A text-book of general astronomy : for colleges and scientific schools (1888)
Young, Charles A. (Charles Augustus), 1834-1908 · Boston ; London : Ginn & Company · leaf 67
res the angular distancebetween two objects ; as, for instance, the sun and the visible horizon,not by pointing first on one and then afterwards on the other, but bysighting them both, simultaneously and in apparent coincidence; whichcan be done even when he has no fixed position or stable footing.… … is usually from five to eight inches radius. It bears a graduation ofhalf-degrees, numbered as ivhole degrees, so that it can measure anyangle less than 120°. An index-arm MNm the figure, is pivoted at the centre of thearc, and carries a vernier which slides along the limb, and can befixed at any…
Astronomy
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