Sextants

The first in a series of articles pertaining to the development and history of surveying instruments. By Leonard Honey

“Captain Nemo, provided with his sextant, took the sun height to find his latitude...” Jules Verne, in his book Twenty Thousand Leagues under the Sea, describes perhaps the traveller and surveyor par excellence. Who invented the sextant? • The mariner's astrolabe It’s between English and Americans. An account of one of these exists Some said the English mathematician, describing how it was used to observe John Hadley (after an idea of Robert the solar eclipse of 3 March 1337. Hooke) developed this instrument in 1731 Amongst early references, the most simultaneously with the American (from famous, especially as it is illustrated, is Pennsylvania) Thomas Godfrey. Others that of Pedro Medina in 1552 in Seville. In said Godfrey invented it in 1730 and John appearance, the mariner's astrolabe Hadley in 1731. resembles a four-spoked, cast bronze wheel, with anointed ring at the top for How did we get to the sextant the navigator's thumb and a pivoted as we now know it? Geodetic measurements. alidade with slits through which he peered • Sea quadrant at the celestial body. Working independently, and in complete One of the first elevation-finding Calibrated 90-0-90 across the two ignorance of each other, John Hadley instruments was the sea quadrant. top quadrants for zenith distances, VPRS (1682-1744) in London and Thomas Originally the tool of the astronomer and usually associated with Portuguese Godfrey, glazier and natural surveyor, this instrument was first used by m a n u f a c t u re or marked for altitude mathematician in Philadelphia, mariners in the 15th century. It was a heights suggesting another school of simultaneously devised an improved form simple arc of a circle made of boxwood makers, they vary considerably in of altitude measuring instrument which (or other close-grained wood) or brass diameter and weight, but an average worked on the same principles. with two sighting pinnules along one size would appear to be 13-18cm (5-7") The Royal Society recognized the straight edge. A plumb bob attached to in diameter and weighing 2-3kg. equality of the two and awarded each a the apex swung across a scale graduated prize of £200. Godfrey received his in 0-90o to show an altitude reading. • The back staff household furniture. This was an ingenious improvement on The Hadley quadrant, as it came to be • The cross staff the quadrant, cross star and mariner’s called, although it was later known as an First described in 1342, it was an astrolabe for taking an elevation. Usually octant, was a brilliantly simple instrument instrument for measuring distances made of wood section 5/8" by 5/8" based on the application of optics. between two stars or the angular (1.6x2cm) in lignum vitae, the instrument

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