For centuries, maritime travel was characterized by a profound challenge: accurately determining a vessel’s location once land was no longer in sight. Early navigation relied on approximate methods, often resulting in significant errors that endangered ships, crew, and cargo. The development of celestial navigation, which uses the positions of the sun, moon, and stars, provided a theoretical solution to this problem. However, this method required instruments capable of measuring the angles of celestial bodies with a precision that was simply unattainable with the existing tools of the era.
Defining the Sextant and Its Purpose
The sextant is a specialized, double-reflecting instrument engineered to measure the angular distance between two visible objects, most commonly a celestial body and the horizon. This measurement, known as the body’s altitude, is the foundation of celestial navigation, allowing a navigator to calculate a line of position on a nautical chart. The instrument is named for the Latin word sextans, meaning one-sixth, because its graduated arc is built on a frame that comprises approximately one-sixth of a circle, or 60 degrees.
The instrument’s design leverages the optical principle that when a light ray is reflected twice, the angle between the initial and final rays is twice the angle between the two mirrors. Because the sextant’s physical arc is 60 degrees, this double-reflection mechanism allows the instrument to measure angles up to 120 degrees. This capability is useful for lunar distance observations, which are necessary for determining longitude. The core function is to precisely align the image of a celestial body with the horizon line, providing the raw data for navigational calculations.
Navigational Instruments Preceding the Sextant
Before the invention of the reflecting instrument, navigators relied on devices like the quadrant and the backstaff, which were inherently limited in their accuracy, especially on the open ocean. Instruments such as the Davis quadrant, or backstaff, required the observer to turn their back to the sun and measure the shadow it cast, a method that was awkward and imprecise. More significantly, these tools required the navigator to observe the celestial body and the horizon sequentially rather than simultaneously.
The constant pitching and rolling of a ship made it difficult for a sailor to hold the instrument steady enough to take a reliable reading. A single measurement error could translate into dozens of miles of positional uncertainty. The octant, a precursor to the sextant, was the first successful reflection instrument, but its arc, one-eighth of a circle, limited its maximum measurable angle to 90 degrees.
The Simultaneous Invention and Key Figures
The breakthrough that led to the sextant was developed independently and almost simultaneously by two men on separate continents in the early 1730s. The English mathematician John Hadley and the American glazier and inventor Thomas Godfrey both arrived at the same solution: the double-reflecting mirror principle. Hadley presented his design to the Royal Society in England in May 1731, while Godfrey’s instrument was tested at sea near Philadelphia in December 1730.
The core innovation of both their designs was the use of an index mirror and a horizon glass, which allowed the celestial body’s image to be reflected down to the horizon. This mechanism created a single, stable view where the celestial body appeared to rest directly on the horizon, even as the ship moved. Because the sight was relative, the instrument maintained accuracy regardless of the motion of the vessel.
Hadley’s initial instrument, often called Hadley’s Quadrant, was technically an octant. The name sextant was adopted later when the instrument’s arc was enlarged to one-sixth of a circle to accommodate the larger angles required for lunar distance measurements, a technique used to determine longitude. Though the double-reflecting principle was established by Hadley and Godfrey in 1731, the first true sextant, designed with the 60-degree arc, was constructed by instrument maker John Bird in 1759 at the request of Admiral John Campbell.
The Royal Society ultimately recognized both Hadley and Godfrey for their independent but nearly simultaneous inventions of the double-reflecting instrument. The controversy over who deserves sole credit persists, but the invention is generally attributed to both men as the co-creators of the principle that revolutionized navigation.
The Enduring Impact on Maritime Navigation
The introduction of the reflecting instrument provided mariners with unprecedented accuracy, allowing them to determine their position within a few nautical miles. This newfound precision drastically reduced the number of shipwrecks and enabled the creation of more accurate and detailed world maps. Paired with a marine chronometer, which provided the accurate time necessary for longitude calculations, the sextant completed the puzzle of oceanic navigation.
The ability to reliably calculate latitude and longitude transformed global trade, exploration, and naval strategy. The sextant remained the standard instrument for determining a ship’s position until the late 20th century, when satellite-based systems like the Global Positioning System (GPS) became widely accessible. Despite the dominance of electronic navigation today, the sextant remains a highly valued tool because it requires no electric power or external signals. It is considered a necessary and reliable backup system in case of electronic failure or GPS jamming.