Newcomb, Simon, 1835-1909, Holden, Edward Singleton, 1846-1914, joint author · New York : H. Holt and company · leaf 115
n. The principle upon which the sextant depends is the following,which is proved in optical works. The angle tetween the first andthe last direction of a ray which has suffered two reflections in the same 94 ASTRONOMY. plane is equal to twice the angle which the two reflecting sutfaees makewith… … Fig. 41. the mirrors, since B G and D G are perpendicular to B and A. Tomeasure the altitude of a star (or the sun) at sea, the sextant is heldin the hand, and the telescope is pointed to the sea-horizon, whichappears like a definite line. The index-arm is then moved untilthe reflected image of the…
Astronomy
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