Sextant

Instrument for determining the angle between the horizon and a celestial body—such as the Sun, the Moon, or a star—used in celestial navigation to determine latitude and longitude. It consists of a metal arc, marked in degrees, and a movable radial arm pivoted at the centre of the arc's circle. A telescope, mounted rigidly to the framework, is lined up with the horizon.

The radial arm, on which a mirror is mounted, is moved until the star is reflected into a half-silvered mirror in line with the telescope and appears, through the telescope, to coincide with the horizon. The angular distance of the star above the horizon is then read from the graduated arc of the sextant. From this angle, the latitude can be determined (within a few hundred metres) by means of published tables, and by consulting an accurate chronometer the longitude can be established. Invented in 1731, the sextant replaced the octant and became an essential tool of navigation.