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Photonics Dictionary

sextant

A handheld navigational instrument used to measure the elevation angle of celestial bodies such as the sun. An image of the sun is viewed through a small telescope via two plane mirrors so that any chance rotation of the instrument in a vertical plane will not cause the sun's image to rise or fall. The altitude of the sun is read from a scale. In some aircraft-bubble sextants, a circular spirit level at the focus of a small collimator replaces the true horizon, since the latter may not always be visible.
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