Navigational sextant. Sextants are used to measure the position of celestial bodies to aid in navigation. Sextants utilise the fact that the exact position in the sky of the Sun and stars depends upon the time and location. If a navigator knows the time, the location (longitude and latitude) can be calculated. The sextant was invented in about 1730 due to independent work by English mathematician John Hadley and American inventor Thomas Godfrey.