 

 difference 45*. 



Semi-diameter 15' 63* (Nautical Almanac). 

 Required the latitude to the nearest minute. 



Ans. Latitude, 58 53' N. 



2. On Jan. 9th, 1840, in longitude 116 \V. , the meridian 

 altitude of the sun's upper limb was found to be 69 14' 

 (zenith N.) ; index error 0, and height of the eye 27 feet. 

 Required the latitude to tho nearest minute. 



Sun's dee. Jan. 9, 22 12' S. (dec.) Correction for 

 long. 116 W., - 3'. Semi-diam. 16'. 



Ans. Latitude, 1 2^ S. 



3. On May 15th, 1828, the meridian altitude of tho 

 star, Spica, was observed to be 30 17' (zenith N. ) : imli-x 

 correction -f 1' 10" ; the star's declination was 10 111 S. 

 Hi-quired the latitude to tho nearest minute, the dip 

 being 6. 



Ans. Latitude, 46 32' N. 



4. The mori<lian altitude of the star Rigel was ob- 

 served to be 85 4' (zenith N. ) ; the index correction was 

 + 2 1 , and the star's dwlinati.m 8 22' 45* S. ; the height 

 of the eye was 20 feet. Required the latitude to the latest 

 minute. 



Ans. Latitude, 3 24' S. 



5. On February 19th, 1823, in longitude 40 W., the 

 meridian altitude of the moon's lower litnl> was observed 

 to be 55 8' (zenith N.) ; the index correction was 2', 

 and tho height of the eye 16 feet. 



