John Hadley

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Born: 16 April 1682 in Enfield Chase (near East Barnet, now in London), Hertfordshire, England
Died: 14 Feb 1744 in Barnet, Hertfordshire, England

John Hadley became a Fellow of the Royal Society in 1716. He built the first Newtonian reflecting telescope in 1721. It had a 6 inch mirror and proved very effective. He built a Gregorian reflector in 1726. It was due to him that reflecting telescopes of sufficient accuracy and power to be useful in astronomy were developed.

In 1730 he invented a quadrant which measured the altitude of the Sun or of a star. In 1731 Hadley showed his new quadrant to the Royal Society. In 1734 he showed his new bubble-level to the Royal Society. It was used to determine position at sea. Edmund Stone wrote

Mr Hadley tells us, that upon trial of one of these instruments, three observations made at sea of the distance between two stars with a brass octant of this kind differed from Mr Flamsteed's at land, only about a minute.

Hadley's design evolved into that of a sextant. A magnifying glass was added to read the scale, a telescopic sight was added with cross-wires to divide the field of view. The arc was extended from an octant to a sextant and a stout handle was added at the back of the instrument.