John Hadley
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| John Hadley | |
 John Hadley
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| Born | April 16, 1682 1682 Bloomsbury, London |
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| Died | February 14, 1744 East Barnet, Hertfordshire |
| Nationality | English |
| Known for | octant |
John Hadley (April 16, 1682 Bloomsbury, London – February 14, 1744, East Barnet, Hertfordshire) English inventor of the octant, the precursor to the sextant, around 1730.
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 improved the reflector telescope, building the first Gregorian telescope in 1721.
Mons Hadley and Rima Hadley on the Moon are named after him.
He is the older brother of George Hadley (meteorologogist).
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