Double A-frame sextant of 12 inch radius, 1788-1803.
© Science Museum / Science & Society Picture Library
This bras sextant was made by the London instrument makers John and Edward Troughton. With its tripod stand and telescopic sights, it was designed for use in navigation, surveying and hydrography. A separate tube can be used for plain sight. Sextants could be used to find the correct latitude by measuring the height of the midday Sun or a suitable star at night. Longitude could also be found by determining local noon from the maximum height of the Sun and comparing this time with a chronometer set to a fixed meridian. Alternatively, the angle between the fixed stars and the Moon could be measured to find longitude, but only after lengthy calculations using tables in the Nautical Almanac.