Captain Cook's sextant, c 1772.
© Science Museum / Science & Society Picture Library
This sextant was made by the optical instrument maker John Bird (1709-1776) and is thought to have belonged to the great sea captain and explorer Captain James Cook. A sextant is an optical instrument for measuring angular distances for determining latitude. The observer can view the horizon (through a telescope) and the sun simultaneously (through a mirror attached to an arm on a graduated arc). The mirror can be moved over the arc to produce a co-incidence of the sun-image and the horizon.