Hadley's double-reflection sextant fixed altitude sights
A sextant measures angular distance between two visible objects—usually a star or Sun and the horizon—to plot a position line. John Hadley and Thomas Godfrey implemented the doubly reflecting principle around 1731. Needing no electricity or outside signal, it remains a trusted backup aboard ships today.
A sextant uses two mirrors to measure the angle between two things in view, most often a star, planet, or the Sun and the horizon. Taking that angle, the altitude, is called shooting or taking a sight, and combined with the exact time it yields a position line on a chart; the noon Sun or, in the north, Polaris gives latitude. John Hadley and Thomas Godfrey put the principle into practice around 1731, and it later turned up in unpublished papers by Isaac Newton. In 1922 the Portuguese naval officer Carlos Viegas Gago Coutinho adapted it for flying.
Because it measures against the horizon rather than against the instrument, it is very precise, and unlike the older backstaff it can sight stars directly, so it works at night. A moving deck does not spoil the reading, since horizon and star drift together in the view while their relative position holds. It needs neither electricity nor any signal controlled by others, as GPS does, which makes it a practical backup on ships.
The frame is a sector of roughly one sixth of a circle, 60 degrees, hence the name from Latin sextans, one sixth; octants and quintants cover other fractions. It carries a horizon mirror, an index arm turning the index mirror, a telescope, Sun filters, and a scale marked so each degree shown equals twice the arm's actual turn. Traditional half-horizon mirrors split the view between horizon and star, while whole-horizon models use half-silvered glass. Modern mirrors are often 5 centimetres or more, where nineteenth-century ones rarely exceeded 2.5.
Good sextants read to a minute of arc, a sixtieth of a degree, and verniers to a tenth of that. An error of one arc minute shifts the position by roughly a nautical mile, so celestial fixes can be good to about a tenth of that distance. When fog or darkness hides the horizon, a sheltered pool of water can serve as an artificial one. Aircraft sextants, now out of production, often averaged hundreds of readings per sight through a flush overhead window.
Source: Sextant
