Naval officer using an octant, late 18th century.
© Science Museum / Science & Society Picture Library
Trade card promoting the octant, or 'artificial horizon', invented and made by R Rust of St Catherine's Stairs, near the Tower of London (formerly of The Minories). An octant is used in surveying and in navigation to measure the angle between two objects. Octants were used by navigators to determine a ship's latitude by measuring the angle between the horizon and a celestial body, such as the Sun.