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John Hadley (16 April 1682 – 14 February 1744) was born in Bloomsbury, the eldest son of George Hadley and Katherine (nee FitzJames), of Enfield Chase near East Barnet, Hertfordshire.
His elder brother George Hadley was a barrister and also a noted meteorologist, while his younger brother Charles was a physician.
In 1717 John became a Fellow of the Royal Society of London. In 1728 he became a Vice President.
On 13 May 1731 Hadley read an account of his reflecting octant before the Royal Society, and two weeks later he exhibited the instrument. It could be used to measure the altitude of the sun or other celestial objects above the horizon at sea. A mirror on a pivoted arm provided a reflected image of the celestial body overlapping the image of the horizon, and the angle of the arm was determined from an accurately-graduated scale. Knowing the position of the object on the sky and the time of the observation allowed the observer to calculate his latitude. A Philadelphian glazier, Thomas Godfrey, independently invented an octant at approximately the same time.
The method had been proposed by Robert Hooke c.1666. Robert Newton invented an octant and communicated an account to Halley at the Royal Society in 1699, but it only became more widely known after Halley's death in 1742.
The name 'octant' derives from the 45 degree angle of the measuring arc. Later instruments favoured an angle of 60 degrees - sextants.
Hadley also developed ways of accurately producing mirrors for reflecting telescopes. In 1721 he showed the first parabolic Newtonian telescope to the Royal Society. He also made Gregorian telescopes with accurately shaped mirrors.
1744 John Hadley died in East Barnet. He had married Elizabeth, daughter of Thomas Hodges, FRS (former Attorney General of Barbados) and had one child, John, born in 1738.