Sextant

A sextant is a precision optical instrument for measuring the angle between two objects — most commonly between a celestial body (sun, moon, star, planet) and the visible horizon. This angle, the *altitude*, is the fundamental measurement of celestial-navigation: combined with accurate time and the appropriate astronomical tables, it gives the navigator's latitude and, if time is known precisely, longitude.

The sextant replaced the astrolabe as the primary instrument of celestial navigation in the mid-18th century and remained the definitive tool for marine navigation until GPS became practical in the 1990s. It is still carried on ships as a backup and is still taught in naval academies.

How It Works

The sextant uses a system of two mirrors to bring the image of a celestial object down to the horizon simultaneously. The navigator looks through the eyepiece and sees two images superimposed: the direct view of the horizon, and the reflected image of the celestial body. By rotating the index arm (the moveable mirror), the navigator adjusts the angle until the bottom of the celestial body appears to rest exactly on the horizon.

The angle is read from the arc — a scale of degrees marked on the frame's curve. The instrument measures up to 120° (a sextant: one-sixth of a circle), though the measurement range covers the typical altitude range needed for navigation.

The key innovation over earlier instruments is the double-reflection principle: because both mirrors move together, the measurement is largely unaffected by the motion of the ship. The observer can hold the celestial body on the horizon even as the vessel pitches and rolls. Earlier instruments — the quadrant, the astrolabe, the cross-staff — required a stable platform.

The Measurement and the Fix

The altitude reading gives the *observed altitude* (Ho). Tables or a calculator give the *calculated altitude* (Hc) — what the altitude of that body *should be* at a predicted position and time. The difference between Ho and Hc gives a *line of position* (LOP): the navigator is somewhere on a circle on the Earth's surface where that altitude is exactly that value.

Two such observations from different celestial bodies give two LOPs that intersect at the fix. In practice, navigators take three observations to get a triangle (a "cocked hat") whose size indicates the accuracy of the fix.

Reduction and Tables

The calculation from raw altitude to position — *sight reduction* — was historically done by logarithm tables. The *Nautical Almanac* provides the positions of celestial bodies for every hour of every day; the *HO 229* or similar tables give the reduction calculation. A skilled navigator can work a sight reduction in 5-10 minutes.

Modern calculators and smartphone apps do the calculation instantly, but the measurement still requires the sextant. No instrument has replaced its function at sea.

Construction and Accuracy

A well-made sextant has an accuracy of about 0.1 arcminute. At sea, this translates to roughly 0.1 nautical miles (185 metres) of position accuracy — exceptionally precise for a hand-held optical instrument. Naval sextants are made of brass or aluminium with precision-ground mirrors. The arc is typically silver-plated for the scale markings.

The instrument is robust and needs no power source. A sextant from 1850 is still a functional navigation tool today.

History

The sextant was independently invented by John Hadley in England and Thomas Godfrey in Philadelphia around 1731, improving on Edmond Halley's earlier double-reflection concept. Hadley's octant (measuring 45°, an eighth of a circle) became the *Hadley's quadrant* in common usage and was quickly adopted by the Royal Navy.

By the 1750s, the arc was extended to 60° (a sextant: one-sixth of a circle) to allow lunar distance measurements for longitude — the technique that preceded John Harrison's chronometer as a solution to the longitude-problem. The name "sextant" dates from this extension.

The instrument reached its definitive form by about 1800 and changed remarkably little over the next 190 years.

End of the Sextant Era

GPS became available to civilian users in 1983 and widely affordable by the early 1990s. Modern ships navigate almost entirely by GPS, radar, and electronic chart systems. Sextant navigation is now a backup skill rather than primary practice.

But backup matters: GPS can be jammed, spoofed, or simply fail. The U.S. Navy reinstated celestial navigation training in 2015 after a period when it had been discontinued, explicitly citing the vulnerability of GPS-dependent navigation to electronic warfare.