John Hadley
| John Hadley | |
 John Hadley
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| Born | April 16, 1682 1682 Bloomsbury, London |
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| Died | February 14, 1744 (aged 61) East Barnet, Hertfordshire |
| Nationality | English |
| Known for | octant |
John Hadley (April 16, 1682 Bloomsbury, London; born to Katherine FitzJames and George Hadley – February 14, 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 that 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].
Mons Hadley and Rima Hadley on the Moon are named after him. It has been announced that the proposed Oasis Trust academy to be opened in Enfield Wash before moving to Ponders End will be named Oasis Academy Hadley in his honour.[6]
He is the older brother of George Hadley (meteorologist).
References
- ^ Institute and Museum of the History of Science, Florence, Italy: Picture of Hadley's octant [1]
- ^ amazing-space.stsci.edu - Hadley’s Reflector
- ^ The complete Amateur Astronomer - John Hadley's Reflector
- ^ [http://books.google.com/books?id=KAWwzHlDVksC&pg=PA77&lpg=PA77&dq=John+Hadley++Gregorian&source=bl&ots=0GRMadGVZ4&sig=ng2mdMGFtRjkrueONC2GO3znmL0&hl=en&ei=g2EDSuEexaS3B93yoYkH&sa=X&oi=book_result&ct=result&resnum=3#PPA77,M1 Henry C. King - The history of the telescope - page 77
- ^ telescopeѲptics.net - 8.2. Two-mirror telescopes
- ^ Oasis Academy in NE Enfield Retrieved 5 February 2009
External links
- mathematicians/Hadley.html" class="external text" title="http://www-gap.dcs.st-and.ac.uk/~history/mathematicians/Hadley.html" rel="nofollow">Biography from the School of Mathematics and Statistics at the University of St Andrews, Scotland