IOM 



NAVIGATION NAUTICAL ASTRONOMY. 



[TIME AT SKA. 



initmoietit, when corrected for index error, U double the 

 altitude from the rational horizon. 



Angle by Inirtniment . 

 Index correction . 



App. alt of star . 

 Refra 



fr 



. 8953'30* 



10 



2)89 53 20 



44 60 40 



68 



True alt .. 44 55 42 



90 



.'.ZS- 45 4 18 



12h. 9m. 

 4m. 



Estimated timo . . . 

 Long. 1 6' NV. ... 



Mean time at Greenwich . 12h. 13m. 

 Sun's R. A. at noon, or the 



sidereal time 

 Correction for 12k 13m. 



4h. 46m. 7s.'l 

 + 2m. Oa. -4 



4h. 48m. 7s.-5 



Sun's II. A. at time of oba. 



Stars R. A. 14h. 8m. 30s. -5, Dec. 20 V 15" N., .'.PS 

 - 69 W 46". Also P Z = 39 12'. 



Z S, 45 4' 18' 



sin. PZ, 39 12 Arith. Comp. MO-'o-.N 

 sin. P 8, 69 69 45 Arith. Comp. -0270256 



2)154 16 3 



sin. s, 77 8 2 

 in.(s ZS), 32 344 



cos. i P, 21 1 1 J 



. 0-0880570 

 . 9-7249635 



2)19-9402089 

 . 9-9701044 



.'. P - 42 



Star's R A. 



2 3 



or 2h. 48m. 8s -2 of sid. time. 

 Uh. 8m. 30s. -5 



I : A. of meridian 

 R. A. of sun . 



Mli. f>Gm. 38s.-7 

 4h. 48m. 7s. -5 



Mean time at ship . 12h. 8m. 31s.-2 



It will be observed here that the hour-angle P is ex- 

 pressed in sidereal time ; and in like manner the right 

 ascensions of the star, the meridian, and the mean sun, 

 are all expressed in sidereal time : hence, 12h. 8m. 31s. -2 

 U the distance of the mean sun from the meridian in 

 sidereal time ; and this time in reference to the mean 

 sun is mean time. 



Esampleifor Exereite. 



1. On January 12th, 1840, in latitude 30 55' N. and 

 longitude 14 W., by account, the mean of a set of alti- 

 tude* of the sun's lower limb was 22 55' 33"; the corre- 

 sponding time by watch was 9h. 31' 33" A.M. ; the index 

 correction ww + 4' 30", and the height of the eye 30 

 feet. Also die K'atitical Almanac gave the following 

 particulars, namely 



': dec., Jan. llth . 21 54' 30" S. 



Hourly variation . . 23" '42 



Kqna. of time, Jan. llth 8" 3" 



Hourly variation . . + 0" 1* 



wmi-diampter . 16' 16" 



Required the mean time at the place of observation. 



Ans. 9h. 41' 24" A.M. 



2. On April 18th, 1844, in latitude 50 48' N., and 

 longitude by account 1 V W. the mean of a set of alti- 

 tude* of the sun's lower limb (with artificial horizon) 

 76 W 46"; the corresponding time by watch was 

 9h. 18m. A.M. : the index correction was 3' 46"; and 



38 



0' 31' '' 

 " n"f>73 

 1"> 



the Nautical Almanac gave the following particulars, 

 namely 



Sun's dec., April 17th . 10 30' 40" N. 



Hourly variation . 



Equa. of time, April 17th 



Hourly variation . . 



Sun's semi-diameter . 

 Required the mean time at the place of observation. 



All*. Oil. ISlll. 1(H. A.M 



3. On April 2Gth, 1840, in latitude 20 47' 45" S., a i..l 

 longitude by account 31 7' E., at 2h. 19m. 41s. A.M. bv 

 watch, the true altitude of the star Alt, iir was 25 11 

 to the east and north. Required the mean time at the 

 place of observation. 



R.A. of Altair 10h. 43m. Decli. 8 20' 47"N. Sun's 

 R.A., or sidereal time at mean noon, 2h. 18m. 21s. -1. 



Ans. Hi. 45m. lla. A.M. 



NOTE. As remarked at page 109:1, the time may be 

 found in a similar way when the object is the moon 

 1 of the sun or a star ; but, oil account of the 

 rapidity of the moon's motion in right ascension ninl 

 declination, this body is the least eligible for the purpose 

 of discovering the time at sea. The approximate time 

 and the approximate longitude which, as we have seen 

 in the case of the sun (page 1095), may be employed with 

 safety, may lead to considerable uncertainty when the 

 object is the moon, of which the declination changes 

 sometimes more than 2' in lOin. of time. 



TIME DEDUCED FROM EQUAL ALTITUDES. There is 

 another way of determining the time, which ought to bo 

 briefly noticed. It is called the method of f/>M/ ultif 

 the following exposition of it is principally from Lieut. 

 Raper's Practice, of Navigation and A'aiitUal Astronomy 

 a work which well deserves the attention of the prac- 

 tical seaman. 



Since the altitude of a body which does not change its 

 declination, varies exactly at the same rate while rising 

 on the E. side of the meridian, as while falling on the 

 W. side, the same altitude occurs at the same hour , 

 on each side of the meridian, and the middle point of 

 time, between the instants of two equal altitudes, is the 

 instant at which the body passes the meridian. 



In the case of the sun, the middle point of time, or tin- 

 mean of the observed times of equal altitudes, A.M. and 

 p.if., is apparent noon. In the case of a star, or other 

 body, the mean of these times corresponds to the R. A. of 

 the star when upon the meridian, or the sidereal time, which 

 may easily be converted into apparent time, or mean time. 



Since the sun changes hi.s declination sensibly in lar-e 

 intervals of time, two equal altitudes, A.M. and P.M , do 

 not correspond to equal hour-angles ; and it therefore 

 becomes necessary to apply to the mean of the obsi -i 

 times a correction which is called the of 



/.',/'// Altitudes. A table of the equation of cqn.il 

 altitudes is to be found in all nautical tables. Also nt 

 sea, the change of place of the ship, in the intervals 

 between the altitudes, will generally render another 

 correction nfecessary. When the course made p->od 

 is true E. or W., the ship changes in her longitude 

 ou!v, by the portion of time, which she gains or loses 

 on the sun in intervals. This change, provided it is 

 made good equally on both sides of the meridian, 

 introduces no correction ; and the only question is, the 

 time by watch when the interval is half expired. 



Hut when the ship changes her latitude, tlve same alti- 

 tude no longer corresponds to the same, time from noon, 

 and the correction adverted to becomes necessary. 



This method has some advantages : it is independent 

 of the terrestrial refraction, provided this remains un- 

 changed in the interval employed: and the correction, 

 when necessary, requires the latitude and altitude to be 

 but roughly known. Within the tropics, the interval may 

 in general bo very small, on account of the rapid change 

 of the altitude ; and the correction for change of latitude 

 may, in such cases, be omitted. In hii;li latitudes, how- 

 ever, the ship's change of latitude considerably alters the 

 time from noon, at which the second altitude (equal to 

 the first) is taken : hence, in such high latitudes the 

 method is loss useful. 



