Stargazing: past and present . Fig. 6.—The First Meridian Circle. And now still another instrument is introduced, andwe begin to find the horizon altogether disregarded infavour of observations made on the meridian. The instrument in c^uestion was probably the inven-tion of Eratosthenes. It consisted of two circles ofnearly the same size crossing each other at right angles,(Fig. 6) ; one circle represented the equator and the otherthe meridian, and it was employed as follows :— The circle A was fixed perfectly upright in the1 28,279 miles. CHAP. II.] THE FIRST INSTRUMENTS. 21 meridian, so that
Stargazing: past and present . Fig. 6.—The First Meridian Circle. And now still another instrument is introduced, andwe begin to find the horizon altogether disregarded infavour of observations made on the meridian. The instrument in c^uestion was probably the inven-tion of Eratosthenes. It consisted of two circles ofnearly the same size crossing each other at right angles,(Fig. 6) ; one circle represented the equator and the otherthe meridian, and it was employed as follows :— The circle A was fixed perfectly upright in the1 28,279 miles. CHAP. II.] THE FIRST INSTRUMENTS. 21 meridian, so that the greatest altitude of the sun eachday could be observed; the circle B was then placedexactly in the plane of the earths equator by adjustingthe line joining c and D to the part of the heavensbetween the Bears, about which the stars appear torevolve. This done, the occurrence of the equinox waswaited for, at which time the shadow of the part of the. Fig. 7.—The First Instrument Graduated into 360° (West Side). circle E must fall upon the part marked f, so as exactlyto cover it. We now come to the time when the circle began tobe divided into 360 divisions or descrees—about thetime of Hipparchus (160 ). There are twoinstruments described by Ptolemy for measuring thealtitude of the sun in degrees instead of in fractions of acircle. They, like the gnomon, were used for determiningthe altitude of the sun. The first, Fig. 7, consisted oftwo circles of copper, one, c d, larger than the other, 22 STARGAZING: PAST AND PRESENT. [book i. having tho smaller one, B, so fitted inside it as to turnround while the larger remained fixed. The larger wasdivided into 360°, and the smaller one carried twopointers. This instrument was placed perfectly up-right and in the plane of the meridian, and with afixed point, c, always at the top by means of aplumb-line hanging from c over a mark, d. On thissmall circle are two square knobs projecting
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Keywords: ., bookcentury1800, bookdecade1870, booksubjec, booksubjectastronomy
