In the summer of 1731, John Hadley in London and Thomas Godfrey in Philadelphia independently invented the same instrument. Neither knew of the other's work. Both submitted their designs within months. The Royal Society examined both claims and declared the honor shared. The instrument they built — a device for measuring the angle between the sun and the horizon using two mirrors and a graduated arc — would remain the primary tool of ocean navigation for more than two centuries.
It is still taught today. The United States Naval Academy reinstated celestial navigation training in 2015 after dropping it in the 1990s. The proximate cause was the USS John S. McCain collision in 2017, which renewed concerns about GPS dependence. The sextant did not cause that accident. But the accident reminded the Navy what sextant competence was for: a fallback that does not require satellites, electricity, or a network.
What Came Before
The history of the sextant is really the history of the problem it solved: measuring angles precisely at sea on a moving platform.
The astrolabe, in use since classical antiquity, required the observer to hold the instrument level while reading it — difficult on a rolling deck. The cross-staff, common in the fifteenth and sixteenth centuries, required the observer to look simultaneously at the horizon and the sun, blinding them if the sun was involved. The backstaff, invented by John Davis around 1594, improved on this by allowing the sun to be read indirectly — the observer faced away from the sun and used shadows — but the instrument remained sensitive to angle errors and limited to sun observations from the stern.
All of these instruments shared a fundamental problem: they measured angles by looking at two things at once and aligning them visually, with no mechanism for stabilizing the reading against the ship's motion.
The Octant, 1731
What Hadley and Godfrey separately understood was the principle of double reflection. If you reflect a celestial body off two mirrors — once off a mirror attached to a movable arm (the index arm), once off a half-silvered mirror — the reflected image of the body appears in the same eyepiece as the direct view of the horizon. You adjust the index arm until the two images align. The angle of the arm tells you the angle between the body and the horizon.
The critical insight: because both images move together when the ship moves, the reading remains stable despite ship motion. The observer is not trying to hold anything level. They are adjusting until two images coincide, which they can do regardless of how the ship is rolling.
Hadley's instrument used an arc of one-eighth of a circle — hence "octant." It could measure angles up to 90 degrees, sufficient for most latitude observations.
The Sextant, 1757
Captain John Campbell of the Royal Navy, working with the instrument maker John Bird, extended Hadley's arc from one-eighth to one-sixth of a circle in 1757. This gave the instrument a 120-degree range rather than 90 degrees — enough to measure the lunar distance, which required angles between the moon and certain stars that could exceed 90 degrees.
The instrument with a sixth-of-a-circle arc was called a sextant. The name stuck even as later instruments were sometimes built with larger arcs.
Ramsden's Dividing Engine, 1775
A sextant is only as good as the accuracy of its graduated arc. The arc must be precisely divided into degrees, minutes, and fractions of minutes. Until the 1770s, this division was done by hand, and the errors in hand-divided scales were the primary limitation on accuracy.
Jesse Ramsden's dividing engine of 1775 changed this. The machine could divide a circle into equal parts mechanically, with errors of less than four arc-seconds — far better than any hand division. Ramsden made the design public, and within a few years, instrument makers across Europe and America were producing sextants with arcs accurate to fractions of a minute.
A well-made sextant with a Ramsden-divided arc could measure angles to within 10 arc-seconds under reasonable conditions. At sea, with a practiced observer and a clear horizon, this translated to a position accuracy of a few miles of latitude.
The Lunar Distance Method
Latitude was always the easier problem. Measure the sun's altitude at local noon, correct for the sun's declination for that date, and you have your latitude. Simple in principle, though requiring good tables and a good instrument.
Longitude was different. The standard solution — the marine chronometer — required a highly accurate clock that could keep Greenwich time over months at sea. Harrison's marine chronometer (H4, completed 1761) solved this, but chronometers remained expensive through the early nineteenth century.
The sextant offered an alternative: the lunar distance method. The moon moves against the background stars at a predictable rate. By measuring the angular distance between the moon and a known star, and consulting the Nautical Almanac (first published 1767), an observer could determine Greenwich time and therefore longitude — without a chronometer.
The calculation was laborious. It required correcting for the different altitudes of the moon and the reference star, and it took perhaps thirty minutes of arithmetic. Nathaniel Bowditch, the American navigator, published simplified methods in his New American Practical Navigator (1802) that made the calculation accessible to ordinary seamen.
By the mid-nineteenth century, chronometers had become cheap enough that most ships carried them, and the lunar distance method faded. But for the fifty-odd years between the sextant's refinement and the widespread adoption of the chronometer, the combination of sextant and lunar distance was the only reliable method of ocean navigation available to those who could not afford Harrison's instrument.
The Sextant and the Chronometer Together
Once chronometers became standard equipment, the sextant's role stabilized. The two instruments worked together: the chronometer kept Greenwich time, and the sextant measured the altitude of celestial bodies. From the measured altitude and the known Greenwich time, the navigator could solve for both latitude and longitude.
This combination made ocean navigation reliably accurate for the first time. Not safe — weather, reefs, and human error remained — but accurate in the sense that a skilled navigator could consistently determine position within a few miles, anywhere on earth, in clear weather.
No other technology would meaningfully improve on this until radar in the 1940s and GPS in the 1980s and 1990s.
290 Years of Continuous Use
The sextant is one of the longest-lived precision instruments in continuous use. The basic optical design has not changed since the 1750s. The graduated arc has been refined. Materials improved from brass to lightweight alloys. Optics improved. Drum micrometers replaced vernier scales for reading fractions of minutes. But a navigator from 1780 could pick up a modern sextant and use it within minutes.
That stability is worth noticing. The sextant solved a hard problem in an elegant way — double reflection stabilizing the reading against platform motion, a moving index arm converting angle to scale position — and the solution turned out not to need improvement. When a tool is this stable for this long, it usually means the design was close to optimal from the beginning.
GPS has not made the sextant obsolete. It has made sextant skill optional. Those are different things. The difference becomes apparent when GPS is unavailable — through jamming, spoofing, solar weather, or equipment failure — and the ship with a trained navigator and a sextant can continue, while the ship without cannot.
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