ueo 



NAVIOATIO PICAL A >T K<>XOMY. 



[MTtTDDl. 



;h kt w the dip generally differs but little through- 

 out the voyage.* 



It my bo remarked, that in oompaungthe dedmiition 

 of the Mm f..r tin- timo of observation, MOOnd* of tiuu- 

 may bo disregarded, and even a few iniir.r. 

 practical coiuoq'i. :>. : indeed, the longHodfl by account 

 u only an approximation to the correct longitude. 



is n little oonaideratiomrin be sufficient to show, 

 the error in the eatimat-d longitude must 1)0 very great 

 indeed to occasion tiny error of practical importai: 

 the dp. -hnation of tlie uu at the instant of OMrtmnon. 

 If only tho altitiulo be observed with proper care, the 

 may trust to the accuracy of the resulting 

 i.lc without any nice determination of his longitude : 

 1 .if longitude is only 4' of time. 



a. On August 14th, 1846, in north latitude, and in 

 longitude 51' W., tho meridian altitude of the sun's 

 upper limb was 47 26' (zenith N.) ; the index correction 

 1* 47', and the height of the eye 20 feet. Required 

 tho latitude. 



1. For the apparent time at Greenwich. 

 Long, by account .... 61 \V. 



App. time at Greenwich 



3)10,2 

 3h. 24m. 



The variation for this time must be subtracted, as the 

 declination is decreasing. 



2. For the sun's declination. 

 Dec. at app. noon (Naut. Aim.) 

 14 25' 28* N. var. iu Ih. . . 40* "51 dec. 

 2'38* . 3 



14 22' 60* N. 



in r.h 

 in 12m. 



iu 12m. 



139-53 



0-30 

 930 



158-13 sec. 



Observed altitude of sun's U. L. 

 Index cor. 1' 47* 

 Dip . . 4' 24" 

 S-mi-diam. 15' 49" 

 A pp. Alt of centre 

 Infraction and parallax 



True alt of centre . 



True zenith distance . 

 Sun's reduced declination 



Latitude . 



47 20' 0" 



2? 0* 



- 



47 4' 0* 



47* 



47 3' 13* 



IMC 



42 f>0' 47* N. 

 14 2^ 50* N. 



57 19' 37* X. 



' >n November 8th, 1840, in longitude 62 E., the 

 iau altitude of the sun's lower limb was 57 12' 30* 

 (zenith S.) ; tin: index correction was + 1' 30*, aud the 

 height of the eye 30 feet. Required the latitude, 



L For the apparent time at Greenwich, 

 Long, by account . . 62 E. 



3)12,4 



App. time at Greenwich 4h. 8m. before noon. 



The variation for thin timo must be subtracted, as tho 

 ition is greater at noon than before noon. 



U i 



r. M ' 



a/Ur 



Trr, when the roraire U Iron, nd <he cnmplr- 

 denblr. the dip mar Mnobly IncrcaM. Captain Tarry, 

 i IUtb.iur.'' in IH10, ntimatnl theoonrampUon 

 oiulo hart tnnnt >l. in the lltcla. to about aevintr 

 tavarafun, hae Incrcaacd. 



2. Far the sun's declination. 



D.-c. at app. noon (.Y.iu<. .1 

 1C :!: ^.ir. inlh. . . 43"'3C inc. 



3' 0' . 4 



31' S. 



in 4h. 

 iu but. 



Observed altitude of sun's L.L. 

 Index tor. + 1' 36" 

 l>ip . . 

 Soiiii-diam. + 10' 10" 



alt. of centre . . 

 ictiou and parallax . 



3' = 179 C3 soo. 



. 07 12' 30* 



. +12' 2." 



Truo alt of centro 



32* 

 > 24' 2J* 



Tme zenith distance . 

 bun's reduced declination 



Latitude . 



90 



3:>'40*S. 

 . 10-' 30' 34*8. 



, 49 G' 14* S. 



It may be remarked hero, that in reducing tho declina- 

 tion to the time at Greenwich when the observation is 

 made, the hourly variation should, in strictness, be taken 

 eijual to 44 1 (Hi, which is the average variation during tho 

 preceding 24 hours, or from the noon of Nov. 7th to the 

 noon of Nov. 8th, because the 4h. 8m., for which tho cor- 

 rection is made, is a portion of thin 24 hours ; but the 

 difference iu the correction would amount only to about 

 2J seconds. Such small quantities are not worth attcnd- 

 iiig to in the present problem, as the altitude of accle 

 object, taken at sea, cannot be measured to within a few 

 seeoiuls of the truth ; and if tho latitude can be deduced 

 to tho nearest minute, it is all that can be reasonably 

 expected. 



To save the trouble of the preliminary reductions here 

 adverted to, n table is given in most nautical collections, 

 by entering which, with the sun's noon-declination and 

 the longitude by account, the correction for declination 

 is found with accuracy sufficient for the purposes of 

 navigation. The table referred to is, in Norie's Tablet, 

 the twenty-first ; we shall use it iu the following 

 example. 



4. On November 21st, 1841, in longitude 105 E., tho 

 meridian altitude of the sun's lower limb was observed 

 to be 47 38' (zenith N.) ; the index correction was 

 1' 15*, and the height of the eye 17 feet. Required 

 the latitude. 



For tlte declination at the GrccntcicJi time of observation. 



Sun's declination at noon, Nov. 21 

 (Naut. Aim.) .... 

 Correction for longitude 105 E. 



Sun's reduced declination . 19*52' 3* S. 



Observed altitude of sun's L.L. . 47 38' 0* 



In. lex cor. 1' 15* \ 



Dip . . 4' 4* . . . +10' 54* 



Semi-diam. + 10' 13* ) ; ; 



App. alt. of centre .... 47 4s 

 Refraction and parallax ... 40" 



True alt. of centre .... 47 48' 8* 



90 



True zenith distance . . . 42 11' 52* N. 

 Sun's reduced declination . . 19" 52' 3* S. 



Latitude. . . . 22 19' 49* N. 

 LATITUDE FBOM ASTAH AIIOVK THK I'OI.E. A fixed star 

 changes its declination so slowly, that its variation, even 

 in a month, is scarcely sensible ; no correction, there- 

 fore, for longitude, will be necessary ; the declination, ag 

 given in the Sautical Alman'M, ou the day of observa- 



19 57' r,:,' s. 



6T.2* 



