John Hadley (16 April 1682 – 14 February 1744) was anEnglish mathematician, and laid claim to the invention of theoctant, two years afterThomas Godfrey claimed the same.^[1]

John Hadley
Born	(1682-04-16)16 April 1682 Bloomsbury,London
Died	14 February 1744(1744-02-14) (aged 61) East Barnet,Hertfordshire
Nationality	English
Known for	Octant

He was born inBloomsbury,London the eldest son of George Hadley ofOsidge,East Barnet,Hertfordshire and his wife Katherine FitzJames.^[2] His younger brotherGeorge Hadley became a notedmeteorologist.

In 1717 John became a member (and later vice-president) of theRoyal Society of London. In 1729 he inherited his father's East Barnet estate.^[3]

He died in East Barnet in 1744 and is buried in the local churchyard with other members of his family. He had married Elizabeth, daughter of Thomas Hodges, FRS (formerAttorney General of Barbados) and had one child, a son and heir John, born in 1738.

In 1730 Hadley invented the reflecting octant, which could be used to measure the altitude of the sun or othercelestial 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.^[4] 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 ownlatitude. The octant proved extremely valuable for navigation and displaced the use of other instruments such as theDavis quadrant. AnAmerican,Thomas Godfrey, independently invented the octant at approximately the same time.

Hadley also developed ways to make precision aspheric andparabolicobjective mirrors forreflecting telescopes. In 1721 he showed the first parabolicNewtonian telescope to the Royal Society.^[5] This Newtonian, with a 6-inch-diameter (150 mm)primary mirror, compared favorably with the largeaerial refracting telescopes of the day.^[6] He also madeGregorian telescopes with accurately shaped mirrors.^[7][8]

Mons Hadley andRima Hadley on theMoon are named after him. TheOasis Trust Academy inPonders End in north London is calledOasis Academy Hadley in his honour.^[9]

• List of largest optical telescopes in the 18th century

• King, H.C. (1970–1980). "Hadley, John".Dictionary of Scientific Biography. Vol. 6. New York: Charles Scribner's Sons. pp. 5–6.ISBN 978-0-684-10114-9.

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