John Hadley

John Hadley 1682-1744, English instrument maker. An optician by trade, Hadley built reflecting telescopes, based on Newton's model, that had greater resolution than the cumbersome refractors then in use. In 1731 he built a reflecting octant, based on Newton's sketch, that prefigured the modern nautical sextant. Hadley was a leading member of the Royal Society.

Hadley's reflecting quadrant, 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.

sextant, a double-reflecting instrument for measuring angles at sea, primarily the altitude of astronomical bodies in celestial navigation. The instrument may also be used for measuring other angles at sea such as for distance-off and horizontal angles for coastal navigation. Although its arc subtends an angle of only 60°, by double reflection the sextant measure angles up to 120°. The instrument was designed by Captain (later Admiral) John Campbell of the British Navy in 1757 and based on Hadley's reflecting quadrant (or octant), a double-reflection instrument proposed in principle by Newton and in 1731 demonstrated to the Royal Society by Robert Hooke. The principal requirement to measure more than 90° arose largely from the need to make lunar distance observations to determine longitude at sea. However, the overwhelming advantage of the double-reflecting principle was that, in a seaway, once the body observed has been brought down to the horizon it will not leave it no matter what the weather. Observations could now be made to within a few minutes of arc. A vernier, which enabled single minutes of arc to be read off the scale, became normal, but was replaced much later by the micrometer drum.

At the same time as the development of Hadley's quadrant a young American optician, Thomas Godfrey, who was engaged with the problem of taking lunar distance observations at sea, had submitted an instrument of his own design based on the same principle as Hadley's, and he is now widely regarded as the joint inventor of the sextant.

The modern sextant employs the same optical principles as Hadley's reflecting quadrant. There are two mirrors: a fixed mirror, the horizon glass, is mounted on the frame of the sextant in line with the observer's telescope. Then there is the index mirror at the top of the arm which, when the index arm is set to zero on the scale, will lie precisely parallel to the horizon glass. The horizon glass is half silvered, so that the horizon can be seen through the plain half and the reflected object seen in the silvered half. To take a sight the navigator, with the index arm set to zero, faces the body to be observed and, with the sextant held vertically, looks through the telescope and moves or rotates the sextant as necessary to get the horizon in view. He then moves the index arm along the scale until the body observed appears in the silvered half of the horizon glass and then adjusts the image so that the limb of the body coincides with the horizon.

See celestial navigation.

Mike Richey