John Hadley Totally Explained

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 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's easy for the user to calculate his 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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