The elements of astronomy; a textbook (1919) (14595652088)

Identifier: elementsofastro00youn (find matches)

Title: The elements of astronomy; a textbook

Year: 1919 (1910s)

Authors: Young, Charles A. (Charles Augustus), 1834-1908 Young, Anne Sewell, b. 1871., ed

Subjects: Astronomy Constellations

Publisher: Boston, New York (etc.) Ginn and Company

Contributing Library: The Library of Congress

Digitizing Sponsor: The Library of Congress

Text Appearing Before Image:

ix of which are alwaysused in the case of a large instrument. Since 1 on the cir-cumference of a circle is ytbt 6T Par^ °^ ^s ra-dius, it followsthat on a circle two feet in diameter 1 is only about 17Q0-g- ofan inch. An error of that amount is now very seldom made byreputable constructors in placing any graduation line. Fig. 154represents a rather small instrument of this kind, having circlestwo feet in diameter, with a four and a half inch telescope. Our limits do not permit a description of the reading micro-scopes seen at A, B, C, and D, by means of which the circle isread. For these, see works on Practical Astronomy. 549. Zero Points. — The instrument is used to measure thealtitude or else the polar distance of a heavenly body at the time 420 APPENDIX. (§549 when it is crossing the meridian. As a preliminary, we mustdetermine a zero point upon the circle, — the nadir point if wewish to measure altitudes or zenith distances, the polar pointif polar distances or declinations.

Text Appearing After Image:

Fig. 154. — A Meridian Circle. The polar point is determined by taking the circle readingfor some star near the pole when it crosses the meridian abovethe pole, and then doing the same thing again twelve hourslater when it crosses it below. The mean of the two readings,corrected for refraction, will be the reading the circle would § 549) COLLIMATING EYE-PIECE. 421 give when the telescope is pointed exactly to the pole ; tech-nically, the polar point. The nadir point is the reading of the circle when the tele-scope is pointed vertically downward. It is determined bymeans of a basin of mercury underneath the instrument,- thetelescope being so set that the image of the horizontal wire ofthe reticle, as seen by reflection from the mercury, coincides withthe wire itself. Since the reticle is exactly in the principalfocus of the object-glass, the rays emitted from any point inthe reticle will form a parallel beam after passing through thelens, and if this beam strikes perpendicularly upo

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1919

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