Sextants vs Satellite Positioning: Two Ways to Fix a Location

A class dropped, then brought back

In the late 1990s the U.S. Naval Academy stopped teaching celestial navigation. Satellites could deliver a position with remarkable ease, and a difficult skill built on tables and arithmetic seemed to belong in a museum. Years later the academy reversed course. Government Executive reported that the school reinstated the subject, with the class of 2017 the first to graduate having received it again.

The reason was not sentiment. Government Executive describes satellite systems as vulnerable to cyber attack, whereas sextants, almanacs and tables are not exposed to that kind of threat. That reversal is the most useful way into this comparison, because it shows that the older method was never really about beating the newer one on convenience. It was about having a second way to answer the same question.

Two mirrors and a horizon

The sextant grew from the reflecting octant. According to the MacTutor biography of John Hadley, he presented his double-reflection instrument to the Royal Society on May 13, 1731. Thomas Godfrey of Philadelphia is credited with developing a similar device independently, though his version reached London in 1732, after Hadley's presentation. Both men arrived at the idea on their own, which is why histories usually name both.

The design was clever. Two mirrors let the observer see the horizon and a celestial body at once, so a moving deck did not spoil the reading. Sea trials aboard the Chatham in 1732 confirmed its value, Hadley patented it in November 1734, and in 1757 Captain John Campbell extended it into the sextant.

From an angle to a place on the chart

Measuring an angle is only the first step. The navigator notes the exact time of the sight and uses almanac data and reduction tables to compute a line on which the ship must lie. Several sights of different bodies, or of the sun at different times, give lines that cross at a fix. Each step requires care, and errors in the clock or arithmetic show up in the result.

The payoff is independence. The whole process needs only light, a horizon and a good timepiece, and none of it depends on anyone's transmitter. Government Executive notes that the technique can put a navigator within about 1.5 miles, or 2.4 kilometers, of the true position, which is more than adequate for a ship at sea with no hazard nearby, even if it would be crude in a harbor.

The practical rhythm of the work was different too. A navigator could not simply glance at an instrument whenever curious. Sights had to wait for a visible horizon and a body worth measuring, so between fixes the crew relied on dead reckoning, estimating position from speed, heading and elapsed time. A celestial fix acted as a periodic correction to that running estimate, rather than a constant readout.

Timing signals from orbit

Satellite positioning replaces the sky's stars with artificial ones. GPS.gov reports 31 operational satellites as of July 2023, while the United States is committed to keeping at least 24 available 95 percent of the time. They circle at about 20,200 kilometers, arranged in six orbital planes, and the arrangement lets a user see at least four of them from nearly anywhere. A receiver uses the timing of their signals to compute where it is.

In practice the receiver does everything. GPS.gov states that smartphones are typically accurate to within about 4.9 meters, or 16 feet, under open sky, though buildings, bridges, trees and indoor locations weaken reception. Selective Availability, the deliberate degradation of civilian signals, was discontinued in May 2000 on the president's direction, after which ordinary receivers could realize much more of the system's accuracy.

The contrast with the older method is striking in one respect: the receiver contributes no observation of its own. It does not look at anything or measure an angle to the sky. It trusts a network built and operated by others, and that trust is efficient because the network is so extensive, yet it is also the reason navies started worrying about what happens if the signals are unavailable.

Accuracy, effort and independence side by side

On accuracy the difference is large. A celestial fix within roughly a mile or two versus a receiver's few meters under open sky makes satellites the clear choice when precision matters, such as approaching a harbor or flying an instrument approach. Speed follows the same pattern: a sextant fix takes minutes of observation and calculation, while a receiver displays a position at once and keeps updating it.

Effort is where the balance shifts. A sextant demands training and steady hands, and its results depend on clear skies. A receiver requires almost no skill and works through clouds and at night. The price is dependence on a powered device and on signals from a system the user does not control. Neither dependence is dramatic on an ordinary day, but both matter when something goes wrong.

Wear, vulnerability and who holds the keys

A sextant is a small mechanical and optical instrument that a specialist can repair, and it can last decades if treated with care. A receiver is sealed electronics that are replaced, not mended. Their vulnerabilities also differ in kind. The sextant's are physical: clouds, haze, a bad horizon, a knocked mirror. The receiver's are systemic: blocked or reflected signals, power loss, or interference with the satellites' transmissions.

Privacy and control run along the same line. A navigator with a sextant decides who learns the position. A receiver listens passively, but the software around it may log or share where it has been, depending on settings. The comparison here is the same one that appears in printed maps and GPS navigation, where independence from infrastructure trades against convenience.

What the sextant taught that the screen does not

Celestial navigation forced a person to understand position as a relationship among angle, time and geometry. That literacy, more than the instrument, is what a screen makes optional. Someone who has worked a sight knows why an error of a few minutes in the clock matters, and why one position line is not a fix. Those insights help in judging any position reading, including one from a satellite.

Timekeeping is the other inheritance. Both methods depend on accurate time, whether it is a chronometer on the ship or an atomic clock in orbit, which is the same theme that runs through analog clocks and smartwatches. What has been lost is not the accuracy of the new method but the general knowledge of how to rebuild a position when it disappears.

Insurance rather than nostalgia

Modern seafarers use satellites as their main tool, and few need celestial navigation day to day. But the Naval Academy's decision shows how the old method has been reframed: not as an alternative but as a backup. Any critical system that depends on external signals benefits from a check that does not. A compass gives a heading even when electronics fail, a topic explored in magnetic compasses and smartphone sensors.

The fairest reading is that the two methods are best used together. Satellite positioning is faster, finer and easier, and it deserves to be the default. The sextant is slower and coarser, yet it offers something a receiver cannot: a position derived from the sky itself, with no one else in the loop. Choosing between them depends less on which is better than on what a failure would cost.