. Astronomy for students and general readers . is 63 30 N., at what altitude does it pass abovethe pole at Washmgton, and at what altitude does it pass below it VAns. 66 23 39 above the pole, and 11 23 39 when below it. (7). If the dechnation of a star is 50° N., what length of siderealtime is it above the horizon of Washington and what length below itduring its apparent diurnal circuit ? Ans. Above, 21 52; below,2 8- g 8. DETERMINATION OF LATITUDES ON THEEARTH BY ASTRONOMICAI. OBSERVATIONS. Latitude from clreumpolar »tars.—In Fig. 16 let iJ represent thezenith of the place of observation, Pth
. Astronomy for students and general readers . is 63 30 N., at what altitude does it pass abovethe pole at Washmgton, and at what altitude does it pass below it VAns. 66 23 39 above the pole, and 11 23 39 when below it. (7). If the dechnation of a star is 50° N., what length of siderealtime is it above the horizon of Washington and what length below itduring its apparent diurnal circuit ? Ans. Above, 21 52; below,2 8- g 8. DETERMINATION OF LATITUDES ON THEEARTH BY ASTRONOMICAI. OBSERVATIONS. Latitude from clreumpolar »tars.—In Fig. 16 let iJ represent thezenith of the place of observation, Pthe pole, and HPZ R the me-ridian, the observer being at thecentre of the sphere. Suppose/Sand S to be the two pomtsat which a circumpolar starcrosses the meridian in tiie de-scription of its apparent diurnalorbit. Then, since P is midwaybetween S and S, zs+zs „„ ?? Z+ Z = 90° - < If, then, we can measure the dis-tances Z and Z\ we have ;90° Z+ Z Fig. which serves to determine f. The distances .Zand Z can be meas- 48 ured by the meridian circle or the sextant—both of which instru-ments are described in the next chapter—and the latitude is thenknown. Z and Z must be freed from the effects of refraction. Inthis method no previous knowledge of the stars declination is re-(juired, provided it remains constant between the upper and lowertransit, which is the case for fixed stars. Latitude by Circum-zenith Observations.—If two starsS and 8, whose declinations i and A are known, cross the meridian,one north and the other south of the zenith, at zenith distances Z 8 and Z8\ which call Z and Z, andif we have measured Z and Z, wecan from such measures find thelatitude ; for (f ^=d -\- Z and 0 =i — Z, whence <f = h\^i + i)^{Z-Z•)?\. It will be noted that in this meth-od the latitude depends simplyupon the mean of two declinationswhich can be determined before-hand, and only requires the differ-FiG. 17. ence of zenith distances to be ac- curately measure
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