Using a Sextant

Sextant Mechanics — How It Actually Works

A sextant measures the angle between two objects — typically a celestial body and the sea horizon. It does this using two mirrors and a precisely machined arc. The index mirror is mounted on the movable index arm, pivoting around the center of the instrument. The horizon mirror is fixed, half-silvered: the top half reflects, the bottom half is clear glass. The observer looks through the telescope at the horizon through the clear half of the horizon mirror, while the index mirror reflects the celestial body down into the silvered half.

When the index arm is moved along the arc, the index mirror tilts, raising or lowering the reflected image of the body. When the reflected image of the body appears to sit exactly on the visible horizon, the angle on the arc is the sextant altitude (Hs). The key property that makes this work — and makes the sextant far more accurate than a simple protractor — is the law of reflection: because the angle of incidence equals the angle of reflection, the angle on the arc is exactly half the angle between the two lines of sight. The arc is graduated to account for this factor of two.

The main arc is graduated in whole degrees, typically from −5° to 120° or more. The micrometer drum (or vernier, on older instruments) reads the minutes within each degree. A standard drum reads to 1 arcminute, with the vernier or digital readout adding tenths of a minute (0.1'). A careful observer can read to 0.1' with practice — that's 6 arc-seconds, corresponding to about 185 meters of position error at the equator.

Optical filters — shade glasses — attach to the index arm (for viewing the body) and to the horizon mirror (for reducing horizon glare). Never look at the Sun through a sextant without shade glasses on the index arm. For the Sun, you typically use two or three shade glasses stacked; for stars and planets, no shades are needed.

Taking a Sight — Technique and Common Errors

Body identification comes first. Know which body you intend to observe before bringing the sextant to your eye. For the Sun, identification is obvious. For stars and planets at twilight, you need to know which bright objects are which — a star finder like HO 229 volume 1, a star chart, or a planning app will show you which stars will be above the horizon during evening or morning twilight and in which direction to look. Set the sextant to approximately the body's altitude before searching — it's faster than scanning from zero.

The rocking technique: Once the body is in view in the sextant, rotate ('rock') the sextant slightly from side to side around the line of sight. As you rock, the body traces an arc — the lowest point of that arc is the true altitude measurement. This works because if the sextant is tilted off-vertical, the body appears higher than the true altitude; rocking finds the minimum, which is the correctly-vertical measurement. A body at the bottom of its rocking arc is properly centered in the vertical plane.

Timing the sight: For a body that is rising or setting quickly (which is everything except Polaris at high latitude), you call 'mark' at the exact moment the body touches the horizon while rocking. Your assistant records the time to the nearest second. Alternatively, note the time yourself immediately after calling mark — practice makes this fast. Write down arc degrees, drum minutes (and tenths), and time (to the second) before moving from your position.

Common errors: The most frequent mistake is rocking on the wrong axis — rotating around the arc instead of the line of sight. This doesn't find the minimum and produces high readings. Another error is not having the horizon mirror perpendicular to the plane of the instrument (side error) — this causes the horizon to appear at an angle in the mirror. A third mistake is observing too low (below 10°) where refraction dominates. Finally, recording the time before reading the arc, or vice versa, risks mixing up observations in a busy workbook.

Index Error — Finding It, Understanding It, Applying It

Index error (IE) is the reading on the arc when the two images of the horizon form a perfectly straight line. If that happens at exactly 0° 00.0', there is no IE. In practice, almost every sextant has some IE, and a well-maintained instrument typically has less than 3 arcminutes. IE is not a defect — it is a known, stable, measurable characteristic that you correct for mathematically.

Testing IE against the horizon: Set the arc to zero, look at the horizon through the telescope, and use the micrometer drum to bring the direct and reflected horizon images into a perfectly straight, unbroken line. If you have to move the drum forward (onto the arc, increasing the reading) to align the images, the IE is on the arc (positive) and is subtracted from all sights. If you move the drum backward (off the arc, below zero) to align, IE is off the arc (negative) and is added.

Testing IE against the Sun: Set the sextant near 0° and look at the Sun. You'll see two images of the Sun in the eyepiece — one direct, one reflected. Bring the two Suns into exact tangency (touching edge-to-edge, lower edge of reflected touching upper edge of direct, or vice versa), then rock and average the two positions. This method is more precise than the horizon method because the Sun's disc gives a sharper reference than the horizon.

IE should be checked at the start of every sight session and recorded. If IE is stable (same value over days or weeks), the instrument is healthy. If IE drifts significantly between sessions, the index mirror may be loose or the instrument may have been dropped. A small, consistent IE — say 2.1' on the arc — is perfectly acceptable. Record it and subtract it every time.

Sextant Care and Maintenance

A sextant is a precision optical instrument. The tolerances that allow it to measure angles to 0.1 arcminute are built into its components at the factory and maintained only by careful handling. Dropping a sextant even once can shift mirror alignment, distort the arc, or crack the arc gearing. Transport it only in its box, with the box secured against motion when underway. At anchor, the box belongs in a locker — not on the chart table where it can slide off.

Plastic sextants (Davis Mark 15 and similar) are functional, affordable instruments well-suited to learning and to backup use. They are less subject to corrosion than brass but are more vulnerable to arc distortion if dropped, and their mirrors are typically less precisely aligned from the factory. Brass/aluminum sextants (Astra III, Tamaya, Freiberger) are professional instruments with finer optics and more durable arc and gearing. They cost more but hold their calibration better over time.

Arc error is a systematic error across the arc — the instrument reads differently at different altitudes. A good sextant has arc error of less than 0.2 arcminutes across its range. Arc error is tested with a specialized autocollimator at a repair facility or by comparison with a known-accurate instrument. It cannot be corrected by the navigator — an instrument with significant arc error needs professional service.

Cleaning the mirrors: Use only clean, soft cotton or the lens tissue provided, with a small amount of lens-cleaning solution. Never use paper towels or abrasive materials. Clean in circular motions, not strokes, to avoid streaks. The silvering on sextant mirrors can corrode in a marine environment — if you see brown edges or pitting on a mirror, the instrument needs re-mirroring. Telescope lenses collect salt spray and need cleaning after every offshore passage. A fogged lens reduces contrast and makes star sights significantly harder.

When to send for overhaul: A sextant showing unstable IE (changing significantly between sessions without having been handled roughly) likely has a loose mirror. Arc error greater than 0.3 arcminutes is unacceptable for serious navigation. If the rack-and-pinion mechanism is stiff or has backlash (play in the drum), the instrument needs service. A good marine sextant, properly maintained, should serve decades without needing more than cleaning and occasional mirror replacement.