sextans

Sextant comes from Latin sextans (genitive sextantis), meaning 'a sixth part,' derived from sextus ('sixth') and the suffix -ans indicating a portion. The name describes the instrument's arc: a sextant spans 60 degrees, which is one-sixth of the full 360-degree circle. The instrument measures the angle between a celestial body — the sun, the moon, a star — and the horizon, and that angle, combined with precise time and a nautical almanac, allows the navigator to calculate the ship's latitude (and, with more complex calculation, longitude). John Hadley and Thomas Godfrey independently invented the double-reflecting quadrant (spanning 90 degrees, one-quarter of a circle) in 1731; the sextant, spanning 60 degrees with increased accuracy, was developed by John Campbell in 1759. The naming follows the same mathematical logic as the quadrant and the octant (45 degrees, one-eighth): the instrument is named for the fraction of the circle its arc represents.

The problem of determining position at sea was the central unsolved problem of navigation for most of maritime history. Latitude — how far north or south of the equator a ship is — was established relatively early using the noon sun or the altitude of Polaris: the angle of Polaris above the horizon equals the observer's latitude, a fact known to Mediterranean navigators by classical antiquity. Longitude — how far east or west — was not solved reliably until the development of accurate marine chronometers in the eighteenth century. A ship at sea could know the local time by astronomical observation (when the sun reaches its highest point, it is local noon), but without knowing the time at a reference meridian (Greenwich, after 1884), it could not calculate how far east or west it had traveled. The sextant provided the angular measurements; the chronometer provided the time reference; together they solved the longitude problem that had sent ships onto reefs for centuries.

The sextant's mechanism is a feat of optical elegance. Two mirrors — an index mirror that can rotate and a fixed horizon mirror — together allow the navigator to bring the image of a celestial body down to the horizon line while viewing the horizon directly. Because the angle of reflection equals the angle of incidence, the angle turned on the index arm equals half the angle of elevation of the celestial body — which is why the arc need only span 60 degrees to measure elevations up to 120 degrees. The navigator reads the angle from the graduated arc, applies corrections for the instrument's own error, atmospheric refraction, and the navigator's height above sea level, and arrives at the true altitude of the celestial body above the geometrical horizon. This altitude, combined with the time of observation, gives position.

The sextant remained the primary navigation instrument from its development in 1759 until the introduction of GPS in the 1990s — an uninterrupted dominance of more than two centuries. Its longevity was not merely inertia: the sextant is independent of any power source, immune to electronic jamming, reliable in all weather that allows a celestial body to be seen, and self-contained. Modern naval vessels still carry sextants and train officers in their use, because GPS can be jammed or spoofed in electronic warfare and a navigator who cannot fall back to celestial navigation is a navigator with a critical vulnerability. The instrument that solved the latitude problem in 1759 is the emergency backup for the satellite system that replaced it.