. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. idian altitude, although, rigor-ously speaking, it is not. The proper corrections for index-error,dip, refraction, parallax, and semi-diameter are next applied tothe sextant reading, and the result is the suns true meridianaltitude, from which the latitude is obtained by the rule given inArt. 76. When cloudy weather prevents the determination of the meri-dian altitude of either the sun or any other celestial body, analtitude obtained within an hour of transit, on either side of themeridian, maybe used t
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. idian altitude, although, rigor-ously speaking, it is not. The proper corrections for index-error,dip, refraction, parallax, and semi-diameter are next applied tothe sextant reading, and the result is the suns true meridianaltitude, from which the latitude is obtained by the rule given inArt. 76. When cloudy weather prevents the determination of the meri-dian altitude of either the sun or any other celestial body, analtitude obtained within an hour of transit, on either side of themeridian, maybe used to find the latitude by the fourth method,Art. 78. Bowditchs Navigator contains special tables by whichthe computation, particularly when the sun is observed, may begreatly facilitated. 80. Reduction of the Latitude.—Owing to the spheroidal form of theearth, the vertical line at any pointof the surface, as Z 0 in Fig. 31,which corresponds exactly withthe normal drawn at that point,does not coincide with the radius ofthe earth, LO, passing through thetame point, excepting at the equator. 76 LONGITUDE. and the poles. It is necessary, then, in refined observations, todistinguish between the geographical zenith, Z\ the point wherethe vertical line, wThen prolonged, meets the celestial spheie, andthe geocentric zenith, Z, the point in which the radius meets thesphere. Since there are two zeniths, there are also two lati-tudes : Zf 0 Q, the geographical latitude, and ZOQ, the geocentriclatitude. The geocentric latitude is evidently smaller than thegeographical by the angle OLO\ which is called the reductionof the latitude. Formulae and tables for finding this reductionare given in Chauvenets Astronomy. Geocentric latitude isconsidered only when the greatest accuracy of result is re-quired. LONGITUDE. 81. Let Fig. 32 represent a projection of the celestial sphereon the plane of the equinoctial ABCG. P is the projection ofc the elevated pole, and PG, PA, and PB are projections of arcs
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