LATITUDE.] 



NAVIGATION NAUTICAL ASTRONOMY. 



1081 



tion, may be taken as that at the time of observation. 

 As a fixed star has no parallax in altitude, the only cor- 

 rections will be those for dip and refraction ; the rule for 

 deducing the latitude is therefore as follows : 



RULE. 1. Apply to the observed altitude the correc- 

 tions for dip arid refraction ; the result will be the true 

 altitude ; and this, subtracted from 90, will give the true 

 zenith distance. 



2. Mark the zenith distance N. or S. according as it is 

 N. or S. of the star ; then, if the declination and zenith 

 distance have the same marks, their sum will be the lati- 

 tude ; if they have different marks their difference will 

 be the latitude. 



Examples. 



1. January 22, 1846, the meridian altitude of Arcturus 

 was observed at sea to be 43 27' (zenith north) ; the index 

 correction was + "H 10'', and the height of the eye 20 feet. 

 Required the latitude. 



Observed altitude of star . . 43 27' 0* 

 Index cor. + 2 10" ) , , . 



Dip . . 4'24"j ' ' ' 



App. alt. of star 

 Refraction 



True alt. of star 



True zenith distance 



Star's declination, Jan. 22, 1846 



Latitude 



43 24' 46* 

 V V 



43 23' 40* 

 90 



46 36' 14"N- 

 19 58' 59 V N- 



06 35' 13"N. 



2. On February 12, 1853, the meridian altitude of a 

 Hydra was observed to be 47 24' 30' (zenith north) ; the 

 index correction was 2 / 10", and the height of the eye 

 17 feet. Required the latitude. 



Observed altitude of star . . 47 24' 30* 

 Index cor. - 2 10" \ ., .., 



Dip . . - 4' 4* j 



App. alt. of star 

 Refraction 



True altitude 



True zenith distance 



Star's declination, Feb. 12, 1853 



Latitude 



47 18' 16* 

 - 52* 



47 17' 24* 

 90 



42 42' 36-N. 

 go j/29's. 



34 41' 7N. 



3. On July 16, 1845, the meridian altitude of Foinal- 

 haut was found to be 73 36i' (zenith north) ; the index 

 correction 30', and the height of the eye 24 feet. 

 Required the latitude. 



Observed altitude of the star . . 73 36' 30* 

 Index cor. - 30* ) 

 Dip . - 449*) 



- 5' 19' 



App. alt. of star 

 Refraction . 



True altitude 



True zenith distance ; 

 Star's declination, July 16, 1845 



Latitude 



73 31' 11" 

 - 16" 



73 30' 55" 

 90 



16 29' 5N. 

 30 26' 20 'S. 



13 57' 15" 8. 



NOTE. In gome nautical tables the corrections for dip 

 and refraction are united under the head of Correction of 

 ttar'i observed altitude. 



LATITCDE FKOM THE MoOIf ABOVE THE POLE. The 



moon's declination is given in the Nautical Almanac for 

 every hour of the day, and the time of her meridian pas- 

 sage from day to day. These elements are computed for 

 VOL. L 



mean, time, the reckoning being from mean noon at 

 Greenwich. Hence, to find the moon's declination cor- 

 responding to the time of taking her meridian altitude at 

 sea, we must know the time at Greenwich at the instant 

 of observation. This is ascertained as follows : 



As the motion of the moon in her orbit is eastward, 

 her transit over the meridian of any place is delayed 

 from day to day. In consequence of this retardation, 

 she will pass the meridian of a place to the west of Green- 

 wich later in the day at that place than she passes the 

 meridian of Greenwich, and her transit over a meridian 

 to the east of Greenwich will take place earlier in the 

 day. How much later, or how much earlier, will be 

 ascertained by converting the longitude of the meridian 

 into time, and applying the corresponding proportional 

 part of the daily variation as furnished by the Nautical 

 Almanac. By help of the following short table, and the 

 daily change in the time of transit, the correction to be 

 added to the Greenwich time of transit, to obtain the 

 time of transit over a meridian west of Greenwich, or to 

 be subtracted to obtain the time of transit over a meridian 

 east of Greenwich, may be at once found. 



The daily variation, as given by the Nautical Almanac, 

 is to be sought for in the top row of figures, and the lon- 

 gitude of the place in the marginal column on the left ; 

 the proper correction of the Greenwich mean time, to re- 

 duce it to the mean time of transit at the place, is then 

 to be taken out from the body of the table. It is suffi- 

 cient that the daily variation be taken to the nearest 

 minute. 



It must be observed, however, that in the case of the 

 moon, there must not be the same indifference as to the 

 accuracy of the longitude by account as is allowable in 

 the case of the sun ; the following examples will show, 

 that on account of the moon's more rapid change in de- 

 clination, a comparatively short interval of time makes 

 a sensible difference in this element. 



Table for finding the Time of the Moon's Transit over a given 

 Meridian when the Time of the Transit at Greenwich 

 is known, and the Daily Variation of the Time. 



By the aid of this table, the latitude from a meridian 

 altitude of the moon, when above the pole, may be found 

 as follows : 



RULE. 1. From the Nautical Almanac take out the 

 time of the moon's passage over the meridian of Green- 

 wich on the given day, as also the daily variation. 



2. From the longitude by account, and the foregoing 

 table, reduce this to the time, at the place, of the moon's 

 passage over the meridian of the ship ; the time of ob- 

 servation, at the place where that observation is made, 

 will thus be found. 



3. From the ship's time and longitude, find the cor- 

 responding time at Greenwich. 



4. Find now the moon's declination at that time from 

 the Nautical Almanac, computing the variation for the 

 odd minutes by means of the difference in declination 

 for 10m. 



5. From page 3 of the month, take out tho moon 



6 Y 



