The sextant has guided mariners across open ocean for centuries, and it remains an essential skill and a required instrument on vessels where GPS alone can’t be trusted as the sole means of navigation. This guide covers the principles, components, error types, usage techniques, and maintenance practices every mariner needs to use a sextant with confidence and precision.
Purpose: Measures the angle between the horizon and a celestial body, including the Sun, Moon, planet, or star, enabling calculation of latitude and longitude at sea through celestial navigation.
Name origin: From the Latin sextus (one-sixth); the arc spans 60 degrees but enables measurement up to 120 degrees through the principle of double reflection.
Core components: Arc-shaped frame, horizon mirror, index mirror, shade glasses, telescope, graduated scale, and micrometer drum gauge.
Two optical principles: The angle of incidence equals the angle of reflection; and after two successive reflections, the angle between incident and reflected rays equals twice the angle between the mirrors.
Adjustable errors: Perpendicularity error, side error, and index error, all correctable onboard using established mirror adjustment procedures.
Non-adjustable errors: Graduation, centering, shade, and optical errors, typically caused by manufacturing defects or wear, requiring professional correction.
What the Sextant Is and How It Works
The sextant is the defining instrument of celestial navigation, a precision optical tool that allows mariners to measure the angle between the visible horizon and a celestial body such as the Sun, Moon, or a navigational star. From that measured angle, combined with accurate time and the appropriate nautical almanac data, a mariner can calculate the vessel’s latitude and longitude at sea without any electronic aids. The instrument derives its name from the Latin sextus, meaning one-sixth, because its arc spans 60 degrees, one sixth of a full circle. Through the optical principle of double reflection, a 60-degree arc enables measurement of angles up to 120 degrees, giving the instrument a range that covers all practical celestial navigation requirements.
To take a sight, the sextant is held vertically. The navigator looks through the telescope and aligns the horizon through the unsilvered portion of the horizon mirror, then adjusts the index arm until the reflected image of the celestial body appears to rest precisely on the horizon line. The altitude, the angle between the horizon and the body, is then read from the graduated scale on the arc and the micrometer drum gauge. Beyond altitude, the same instrument can measure Vertical Sextant Angles for estimating distance off a charted object, and Horizontal Sextant Angles for fixing position by three-point bearing.
A Brief History of the Instrument
The sextant emerged from a long lineage of celestial measurement tools. Arab navigators used the kamal, a simple card-and-string device, from the 6th century to measure star altitudes for latitude determination. The astrolabe and quadrant followed, becoming the instruments of choice for European ocean exploration. By the 15th century, Portuguese navigators were using quadrants to measure the Sun’s altitude and determine latitude from its declination, a technique Christopher Columbus employed extensively on his Atlantic voyages. The reflecting sextant, which combined the optical principle of double reflection with a precise graduated arc, emerged in the 18th century and rapidly superseded its predecessors as the standard instrument for celestial navigation.
The sextant has remained fundamentally unchanged for nearly three centuries, not because innovation has stalled, but because the design achieved such precision and reliability that improvement has been incremental rather than transformative. A mariner who can use a sextant well will never be without a means of fixing position at sea, regardless of what electronic systems fail around them.
Key Components of the Sextant
Understanding and Correcting Sextant Errors
Accurate celestial navigation demands that the sextant is free from correctable errors before use, and that non-correctable errors are known and accounted for. Errors fall into two distinct categories, each requiring a different response from the navigator.
Adjustable errors, including perpendicularity error (index mirror not perpendicular to the frame), side error (horizon mirror not perpendicular to the frame), and index error (the reading when the index arm is set to zero doesn’t coincide with the true zero), can all be corrected onboard using established mirror adjustment procedures. Index error in particular should be checked and noted before every set of sights, as it’s the most commonly encountered and the most directly significant to altitude readings.
Non-adjustable errors, including graduation errors in the arc, centering errors in the index arm pivot, shade glass errors, and optical errors in the telescope or mirrors, are typically caused by manufacturing tolerances or long-term wear and require professional attention rather than onboard correction. Their magnitudes should be known from the instrument’s certificate and applied as corrections to observed altitudes.
Dip correction: One additional correction applies universally, dip, the angle between the sensible horizon and the visible horizon. Dip varies with the observer’s height of eye above sea level and is always applied as a negative correction to all observed altitudes. Standard dip tables are provided in the nautical almanac. Omitting dip from the correction chain is one of the most common sources of systematic error in celestial position fixes.
Accurate sextant work begins before the sight is taken, by checking for errors, setting the instrument correctly, and understanding which corrections to apply to the raw altitude reading. A navigator who takes sights without this preparation may achieve precision in the observation and still arrive at a wrong position.
Usage Tips and Maintenance
- Always check for and record errors before taking sights
- Focus the telescope on the horizon and mark the telescope stem before beginning
- Maintain a steady stance with feet slightly apart to absorb vessel motion
- Swing the sextant in a small arc to identify the celestial body’s lowest point, the true altitude
- Use appropriate shade glasses when observing the Sun; never observe unfiltered
- Avoid nighttime observations in conditions where a false horizon may form above the true horizon
- Avoid stressing the index bar and never touch the arc with bare fingers
- Keep the worm and rack mechanism clean and lightly lubricated with Vaseline
- Clean mirrors, lenses, and shade glasses only with a soft, lint-free cloth
- Store the sextant in its fitted box, away from direct sunlight, dampness, and heat sources
- Have the instrument professionally checked and certificated at regular intervals
- Record known non-adjustable errors from the certificate and apply them consistently
Frequently Asked Questions
Why is the sextant called a sextant if it can measure up to 120 degrees?
The name derives from the Latin sextus (one-sixth), referring to the instrument’s arc which spans 60 degrees, one-sixth of a full circle. Because of the principle of double reflection, this 60-degree arc enables measurement of angles up to 120 degrees, but the traditional name was retained from the instrument’s original design.
What is index error and how is it corrected?
Index error is the discrepancy in the sextant’s reading when the index arm is set to zero, caused when the index and horizon mirrors aren’t exactly parallel. It’s detected by observing the horizon with the index arm at zero and noting any offset between the direct and reflected images. Index error is either corrected by mirror adjustment or noted and applied as a correction to all altitude observations.
What is the dip correction and why is it always negative?
Dip is the angle between the sensible horizon (perpendicular to the observer’s vertical) and the visible horizon at sea. Because the observer’s eye is above sea level, the visible horizon always appears lower than the sensible horizon, meaning observed altitudes are always slightly too high. Dip is therefore always subtracted from the observed altitude. Its value varies with height of eye and is tabulated in the nautical almanac.
Is it still necessary to learn celestial navigation when GPS is available?
Yes. Celestial navigation remains a required competency under STCW for officers in charge of a navigational watch on vessels operating in international waters. GPS and electronic chart systems can fail, be jammed, or be deliberately spoofed, and a navigator who can’t fix position by celestial means in such circumstances leaves the vessel without a reliable backup. The sextant is the only navigational instrument that requires no external signal, power source, or database to function.
Sources: Bowditch — The American Practical Navigator (NIMA Publication No. 9) · Admiralty Manual of Navigation Vol. 2 (celestial navigation and sextant use) · UK Hydrographic Office Nautical Almanac · IMarEST professional navigation references
