Sextant

A sextant is an optical instrument that measures the angle between two visible objects, commonly a celestial body and the horizon. Its mirrors let the observer bring both objects into a single view.

Double reflection: An optical arrangement in which light reflects successively from two mirrors before reaching the eye. The sextant uses successive reflections to align the horizon and a distant object in one view.
Celestial navigation: Navigation that determines position or direction by observing celestial bodies. A sextant supplies the angular observations on which traditional celestial fixes depend.
Backstaff: A navigational instrument that measures a celestial body's altitude by observing the shadow it casts. It offered a safer way to measure solar altitude before reflective instruments became widespread.
Octant: A reflective navigational instrument whose graduated arc spans one eighth of a circle, or 45 degrees. Its smaller arc limits the angles it can measure compared with a sextant.
Index mirror: The movable mirror in a sextant that reflects light from the observed object toward the horizon glass. Its motion sets the measured angle by changing the reflected object's apparent position.
Altitude (astronomy): The angular height of a celestial object above an observer's horizon. A sextant measures this angle when its horizon view is aligned with the object.
Cross-staff: A navigational instrument that measures angles by aligning a sliding crosspiece with two objects. Unlike the sextant's reflected image, it requires direct alignment that can strain the eye.
Theodolite: A surveying instrument that measures horizontal and vertical angles using a telescope and graduated circles. Unlike a sextant, it is designed for precise land surveying and does not depend on a visible horizon.
Horizon glass: A partly transparent, partly reflective plate that lets a sextant user view the horizon and a reflected object together. It combines the direct horizon view with light redirected by the index mirror.
Line of position: A line on a chart along which an observer's position must lie, based on a measurement or observation. A corrected celestial altitude gives a line of position rather than a location by itself.

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