An Octant is a measuring instrument used primarily in navigation. The name “octant” derives from the Latin “octans” meaning eighth part of a circle, because the instrument’s arc is one eighth of a circle. It is a type of reflecting instrument.

Two men independently developed the octant around 1730: John Hadley (1682–1744), an English mathematician, and Thomas Godfrey (1704–1749), a glazier in Philadelphia. While both have a legitimate and equal claim to the invention, Hadley generally gets the greater share of the credit. This reflects the central role that London and the Royal Society played in the history of scientific instruments in the eighteenth century.

The octant provided a number of advantages over previous instruments.

Octants were produced in large numbers into the 19th century. In wood and ivory, their relatively low price compared to the all-brass sextant (which was developed subsequently) made them a popular instrument. The design was standardized with many manufacturers using the identical frame style and components. Different shops could make different components, with woodworkers specializing in frames and others in the brass components.

The octant revolutionized maritime navigation by making a high-quality product that was affordable and simple to use. The Octant allowed countries and individuals to explore, improve global trade and expand industry. More modern devices are enhancements to the functional use and broad adoption of the Octant.