- John Hadley
Infobox Scientist
name = John Hadley
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caption = John Hadley
birth_date = birth date|1682|4|16
birth_place = 1682Bloomsbury ,London
death_date = death date and age|1744|2|14|1682|4|16
death_place =East Barnet ,Hertfordshire
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nationality = English
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known_for = octant
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John Hadley (April 16 ,1682 Bloomsbury ,London –February 14 ,1744 ,East Barnet ,Hertfordshire ) English mathematician inventor of the octant, the precursor to thesextant , around 1730.He also worked , along with
Edmund Halley , on the subject of thetrade winds Fact|date=July 2008. In 1717 he became member ( and later vice-president ) of theRoyal 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 [Institute and Museum of the History of Science, Florence, Italy: Picture of Hadley's octant [http://brunelleschi.imss.fi.it/museum/esim.asp?c=413024] ] . 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 theDavis quadrant .An American, Thomas Godfrey, independently invented the octant at approximately the same time.
Hadley also improved the
reflecting telescope , building the firstGregorian telescope in 1721.Mons Hadley andRima Hadley on theMoon are named after him.He is the older brother of
George Hadley (meteorologogist).References
External links
* [http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Hadley.html Biography from the School of Mathematics and Statistics at the University of St Andrews, Scotland]
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