Cambridge Encyclopedia » Cambridge Encyclopedia Vol. 67

sextant - Advantages, Adjustment, Anatomy of a sextant, The "Bris" sextant

horizon sun arc mirror

An optical instrument for measuring angular distances; in particular, the elevation of the Sun above the horizon at noon, for determining latitude. The observer views the horizon through a telescope, and simultaneously (through a mirror attached to an arm on a graduated arc) the Sun. The mirror can be moved over the arc to produce a coincidence of the Sun-image and the horizon. For land use, an artificial horizon is provided by a pool of mercury; in aircraft, a bubble-level horizon is used.

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