John Hadley (Bloomsbury, London), April 16, 1682 - East Barnet (Hertfordshire), February 14, 1744) was an English mathematician. He found out the octant, the precursor of the sextant around 1730.
His father was George Hadley and his mother Katherine FitzJames. In 1717 he became a member of the Royal Society, of which he later became vice president.
The octane is used to measure the height of the Sun or other celestial bodies above the horizon. A moveable arm with a mirror revolves around an arcuate arc, thus providing a reflection of the celestial body that overlaps the image of the horizon. If the position of the celestial body and the time of observation are known, the user can easily calculate his own geographical width. The octant proved very valuable for navigation and replaced the use of other instruments.
The American Thomas Godfrey found out octant independently and at the same time.
Hadley also developed ways to make accurate aspheric and parabolic mirrors for mirror telescopes. In 1721 he showed the first parabolic Newtontelescope at the Royal Society. This Newtontelescope with a 6-inch primary mirror turned out to be better than the big airline scopes from that time. He also made a Gregorian telescope with accurately shaped mirrors.
He was the older brother of meteorologist George Hadley.
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