Stargazing: past and present . ^ ^r~ bo- .-^^ Fig. 13. - Showing how the Vernal E-iumox has now passed Iroui Taurus and Ai ies. The two accompanying drawings by Professor PiazziSmyth of the position of the vernal equinox among thestars in the years 2170 and 1883 will show howprecession has brought about celestial changes which have CHAP. III.] HIPPARCHUS AND PTOLEMY. 35 not been unaccompanied by changes of religious ideasand observances in origin connected with the stars. We now come to Ptolemy. There was another instru-ment used by Ptolemy, and described by him, which wemay mention
Stargazing: past and present . ^ ^r~ bo- .-^^ Fig. 13. - Showing how the Vernal E-iumox has now passed Iroui Taurus and Ai ies. The two accompanying drawings by Professor PiazziSmyth of the position of the vernal equinox among thestars in the years 2170 and 1883 will show howprecession has brought about celestial changes which have CHAP. III.] HIPPARCHUS AND PTOLEMY. 35 not been unaccompanied by changes of religious ideasand observances in origin connected with the stars. We now come to Ptolemy. There was another instru-ment used by Ptolemy, and described by him, which wemay mention here; it was called the Parallactic Rules,so named perhaps because that ancient astronomer usedit first for the observation of the parallax of the consists of three rods, t) E, D F, e f, Fig. 14, two ofwhich formed equal sides of an isosceles triangle ; and. Fk;. 14.—Instrument for Measurini< Altitudes. tlie third, which had divisions on it, made the one at thebase, or was the chord of the angle at the summit. Oneof the equal sides, D F, was furnished with j)ointers, overwhich a person observed the star, whilst the other, d e,was placed vertically, so that they read off the divisionson E F, and then, by means of a table of chords, the anglewas found ; this angle was the distance of the star fromthe zenith. Ptolemy, wishing to observe with greataccuracy the position of the moon, made himself an u 2 36 STARGAZING : PAST AKD IKESENT. [book i. instrument of this kind of a considerable size; for theequal rulers were four cubits long, so that its divisionsmight be more obvious. He rectified its position bymeans of a plumb-line. Purbach, Regiomontanus, andAValther, astronomers of the fifteenth century, employedthis manner of obser\dng, which, considering the youthof astronomy, was by no means to be despised. Thisinstrument, constructed with
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Keywords: ., bookcentury1800, bookdecade1870, booksubjec, booksubjectastronomy
