The sextant as we know it emerged in the late 1750s, but its origin story begins in 1731, when two men working independently on opposite sides of the Atlantic each built a reflecting instrument that used mirrors to measure the angle between a celestial body and the horizon. John Hadley, a wealthy English gentleman and Fellow of the Royal Society, and Thomas Godfrey, a Philadelphia glazier with a self-taught gift for mathematics, both arrived at essentially the same design without knowing of each other’s work. Their instruments were technically octants, measuring arcs of up to 90 degrees, but they laid the groundwork for the true sextant that followed a generation later. The full story involves a forgotten sketch by Isaac Newton, a bitter priority dispute, and a problem that had killed thousands of sailors.
Hadley, Godfrey, and an Unpublished Sketch by Newton
In May 1731, John Hadley demonstrated a new instrument before the Royal Society in London. It used two mirrors to bring the reflected image of the sun or a star into alignment with the horizon as seen directly through a half-silvered glass. By reading the angle on a graduated arc, a navigator could determine the altitude of a celestial body with a precision that far exceeded anything available at the time. Hadley called it a “reflecting quadrant,” and it quickly became known as Hadley’s quadrant.
Around the same time, Thomas Godfrey in Philadelphia had independently constructed a similar device, attaching mirrors to a mariner’s wooden crossbow frame. Godfrey’s instrument was tested at sea by acquaintances in 1730 and 1731, and word of it reached the Royal Society through James Logan, a prominent Pennsylvanian who corresponded with London’s scientific circles. A priority dispute erupted. The Royal Society ultimately acknowledged both men, but Hadley’s demonstration had come first in the Society’s records, and his name became the one most associated with the invention in Britain and across Europe. Godfrey received a reward of 200 pounds from the Society, though his contribution faded from popular memory for a long time.
What neither man knew was that Isaac Newton had sketched a nearly identical reflecting instrument around 1699. The design sat in Newton’s papers, unpublished, until Edmond Halley brought it to the Royal Society’s attention in 1742, over a decade after both Hadley and Godfrey had built working versions. Newton’s sketch showed the same principle of double reflection, but because it never left his notes during his lifetime, it played no role in the instrument’s actual development. It does, however, make the invention story stranger: three people arrived at the same optical solution, and the one who got there first never told anyone.
From Octant to Sextant
Hadley’s quadrant and Godfrey’s instrument were octants. The name comes from the shape of the arc: one-eighth of a full circle, spanning 45 degrees on the graduated scale. Because the double-reflection principle effectively doubles the angle, an octant could measure celestial altitudes up to 90 degrees, which was sufficient for most routine observations of the sun or stars above the horizon.
The limitation became apparent when navigators tried a technique called lunar distances, which required measuring the angle between the moon and a reference star or the sun. Those angles could easily exceed 90 degrees, and an octant simply could not reach far enough. In the mid-1750s, Captain John Campbell of the Royal Navy, who had been testing the lunar distance method at sea, urged London instrument makers to build a larger instrument. John Bird, one of the finest instrument makers of the era, constructed the first sextant around 1757. Its arc covered one-sixth of a circle (60 degrees on the scale), which doubled to 120 degrees of measurable angle. That was enough to handle lunar distances comfortably.
The sextant caught on rapidly. It was heavier and more expensive than the octant, but the expanded range made it far more versatile. By the 1780s, sextants had become the standard instrument for serious navigation, while octants lingered on among coastal sailors and those who could not afford the upgrade.
What Came Before and Why It Was Not Good Enough
To appreciate why the reflecting instrument mattered so much, consider what navigators had been using. The mariner’s astrolabe, a simplified version of the astronomer’s astrolabe, had been in use since the fifteenth century. It was a heavy brass ring with a rotating sighting arm, and it worked reasonably well in calm conditions. In any kind of sea state, the rolling of the ship made accurate readings nearly impossible. Errors of two degrees or more were common, which could translate to over a hundred nautical miles of positional uncertainty.
The cross-staff, another old tool, required the observer to look in two directions simultaneously: at the horizon and at the celestial body. This was awkward and strained the eyes, especially when observing the sun. The backstaff, invented by the English navigator John Davis around 1594, improved on the cross-staff by letting the observer stand with their back to the sun and work with shadows instead of direct sighting. It was more comfortable but still limited in precision and troubled by the ship’s motion.
The genius of the reflecting instrument was that it brought both images, the celestial body and the horizon, into a single line of sight through the eyepiece. Because both images moved together as the ship rolled, the observer could take an accurate reading even on a heaving deck. This was a transformative change. A skilled navigator with a well-made sextant could measure angles to within about one arc-minute, a dramatic improvement over the several-degree errors common with earlier instruments.
The Longitude Connection
Knowing your latitude at sea had been possible for centuries. You measured the altitude of the North Star or the noon sun, applied some arithmetic, and got a reasonably good answer. Longitude was the nightmare. Without an accurate way to determine how far east or west you were, ships routinely sailed hundreds of miles off course, sometimes with catastrophic consequences. The famous Scilly naval disaster of 1707, in which four Royal Navy ships struck rocks and nearly two thousand men drowned, was largely a failure of longitude determination.
Two solutions emerged in the eighteenth century, and both depended on precise angular measurement. The first was the lunar distance method. The moon moves against the background stars at a predictable rate, and by carefully measuring the angle between the moon and a known star, a navigator could work out Greenwich time and compare it to local time to find longitude. This method required accurate lunar tables, which took decades to develop. Nevil Maskelyne, the fifth Astronomer Royal, compiled the first Nautical Almanac for the year 1767, which finally made it possible to determine longitude at sea to an acceptable precision using lunar distances.1Vistas in Astronomy. Lunar distances and the Nautical Almanac The sextant was the essential tool for taking those measurements. An octant could not reach the wide angles that lunar distances often required, which is exactly why Campbell pushed for the sextant’s development in the 1750s.
The second solution was the marine chronometer, a precision timepiece that could keep Greenwich time throughout a long voyage. John Harrison’s famous chronometers proved the concept, and by the early nineteenth century, affordable chronometers had become available to merchant captains. Even after chronometers became widespread, the sextant remained indispensable: a chronometer tells you time, but you still need the sextant to observe the sun or stars and convert those observations into a position fix. The two instruments were partners, not competitors. And when a chronometer’s rate drifted or it stopped, lunar distances taken with a sextant were the backup method for checking or recovering accurate time.
How a Sextant Actually Works
The instrument is built around a rigid frame shaped like a pie slice, roughly one-sixth of a circle. At the top sits an index mirror, mounted on a movable arm that sweeps along the graduated arc at the bottom. A second mirror, called the horizon glass, is fixed in position near the eyepiece. The horizon glass is half-silvered (or half-clear): one side reflects and the other side lets light pass straight through.
When you look through the eyepiece, you see the horizon directly through the clear half of the horizon glass. Meanwhile, light from a star or the sun bounces off the index mirror, hits the silvered half of the horizon glass, and reflects into your eye alongside the horizon view. By moving the index arm, you tilt the index mirror until the celestial image sits right on the horizon line. Then you read the angle off the arc. The double reflection means the measured arc is half the actual angle, so the scale is marked to compensate: when the index arm moves 30 degrees along the arc, the instrument is reading 60 degrees of actual angle.
Colored shades swing in front of either mirror to protect the observer’s eyes during sun sights. A small telescope or monocular attached to the eyepiece magnifies the view for finer alignment. The frame was originally made of wood with brass fittings, but by the late eighteenth century, all-brass construction became standard. Later instruments used aluminum alloy to save weight.
Precision, Craftsmanship, and the Instrument-Making Trade
The accuracy of a sextant depended entirely on the quality of its manufacture. Dividing the arc into precise graduations was an exacting craft. In the early decades, each graduation was cut by hand using a dividing engine, a specialized machine with a precisely calibrated worm gear. Jesse Ramsden, a London instrument maker, built an improved circular dividing engine in 1775 that could engrave arc divisions with unprecedented accuracy. His machine and its successors transformed the instrument trade and made affordable, high-quality sextants available in quantity for the first time.
London became the center of the maritime instrument trade in the eighteenth and early nineteenth centuries. Firms clustered around the docks and the Admiralty, and the best makers, including Ramsden, Peter Dollond, and later firms like Troughton and Simms, built reputations that captains and naval officers trusted their lives to. A well-made sextant was expensive but durable. Many instruments from the 1800s survive today in working condition, still capable of producing an accurate sight.
The trade was not exclusively male. Janet Taylor, a remarkable figure of the nineteenth century, ran a successful navigation instrument business in London from the 1830s onward, selling and adjusting sextants, compasses, and chronometers while also publishing textbooks on navigation and teaching mariners. She held an agency for chronometer rating and earned respect in a trade that rarely welcomed women. Her work is a reminder that the history of navigation instruments includes not just the inventors but the wider network of makers, teachers, and retailers who kept the tools in sailors’ hands.
Why the Sextant Survived GPS
You might expect that satellite navigation would have made the sextant completely obsolete. GPS does provide position fixes accurate to within a few meters, which is orders of magnitude better than any celestial observation. Yet the sextant has not disappeared. The U.S. Naval Academy reintroduced celestial navigation into its curriculum in 2015 after a period in which it had been de-emphasized. The Royal Navy and several other navies have maintained celestial navigation training without interruption. Large commercial vessels still carry a sextant aboard as a regulatory or practical backup.
The reasoning is straightforward. GPS signals can be jammed, spoofed, or lost. Military planners worry about electronic warfare scenarios in which satellite navigation becomes unavailable. Even in peacetime, there have been incidents of GPS outages affecting large areas, and solar storms can degrade signal quality. A sextant requires no batteries, no satellites, no software updates, and no data link. If the sky is clear and you know the procedure, you can fix your position. The accuracy, within a nautical mile or so for a competent observer, is more than sufficient to keep a ship from running aground or to navigate across open ocean.
Recreational sailors also maintain a quiet affection for the sextant. Learning celestial navigation is part of the culture of long-distance sailing, and there is a genuine practical argument for having a skill that works when electronics fail. Sextants manufactured today, both metal instruments from makers like Astra and plastic training models from Davis Instruments, continue to sell in modest but steady numbers.
Common Misconceptions About the Sextant’s History
A few myths cling to the sextant story. One is that it was a single dramatic invention, a eureka moment by one genius. In reality, the development was gradual and collective. Robert Hooke had proposed a reflecting instrument as early as the 1660s, though it was not practical enough to catch on. Newton sketched one but never shared it. Hadley and Godfrey independently built working versions, and then a generation of instrument makers, naval officers, and astronomers refined the design into the sextant proper. It was an evolution driven by practical need at every step.
Another misconception is that the sextant “solved” the longitude problem. It was necessary but not sufficient. Without the accurate lunar tables that took decades of astronomical observation to compile, and without the Nautical Almanac that put those tables into usable form, a sextant alone could not determine longitude.1Vistas in Astronomy. Lunar distances and the Nautical Almanac And the chronometer provided a separate, simpler path to the same answer. The sextant was a critical piece of a larger system, not a standalone solution.
A third confusion involves the word itself. People sometimes assume a sextant can measure any angle, or that “sextant” is a generic term for any handheld navigation instrument. Strictly, a sextant has an arc of 60 degrees and a usable range of about 120 degrees. A quintant has a 72-degree arc (one-fifth of a circle) and a range of 144 degrees. An octant has a 45-degree arc and a range of 90 degrees. The names describe the geometry. All three are reflecting instruments using the same double-mirror principle, but they are sized for different observational tasks.
Sextant Use Beyond the Sea
Though the sextant is primarily associated with maritime navigation, it found uses well beyond the deck of a ship. Surveyors adopted sextants for angle measurement on land, particularly in areas where setting up a full theodolite was impractical. Explorers carried sextants into the interior of continents throughout the nineteenth century, fixing positions of rivers, mountains, and boundaries. Much of the early mapping of Africa, Australia, and the American West depended on sextant observations of the sun and stars.
Aviation took up the sextant in the early twentieth century. Long-range aircraft flying over oceans had no landmarks to navigate by, and before electronic navigation aids like LORAN became widespread in the 1940s and 1950s, a bubble sextant was standard equipment in the navigator’s station. Unlike a marine sextant, which uses the natural sea horizon, a bubble sextant contains a small spirit level that creates an artificial horizon. This allowed observations from an aircraft where the true horizon might be obscured by clouds or simply too indistinct at high altitude. Navigators aboard B-29 bombers, flying boats, and early transatlantic airliners all relied on bubble sextants to cross featureless expanses of ocean. Even early spacecraft carried sextants: the Apollo command module included a sextant and scanning telescope that astronauts used for navigation checks during the journey to the moon and back.
The thread connecting all these uses is the same one that motivated Hadley and Godfrey in 1731: when you need to know where you are, and you can see the sky, a pair of mirrors and a graduated arc can give you an answer. The principle has not changed in nearly three centuries. The optics are better, the materials lighter, and the tables have been replaced by phone apps, but the geometry is the same one Newton sketched and never showed anyone.