Familiar talks on astronomy, with chapters on geography and navigaton . he suns polar distance (that is, 900 minusthe suns declination.) This gives us the dis-tance of the pole above the horizon, and this isequal to the latitude. At the equator the pole is on the horizon; andfor every degree we move north, the pole risesone degree above the horizon. Hence we havefor another definition: the latitude of a placeis equal to the altitude of the elevated pole. The theory by which we find the latitude, then,you observe, is very simple. We generally usethe sun, but we use also the moon, planets, andst
Familiar talks on astronomy, with chapters on geography and navigaton . he suns polar distance (that is, 900 minusthe suns declination.) This gives us the dis-tance of the pole above the horizon, and this isequal to the latitude. At the equator the pole is on the horizon; andfor every degree we move north, the pole risesone degree above the horizon. Hence we havefor another definition: the latitude of a placeis equal to the altitude of the elevated pole. The theory by which we find the latitude, then,you observe, is very simple. We generally usethe sun, but we use also the moon, planets, andstars. The observed altitude, you must under-stand, is corrected for index-error, semi-diameter,dip, refraction, and parallax. The altitude of astar, however, requires no correction for semi-diameter and parallax, as it has none. 1 It depends, of course, upon its declination. Finding the Latitude. 231 What I have said concerning the method offinding the latitude by the meridian altitude of acelestial body appears very clear from an in-spection of the following Fig. 15. Let the diagram be a projection of the celes-tial sphere on the plane of the meridian N. Z. is the zenith ; N S the horizon; P the elevatedpole; P P the axis of the sphere; E Q the equa-tor ; Z Q the distance of the zenith from theequator, and P N the altitude of the pole. Z Qand P N are each equal to the latitude L. LetM, M7, M/f, be three positions of the sun on themeridian; Q M, Q M, and EM represent itsdeclination, d; and Z M, Z M, and Z M, repre-sent its zenith distance, z. When the sun is at M, north of the equator,L = z + d; when the sun is at M, south of the 232 Familiar Talks on Astronomy, etc. equator, L = z — d\ and when it is at M, be-low the pole, L = altitude + /, the polar dis-tance, as I have said. I will now tell you how we find the longitudeby A chronometer is a largewatch of peculiar construction, made especiallyfor the use of navigators. It is suspended bygimbals, a
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