Image from page 64 of "Astronomy. Determination of time, longitude, latitude, and azimuth" (1913)
Subjects: Spherical astronomy
Publisher: Washington, U.S. Govt. print. off.
Contributing Library: Wellesley College Library
Digitizing Sponsor: Boston Library Consortium Member Libraries
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Text Appearing Before Image:
ally a theodolite with the graduated circle in a vertical positionand the axis horizontal, with the telescope fastened rigidly to the alidade. The circle andalidade are fastened to a horizontal support which rests upon the top of a vertical axis, the latterfitting into a stand. There is a counterpoise to the circle and alidade on the opposite side of thevertical axis. The stand has three leveling screws, and there may be a graduated circle near itsbase for measuring horizontal angles approximately. 1 For convenient instructions, formulae, and tables for sextant observations for time and other approximate astronomic methods, see BowditchsAmerican Practical Navigator, published by the U. S. Navy Department. 2 Such an instrument is used in observing vertical angles or zenith distances in primary triangulation. The circles of these instruments arefrom 8 to 10 inches in diameter and are graduated very accurately. 3 See p. 45, Directions for Magnetic Measurements, Coast and Geodetic Survey.
Text Appearing After Image:
VERTICAL CIRCLE. DETERMINATION OF TIME. 53 Before starting observations the usual adjustments of the eyepiece and object glass shouldbe made and the crosswires should be brought approximately into the center of the field. Thereis no adjustment for collimation in either the vertical or horizontal plane. A coarse stride levelis used to make the horizontal axis of the circle truly horizontal and, consequently, the circlevertical, and a sensitive level is placed parallel with and fastened to the circle to define a hori-zontal line through the instrument. If, after leveling by the two levels, the instrument isrotated on its vertical axis through 180° and the bubbles remain on the graduated scales of thelevel vials then the adjustments for level are satisfactory. TIME FROM OBSERVATIONS ON A STAR WITH A VERTICAL CIRCLE. When making the observations the stars image is brought into the field of the telescopeand the telescope clamped with the horizontal wire slightly ahead of the star. As the
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