The elements of astronomy; a textbook . average conditions the refraction elevates a body at thehorizon about 35, so that the sun and moon in rising both appear 32 REFRACTION. [§50 clear of the horizon while still wholly below it. At an altitudeof only 5° the refraction falls off to 10; at 44°, it is 1; and at the zenith, zero. Its amount at anygiven altitude varies quite sensibly how-ever, with the temperature and barometricpressure, increasing as the thermome-ter falls or as the barometer rises; sothat whenever great accuracy is re-quired in measures of altitude of aheavenly body, we must ha
The elements of astronomy; a textbook . average conditions the refraction elevates a body at thehorizon about 35, so that the sun and moon in rising both appear 32 REFRACTION. [§50 clear of the horizon while still wholly below it. At an altitudeof only 5° the refraction falls off to 10; at 44°, it is 1; and at the zenith, zero. Its amount at anygiven altitude varies quite sensibly how-ever, with the temperature and barometricpressure, increasing as the thermome-ter falls or as the barometer rises; sothat whenever great accuracy is re-quired in measures of altitude of aheavenly body, we must have observa-tions both of the thermometer andbarometer to go with the readings ofthe circle. In works on PracticalAstronomy tables are given by which the refraction can be computedfor an object at any altitude and in any state of the weather. It is hardly necessary to say that this indispensable refraction-correction of nearly all astronomical observations makes a great dealof trouble and involves more or less error and 51. Second Method of Determining the Latitude. — By the meridian altitude or zenith distance of a body whose declinationis accurately known. In Fig. 13 the circle AQPB is the meridian, Q and P beingrespectively the equator and thepole, and Z the zenith. QZ isobviously the declination of thezenith, or the latitude of the ob-server (Art. 47). Suppose nowthat we observe Zs, the zenithdistance of a star, s, south of thezenith as it crosses the meridian, and that Qs, the declination of the star, is known. Then, evi-dently, QZ equals Qs + sZ\ , the latitude equals the declina-tion of the star plus its zenith distance. If a star were at «, south of the equator, the same equation wouldstill hold algebraically, because the declination Qs is a minus quantity.
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