. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. to the south point, thereading of the horizontal circle is 0°; when it is directedto the zenith, the reading of the vertical circle is 0°. Con-sequently, if we direct the telescope to any particular star,one circle gives the zenith distance of the star (or its alti-tude) ; the other gives its azimuth. If we fix or clamp the telescope to the vertical circle,we can turn the axis which carries both
. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. to the south point, thereading of the horizontal circle is 0°; when it is directedto the zenith, the reading of the vertical circle is 0°. Con-sequently, if we direct the telescope to any particular star,one circle gives the zenith distance of the star (or its alti-tude) ; the other gives its azimuth. If we fix or clamp the telescope to the vertical circle,we can turn the axis which carries both round, and ob- micrometer? What is the use of the Position-circle? How are the micrometerwires illuminated at night ? 440. If we want simply to measure the angular dis-tance of one celestial hody from another, what do we use ? Generally speaking,what else is to he determined ? 441. What is used when we wish to determinepositions on the visihle portion of the sky ? 442. Descrihe the altazimuth and its THE ALTAZIMUTH. 243 serve all stars having the same altitude, and the horizontalcircle will show their azimuths. If we clamp the axis tothe horizontal circle, we can move the telescope so as to. Fig. 99.—Portable Altazimuth. make it travel along a vertical circle, and the circle at-tached to the telescope will give us the zenith-distances 244 ASTRONOMICAL INSTRUMENTS. of the stars (or the altitude), which, in this case, will lie intwo azimuths 180° apart. Fig. 99 represents a portable altazimuth, the variousparts of which will be recognized from the foregoing de-scription. 443. The Transit-circle.—When we wish to determinedirectly the position of a heavenly body on the celestialsphere, a Transit-circle is used. This instrument consistsof a telescope movable in the plane of the meridian, beingsupported on two pillars, east and west, by means of ahorizontal axis. The ends of the axis are of exactly equalsize, and move in pieces, which, from their shape, arecalled Ys. When the
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectastronomy, bookyear18
