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Astronomical sextant. Historical artwork of an astronomical sextant designed by John Hadley (1682-1744) in about 1730. Sextants were used for measuring the angle of an object, usually the Sun, above the horizon. This was achieved through the use of two mirrors, one of which was fixed. The second mirror was moved until the two images of the object matched up. The angle can then be read from the curved scale (at bottom) to which the moveable mirror was attached. From this the latitude, which was vital for ship navigation, can be calculated.
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