English mathematician and inventor who built the first working reflector telescope. Hadley built his Gregorian reflector telescope in 1721. The device contained a 6-inch (15.2-cm) mirror, and proved accurate enough to be successfully used by astronomers. The response to Hadley's invention was so favorable, he later built a larger, more sophisticated version. In 1730 he invented a quadrant, known as Hadley's quadrant, which was used to measure the altitude of the Sun or a star in order to ascertain a ship's geographic position while out at sea. His design eventually evolved into a sextant, a navigational instrument that measured the distance between the horizon and either the Sun or stars.
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