File:Elements of theoretical and descriptive astronomy, for the use of colleges and academies (1901) (14597760587).jpg

Identifier: elementsoftheore00whit_0 (find matches)
Title: Elements of theoretical and descriptive astronomy, for the use of colleges and academies
Year: 1901 (1900s)
Authors: White, Charles J
Subjects: Astronomy.
Publisher: New York: J. Wiley & sons
Contributing Library: University of Florida, George A. Smathers Libraries
Digitizing Sponsor: University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation

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of the body, and the side PZy theco-latitude of the place of observation. We can, therefore, com-pute the side ZA, which is the true zenith distance of the bodyobserved; and the difference between this and the observed zenithdistance, (corrected for parallax, (Art. 55,) and instrumentalerrors,) will be the amount of refraction for that zenith distance. The construction of the instrument enables us to follow acelestial body through its whole course from rising to sett-ag,measuring altitudes and noting the c orresponding times to anyextent that we choose; and the amoun t of refraction correspond-ing to each altitude can afterwards be computed at our leisure. THE EQUATORIAL. 38. The equatorial is similar in general construction to thealtitude and azimuth instrument. It is, however, differentlyplaced, the plane of the principal graduated circle, CC, coin-ciding, not with the plane of the horizon, but with that of thecelestial equator, from which peculiarity of position comes the PLATE II.
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EQUATORIAL OF HARVARD COLLEGE. EQUATORIAL. 41 name of equatorial. The circle C C> when thus placed, is calledthe hour circle of the instrument, and the axis J., at right anglesto it, is called the hour or polar axis. It is evident that the axisis directed towards the poles of the heavens. The circle CC,the plane of which is perpendicular to the plane of the circleC C, will lie continually in the plane of a circle of declination,as the instrument is turned about the polar axis, and is hencecalled the declination circle. The axis on which this latter circle13 mounted is called the declination axis. 39. Use of the Equatorial.—The equatorial is employed prin-cipally in that class of observatious which requires a celestialbody to remain in the field of view during a considerable lengthof time. The manner in which this requirement is met is explainedin Fig. 13. Let A A be the polaraxis of an equatorial, directed to-wards the pole of the heavens, P.Let sss,f be the diurnal circle inwhic

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