This is a traditional maritime navigation instrument which today has been made redundant by GPS, although ships' officers are still expected to know how to use one.
According to Encyclopedia Britannica it determines "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. The device consists of an arc of a circle, marked off in degrees, and a movable radial arm pivoted at the centre of the 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 and the exact time of day as registered by a chronometer, the latitude can be determined (within a few hundred metres) by means of published tables.
"The name comes from the Latin sextus, or one-sixth, for the sextant’s arc spans 60°, or one-sixth of a circle. Octants, with 45° arcs, were first used to calculate latitude. Sextants were first developed with wider arcs for calculating longitude from lunar observations, and they replaced octants by the second half of the 18th century."
All trainees are taught how to use one in case of a major blackout on board with no power for electronic instruments ,also handy in a lifeboat if seas are calm enough on clear days for a reading to position etc.. This young lady would be shouting the lads 'a round' for no uniform on duty back in my day.
ReplyDelete