Hadley's reflecting quadrant

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The replacement of the far less accurate Davis's quadrant for taking a sight for the purposes of celestial navigation. It was invented by the English mathematician and scientist John Hadley (1682–1744) and was in fact an octant. However, as the principle of double reflection made one degree of the octant's arc represent two degrees between the observed objects, it was called a quadrant. After details of Hadley's new invention had been announced in the Royal Society in 1731 the British Admiralty ordered a series of observations to be made to test the instrument, and as a result the quadrant was widely accepted at sea as a vastly improved aid to navigation and hydrography. The incorporation of a spirit level in 1734 made it possible to take a meridional altitude at sea without the horizon being visible.

Subjects: Maritime History.

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