16 April 1682|
1682 Bloomsbury, London
|Died||14 February 1744
East Barnet, Hertfordshire
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. 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 he showed the first parabolic Newtonian telescope to the Royal Society. This Newtonian, with a 6-inch-diameter (150 mm) primary mirror, compared favorably with the large aerial refracting telescopes of the day. He also made Gregorian telescopes with accurately shaped mirrors.
His parents were Katherine FitzJames and George Hadley. His younger brother George Hadley was a noted meteorologist. On 6 June 1734 he married Elizabeth Hodges who herself was wealthy. They had one child, a son John born in 1738.
- Correspondence between William Penn and James Logan. Philadelphia: J. B. Lippincott & Co. 1870.
- Institute and Museum of the History of Science, Florence, Italy: Picture of Hadley's octant 
- amazing-space.stsci.edu - Hadley’s Reflector
- The complete Amateur Astronomer - John Hadley's Reflector
- 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
- King, H.C. (1970–80). "Hadley, John". Dictionary of Scientific Biography 6. New York: Charles Scribner's Sons. pp. 5–6. ISBN 978-0-684-10114-9.
|Wikisource has the text of a 1920 Encyclopedia Americana article about John Hadley.|