April 16, 1682 – February 14, 1744
John Hadley was an English mathematician, inventor of the octant and precursor to the sextant around 1730.
In 1717 he became member (and later vice-president ) of the Royal Society of London.
The octant is used to measure the altitude of the Sun or other celestial objects above the horizon at sea. A mobile arm carrying a mirror and pivoting on a graduated arc provides a reflected image of the celestial body overlapping the image of the horizon, which is observed directly. 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 own latitude. The octant proved extremely valuable for navigation and displaced the use of other instruments such as the Davis quadrant.
An American, Thomas Godfrey, independently invented the octant at approximately the same time.
Hadley also improved the reflecting telescope.