The Sextant Explained

The /learn/celestial-navigation article covers the navigation method — the sextant + chronometer + Nautical Almanac + sight reduction. This article covers the instrument: how the sextant works, what came before, the components and corrections, and the modern manufacturers who still produce them.

The optical principle: double reflection

A sextant measures the angle between two visible objects by optically bringing them into the same field of view. The trick is double reflection.

Light from the celestial body strikes the index mirror (fixed to a movable arm), reflects to the horizon mirror (half-silvered, fixed to the frame), and reflects again into the telescope. The other half of the horizon mirror is transparent, so the navigator sees the real horizon through it directly. The sextant brings the reflected celestial body into apparent contact with the horizon line.

The geometric principle: when light is reflected twice, the total angle of deviation is twice the angle between the mirrors. So a sextant with a 60° physical arc can measure angles up to ~120° by reading the index-arm position on a scale graduated to twice its physical angle.

This double-reflection principle was first described in an unpublished 1699 paper by Newton (rediscovered after the 1730s inventions). Hadley and Godfrey worked independently of Newton; both arrived at the principle through their own optical reasoning.

Predecessors

Several instruments preceded the sextant. Each was displaced by its successor as accuracy and ease of use improved.

The cross-staff (medieval through 17th century)

A long staff with a sliding crosspiece. The navigator aligned the staff with the horizon, slid the crosspiece until its top edge touched the celestial body and bottom edge touched the horizon, and read the angle from graduations on the staff. Accuracy: roughly 1–2 degrees. The navigator had to look directly at the sun, risking eye damage.

The mariner's astrolabe (15th–17th century)

A circular brass or bronze disc with an alidade (sighting arm) pivoted at the centre. Used by Portuguese and Spanish navigators on the early voyages of discovery. Accuracy on land: ~10 arcminutes; at sea: ~30 arcminutes due to ship motion. The astrolabe was the standard navigation instrument from roughly 1480 to 1700.

The backstaff / Davis quadrant (1594 onwards)

Invented by English navigator John Davis in 1594. The navigator faced away from the sun and observed its shadow on a graduated arc. Eliminated the eye-damage risk of the cross-staff. Accurate to ~3–5 arcminutes. The Davis quadrant dominated English-speaking navigation through the early 18th century.

The Hadley quadrant / octant (1731)

John Hadley's 1731 instrument was a 45° arc (one eighth of a circle, hence “octant”) with double-reflection optics. The double-reflection principle let it measure angles up to 90°. The Hadley quadrant was accurate to ~1 arcminute and rapidly displaced earlier instruments. The sextant (Hadley's and other makers' later 60° version) extended the same optics to a larger arc.

The components

A modern marine sextant has these key parts.

The frame

The structural arc, traditionally bronze or brass, modern versions in aluminium or magnesium for lighter weight. The arc spans 60° (sometimes 70° for “long-arc” sextants that measure higher altitudes). The arc is graduated in degrees, typically to 0.5° divisions; the drum

• vernier read finer divisions.

The index arm and index mirror

A pivoted arm extending from the centre of the arc. The index mirror is rigidly attached to the index arm; as the arm moves, the mirror rotates. The index arm slides along the graduated arc; the position of the arm reads the gross angle (to half a degree).

The horizon mirror

A half-silvered mirror fixed to the frame near the telescope. Half the mirror is silvered (for the reflected celestial body); the other half is transparent (for the direct view of the horizon). The two views — direct horizon and reflected body — appear adjacent in the telescope's field of view.

The telescope (or sighting tube)

A small telescope (typically 4× to 7× magnification) mounted along the optical path. The navigator looks through the telescope, sees both the horizon and the reflected body, and adjusts the index arm until the body's lower edge (limb) appears to touch the horizon.

The drum and vernier (or micrometer)

The fine-adjustment mechanism. The drum is a numbered wheel attached to a worm gear that drives the index arm through small angles. One rotation of the drum corresponds to 1 arcminute (typically). The vernier scale next to the drum reads fractions of an arcminute (typically to 0.2'). Modern micrometer drums replace the older vernier.

The shades (filters)

Coloured glass filters that swing into the optical path to attenuate sunlight. Multiple shades of varying density let the navigator look at the sun safely. Modern sextants also have shades over the horizon mirror to attenuate glare from bright sea or low-sun horizons.

Handle and grip

The sextant is held in one hand by a wooden or polymer grip; the other hand operates the drum. The instrument is swung gently from side to side to find the true vertical plane through the body — the lowest point of the swing is the correct measurement.

How to take a sight

The full procedure for taking a sextant sight of the sun:

1. Pre-set the index arm to roughly the expected altitude (e.g., 35° if the sun is well up).
2. Add shades over the index mirror to dim the sun safely; add lighter shades over the horizon mirror if glare is bright.
3. Aim the telescope at the horizon below the sun.
4. Lower the sun by adjusting the index arm: rotate the index arm to make the reflected sun image appear to come down in the field of view, until it's close to the horizon.
5. Fine-tune with the drum: rotate the drum until the sun's lower limb just touches the horizon.
6. Rock the sextant: gently tilt the instrument left and right; the sun image traces a small arc. The measurement is at the lowest point of the arc — this ensures the sextant is in the true vertical plane through the sun.
7. Note the time: at the instant of contact, note the exact time from the chronometer (to the second).
8. Read the angle: degrees from the index arm position; arcminutes from the drum; tenths from the vernier.

A skilled navigator takes the sight in 30–60 seconds, including the time call. The angle is the sextant altitude (Hs) — uncorrected; the five corrections give the observed altitude (Ho) used in sight reduction.

The five corrections

Index correction

The sextant's zero reading may not be true zero, due to mechanical imperfections or temperature changes. The navigator measures index error by sighting the horizon itself (sea horizon at the geometric horizontal) and applying the correction (typically ±1' to ±3'). Modern sextants have an adjustment screw to zero the mirrors; even with adjustment, a residual index error is checked and applied to every sight.

Dip correction

The visible horizon is below the geometric horizontal by an amount that depends on the observer's height of eye:

dip ≈ 1.06 × √(height_in_metres) arcminutes

So a navigator with eye height 5 m sees the horizon at about 2.4 arcminutes below horizontal. Dip is subtracted from the sextant altitude.

Refraction correction

Light from a celestial body bends through Earth's atmosphere, making the body appear higher than it really is. At the horizon, atmospheric refraction lifts the apparent altitude by ~34 arcminutes; the correction decreases with altitude (~5' at 10° altitude, ~1' at 30°, ~0' at the zenith). Refraction is subtracted from the sextant altitude. The Nautical Almanac tabulates refraction by altitude.

Parallax correction

The sun, moon, and planets show measurable parallax — the apparent altitude depends on the observer's position on Earth's surface relative to the centre of the Earth. Most pronounced for the moon (up to 1° of horizontal parallax), much smaller for the sun and planets (under 9'). The Almanac provides the correction.

Semi-diameter correction

The sun and moon are visible discs (not points). The navigator typically measures the lower limb touching the horizon and adds the semi-diameter to get the centre's altitude. The sun's semi-diameter is ~16 arcminutes; the moon's varies from ~15 to ~16.5 arcminutes due to its elliptical orbit. The Almanac tabulates daily.

The bubble sextant

For aviation use — where the horizon may not be visible (above clouds, at night) — the bubble sextant uses an artificial horizon. A small spirit level inside the instrument provides a horizontal reference; the navigator aligns the bubble with the celestial body. The U.S. Army Air Forces used bubble sextants extensively in World War II for B-29 navigation across the Pacific.

Bubble sextants are less accurate than marine sextants (~5–10 arcminutes vs. ~0.2 arcminutes) because the bubble is sensitive to aircraft acceleration and turbulence. The errors translate to ~5–10 nautical miles of position error. Modern aviation has replaced celestial navigation with GPS-based systems, but bubble sextants remain in some emergency-survival kits.

Modern manufacturers

Despite GPS's dominance, several manufacturers still produce sextants:

• Cassens & Plath (Bremen, Germany, founded 1916). The premium maker, used by Lloyd's-classed shipping and many world navies. Models include the Horizon (entry- level professional), Professional, and Ultra. Price range €1,500–3,500.
• Tamaya (Tokyo, Japan). Mid-range; the MS-833 is widely used in Japanese commercial shipping. Price range $800–1,500.
• Astra IIIB (Shanghai, China). Affordable; the most common sextant in amateur sailing. Price range $250–400.
• Weems & Plath (USA). Distributes Cassens & Plath and produces some plastic-bodied training sextants.

The U.S. Naval Academy issues sextants to midshipmen as part of its 2015-restored celestial-navigation curriculum. The U.S. Merchant Marine Academy and the Royal Navy similarly include sextant proficiency in their training.

Modern use

Sextants are still used for:

• Commercial-ship GPS-backup navigation: most ocean- going vessels carry one or two sextants and chronometers, with periodic bridge-officer training.
• Naval training: U.S. Naval Academy (since 2015), U.S. Merchant Marine Academy, Royal Navy, and many other navies require celestial-navigation proficiency.
• Offshore yacht racing: many race rules require celestial-capable navigation.
• Amateur ocean sailing: many circumnavigators learn celestial navigation as a redundancy.
• Surveying and astronomy: occasionally used as a precision angle instrument outside navigation contexts.

Sextant collecting and restoration is an active hobbyist community. Antique sextants from major makers (Plath, Heath, Stanley, Spencer Browning & Rust) command prices ranging from a few hundred dollars to tens of thousands for rare presentation pieces.

Common misconceptions

“A sextant has a 60° arc and measures up to 60°.” It has a 60° physical arc but measures up to 120° because of double reflection. The graduated scale on the arc reads twice the physical arc angle.

“A sextant measures latitude directly.” It measures the angle of a celestial body above the horizon. Latitude must be computed from that angle plus the body's declination plus other corrections. The exception is Polaris in the Northern Hemisphere — Polaris' altitude (with a small correction) equals the observer's latitude. For any other body, latitude requires calculation.

“A sextant is obsolete in the GPS era.” Most ocean-going commercial and naval ships still carry sextants and require their officers to maintain proficiency. The reasoning: GPS can fail (jamming, spoofing, satellite outages, solar storms), and the sextant is a robust electromagnetism-independent fallback. The U.S. Naval Academy reinstated celestial- navigation training in 2015 explicitly to address GPS- vulnerability concerns.

“Sextants are simple.” They're optically simple but mechanically demanding. A precision sextant requires careful manufacture (mirror alignment to arcsecond tolerances, drum to sub-arcminute precision) and careful use (rocking technique, correct shade selection, awareness of all five corrections). A novice's first sights are typically 5–10 arcminutes off; arc-minute accuracy requires practice and patience.

“The sextant was invented by Newton.” Newton described the double-reflection principle in an unpublished 1699 paper, but he never built an instrument. The sextant was invented by Hadley and Godfrey in 1731, working independently of Newton; the discovery of Newton's priority paper was after both inventions.

“Modern sextants use digital displays.” Some experimental electronic sextants exist, but the standard production sextants from Cassens & Plath, Tamaya, and Astra are entirely mechanical — drum + vernier scale, no electronics. The mechanical instrument is more reliable in marine conditions (no batteries, no electronic failure modes) and the optical principle is unimprovable by digitization.

“A bubble sextant is as accurate as a marine sextant.” It isn't. Bubble sextants are typically 25–50× less accurate due to the bubble's sensitivity to motion. Marine sextants use the real horizon, which is far more stable. Bubble sextants exist specifically for situations where the real horizon is unavailable; the accuracy trade-off is accepted.