What the Sextant Actually Does
A sextant measures the angle between two visible objects — most commonly the sun or a star and the horizon. That angle, combined with the exact time of the observation and printed almanac tables, produces a position line. Cross two such lines from observations on different bodies and you have a fix. The instrument itself is mechanically simple: a calibrated arc, a moveable index arm, two mirrors, and a small telescope. A navigator could learn to take a sun sight in an afternoon and produce results accurate to within a mile or two with practice.
The arc of the standard instrument spans 60 degrees — one sixth of a circle — which is the origin of the name. Early octants, which preceded the sextant, used a 45-degree arc. The expanded range of the sextant allowed the measurement of lunar distances, a method of finding longitude by measuring the moon's angular distance from a nearby star and comparing the result against pre-computed tables. This technique, though labour-intensive, was the only independent means of finding longitude at sea before reliable chronometers became widely available.
Development and the Canadian Connection
The reflecting quadrant was invented around 1730, with credit attributed separately to John Hadley in England and Thomas Godfrey in Pennsylvania. The sextant as a distinct instrument appeared by the 1750s. By the time French and British survey vessels were working the Gulf of St. Lawrence in the 1760s and 1770s, the sextant was standard equipment.
Samuel Holland, Surveyor General of Quebec after 1764, carried sextants and an astronomical quadrant on his surveys of Prince Edward Island and the Gulf coast. His position data, combined with Joseph Frederick DesBarres's subsequent Atlantic Neptune charts, gave the Royal Navy the first reliable angular measurements of the Canadian Maritime coastline. The accuracy of those charts depended directly on the quality of the celestial observations made with these instruments.
How a Sun Sight Was Taken
At apparent noon, when the sun reached its highest point above the horizon, a navigator took what was called a meridian altitude. The procedure: hold the sextant to the eye, swing the index arm until the sun's reflected image in the horizon glass aligns with the visible horizon, and read the angle from the arc. Tables in the Nautical Almanac, published annually by the British Admiralty from 1767 onward, gave the sun's declination for every day of the year. A short calculation combining the observed altitude and the declination produced the observer's latitude.
Longitude required time. A ship's chronometer was set to Greenwich Mean Time before departure. The navigator observed local noon — when the sun reached its meridian — and noted the chronometer time at that moment. Every four minutes of time difference corresponded to one degree of longitude. A chronometer accurate to within a few seconds could produce longitude to within a nautical mile.
The Morning Star Sight
Latitude at noon was a single line of position. For a full fix, navigators also took sights at morning and evening twilight, when both stars and the horizon were simultaneously visible. Three star sights taken in quick succession and plotted on a chart gave a small triangle of position lines; the centre of the triangle was the estimated position. This technique, called a running fix when observations were spread over time, remained the backbone of offshore navigation until satellite systems became available in the 1980s.
Instruments Held by Canadian Maritime Collections
Several institutional collections in Canada hold notable sextant examples. The Maritime Museum of British Columbia in Victoria includes 19th-century examples from the Pacific coastal trade. The Musée de la mer on Île Bonaventure in Quebec holds instruments connected to the Gulf of St. Lawrence fishery. The Naval Museum of Halifax maintains documentation of the instruments used by the Royal Canadian Navy during the two World Wars, a period when celestial navigation was still the primary offshore method.
Most maritime sextants found in Canadian collections were made in London by instrument makers such as Heath & Co., Kelvin & Hughes, and Henry Hughes & Son. These firms dominated naval instrument supply from the mid-1800s through the mid-1900s. A few examples survive from Canadian makers, particularly from Halifax workshops that repaired rather than manufactured navigational instruments.
When the Sextant Stopped Being the Primary Tool
The Transport Canada certificate of competency for watch officers required demonstrated celestial navigation ability until the late 1990s. LORAN, Omega, and Decca radio navigation systems had been available since the 1940s and 1950s, but their coverage gaps meant the sextant remained necessary for ocean passages. GPS, declared operational in 1995, effectively ended the routine use of celestial navigation in commercial shipping within a decade. The Royal Canadian Naval College at Halifax retained celestial navigation in its curriculum as a backup method until 2013.