The sextant is an instrument that measures angular distance of the sun and stars from the horizon to determine latitude. It is used with a chronometer and nautical charts by navigators of ships at sea to determine location. The sextant consists of a telescope rigidly mounted to a bar and aligned with the horizon. A pair of mirror s, one fixed in line with the horizon, the other mounted on an adjustable arm, reflect the celestial object. When the object aligns with the horizon in the fixed mirror, the angular reading is taken from the sextant's arc.

Mariners originally took sightings with an astrolabe, a flat circular instrument that had a map on its face and an adjustable ring that represented the local horizon. They were compact, but complicated to use. Many seafarers preferred the simpler cross-staff. It had a staff topped with a peep-sight and a graduated...