The Mariner’s Sextant and the Royal Society

ONE would be hard pressed to name a device superior to the mariner’s sextant by which physical principles are better adapted to solve a relatively simple practical problem. The sextant, an instrument superbly elegant in its simplicity, designed merely to measure accurately the altitude of a heavenly body from a platform as unstable as the heaving deck of a ship at sea, is ideal for its purpose. The first part of this paper describes the principal altitude measuring devices employed during the Golden Age of Discoveries. It covers a period of about a quarter of a millennium from the time when Portuguese mariners under the sponsorship of Prince Henry the Navigator (1394-1460) first struck out to navigate the open Atlantic, to the time when Robert Hooke (1635-1703), the eminent experimental philosopher of the seventeenth century, first described, in 1666, a reflecting instrument for measuring altitudes at sea.