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**John Hadley**
John Hadley
	; John Hadley
Born	16 April 1682; 1682 Bloomsbury, London
Died	14 February 1744 (aged 61); East Barnet, Hertfordshire;
Nationality	English
Known for	octant

John Hadley (16 April 1682 Bloomsbury, London; born to Katherine FitzJames and George Hadley – 14 February 1744, East Barnet, Hertfordshire) 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 ^[1]. 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 developed ways to make precision aspheric and parabolic objective mirrors for reflecting telescopes. In 1721 he showed the first parabolic Newtonian telescope to the Royal Society^[2]. This Newtonian with a 6 inch diameter primary mirror compared favorably with the large aerial refracting telescopes of the day^[3]. He also made Gregorian telescopes with accurately shaped mirrors^[4]^[5].