John Hadley (April 16, 1682 – February 14, 1744) invented the sextant around 1730. The sextant allows its user to determine the elevation of celestial objects with respect to the horizon. If the position of the object on the sky and the time of the observation are known, it is easy for the user to calculate his latitude. The sextant proved extremely valuable for navigation and displaced the use of the astrolabe.

An American, Thomas Godfrey, independently invented the sextant at approximately the same time.

Hadley also improved the reflector telescope, building the first Newtonian telescope in 1721.

Mons Hadley and Rima Hadley on the Moon are named after him.

External link

• Biography from the School of Mathematics and Statistics at the University of St Andrews, Scotland