| John Hadley |
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| 1682 births | |
| 1744 deaths | |
| people from hertfordshire | |
| english astronomers | |
| english inventors | |
| fellows of the royal society | |
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John Hadley ( April 16 , 1682 Bloomsbury , London – February 14 , 1744 , East Barnet , Hertfordshire ) English inventor of the Sextant , around 1730 . The sextant allows its user to determine the elevation of Celestial objects with respect to the Horizon . 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 Latitude . The sextant proved extremely valuable for navigation and displaced the use of the Astrolabe . An American , Thomas Godfrey , independently invented the sextant at approximately the same time. Hadley also improved the Reflector Telescope , building the first Newtonian Telescope in 1721 . Mons Hadley and Rima Hadley on the Moon are named after him. He is the older brother of George Hadley . EXTERNAL LINK |