Stargazing: past and present . d extra-meridional observa-tions. He followed Meton, Anaximander, and others inobserving on the meridian instead of on the horizon, andthen it struck him that it was not necessary to keep tothe meridian, and he conceived an instrument, called anAstrolabe, fixed on an axis so that the axis would pointto the pole-star, like the one represented in Fig. 8. Thisengraving is of one of Tycho Brahes instruments, whichis similar to but more elaborate than that of Hipparchusno drawing of which is extant, c, D, is the axis ofthe instrument pointed to the pole of the heavens
Stargazing: past and present . d extra-meridional observa-tions. He followed Meton, Anaximander, and others inobserving on the meridian instead of on the horizon, andthen it struck him that it was not necessary to keep tothe meridian, and he conceived an instrument, called anAstrolabe, fixed on an axis so that the axis would pointto the pole-star, like the one represented in Fig. 8. Thisengraving is of one of Tycho Brahes instruments, whichis similar to but more elaborate than that of Hipparchusno drawing of which is extant, c, D, is the axis ofthe instrument pointed to the pole of the heavens ;E, B, c, the circle placed North and South representingthe meridian ; R, Q, N, the circle placed at right angles 26 STARGAZING ; JAST AND JRESENT. [book r. to the polar axis, representing the equator, but in theinstrument of Hipparchus it was fixed to the circleE, B, c, and not movable in its own plane as this one , L, K, is a circle at right angles to the equator, andmoving round the poles, being a sort of movable meri-. Fiu. 8.—Astrolabe (Armillfe Aequatoiite of Tyeho Brahe) similar to the ouecontrived by Hipparchus. dian. Thus, then, if the altitude of a star from theequator (or its declination) was required to be observed,the circle was turned round on the axis, and thesights, Q, M, moved on the circle till they, together withthe sight A, pointed to the star ; the number of degreesbetween one of the sights and the equator, was then CHAP. 111.] IIlPrARCHUS AND PTOLEMY. 27 read off, giving the declination required. The numberof degrees, or hours and minutes, of Right Ascension,from K to E could be then read off along the circle R, Q, N,gi^dng the distance of the object from the meridian. As the stars have an apparent motion, the difference inright ascension between two stars only could be obtainedby observing them directly after each other, and allowingfor the motion during the interval between the twoobservations. In this manner, then, HijDparchus could point to anyp
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