© Science Museum / Science & Society Picture Library
Octant with a wooden frame inlaid with ivory scales and naked-eye sights, made by S W Silver & Company of London. This instrument would have been used for navigation on a ship. The octant could be used to find latitude by measuring the height of the midday Sun or a suitable star at night. Longitude could also be found from determining local noon from the maximum height of the Sun and comparing it with a chronometer set to a fixed meridian. The angle between the fixed stars and the Moon could also be measured to discover longitude using lengthy calculations and tables from the Nautical Almanac.