Celestial Navigation Using a Sextant
At-sea guide to taking, correcting, and reducing a sextant sight — sun, stars, and moon — for position offshore when GPS is unavailable.
AT SEA · SEXTANT · ALMANAC · TIME
A sextant measures the angle between a celestial body and the sea horizon. With that angle, the exact time (UTC), and a nautical almanac, you can plot a position line — and crossing two or more gives your position anywhere on the ocean.
Parts of the Sextant
The rigid base — everything mounts to it. Keep it protected from knocks and salt.
The pivoting bar you move to bring the body down to the horizon. Driven by the worm gear.
Mounted on the index arm; reflects the celestial body toward the horizon glass.
Half-silvered mirror; you see the horizon directly through the clear half and the reflected body through the silvered half.
Magnifies the view so you can judge contact precisely.
The arc reads whole degrees (0–90°); the drum reads minutes (′) — one full turn = 1°.
Dark glasses for the index mirror and horizon glass — essential for sun sights.
Light from the body bounces off the index mirror, through the horizon glass, into the telescope — where it lines up with the sea horizon.
Before You Sight: Checks & Errors
Check for Index Error
Set the arc to 0°00′. Hold the sextant vertical and sight the horizon (or a distant straight edge). The direct and reflected horizon should form one unbroken line. If they step up or down, use the micrometer to align them — that reading (with sign) is your index error. Subtract it from (or add it to) every sight.
Check the Mirrors are Perpendicular
With the arc near 0°, look into the index mirror — the arc reflected should line up perfectly with the arc seen directly. Any kink means the mirror is out of adjustment; have it serviced before relying on sights.
Protect from Heat & Salt
Keep the sextant in its case out of direct sun before and between sights — heat warps the arc. Rinse off salt spray with fresh water and dry gently; salt crystals scratch optics and mirrors.
Taking the Sight — Step by Step
The goal: bring the celestial body down until it just touches the sea horizon, hold the sextant truly vertical (rock it), and note the exact UTC time.
Select the Body & Set Shades
For a sun sight, pick the lower limb (bottom edge) — it's easier to judge than the upper. Swing in the appropriate shades on both the index and horizon mirrors so the sun is comfortable to view.
Bring the Body Down to the Horizon
Set the arc to 0°, point the telescope at the horizon directly beneath the body, then push the index arm forward (increase the angle) until the sun (or star) appears in the field of view just above the sea horizon.
Rock to Find Vertical (the Swing)
Rotate the sextant gently left–right about the line of sight. The body will trace an arc; it dips lowest at one point. That lowest point is true vertical — adjust the micrometer so the body just kisses the horizon at the bottom of the swing.
Nail the Contact
Bring the lower limb of the sun to just touch the horizon line (a tiny sliver of light between sun and sea). For stars, place the point of light on the horizon. For the moon, use the near limb.
Note the Exact Time
The instant you are satisfied with the contact, read the time to the whole second (UTC). Time error of 4 seconds ≈ 1′ of latitude for a sun sight — so a good watch is critical.
Read the Altitude
Read the degrees off the arc and the minutes off the micrometer drum. Record: body, limb used, altitude (Hs), time (UTC), date, and your height of eye above the waterline.
The Swing — Finding Vertical
Rock the sextant left–right; the body traces an arc. The lowest point is true vertical — set the contact there.
Correcting the Altitude (Hs → Ho)
The raw reading (Hs) is never the true altitude of the body's centre. Apply these corrections in order, from the almanac and your height of eye, to get Ho.
The raw reading off the arc and drum.
Add or subtract the index error found at 0°. A positive IC is subtracted; a negative IC is added.
Subtract dip — the height of your eye above the water makes the visible horizon sit below the true horizon. Dip (′) ≈ 1.76 × √(height of eye in metres).
Subtract atmospheric refraction (greatest near the horizon, ~0 at 90°). Look it up in the refraction table for the apparent altitude.
For sun and moon limb sights, add (lower limb) or subtract (upper limb) the semi-diameter from the almanac — so the limb reading becomes the centre reading.
For the moon (small for the sun, negligible for stars), add parallax in altitude from the almanac — the moon's large parallax is why lunar sights need the full correction.
The fully corrected altitude of the body's centre — what you take to the sight reduction tables.
Turning the Sight into a Position
Noon Sight (Meridian Passage)
The simplest fix. Observe the sun's altitude at local noon (when it peaks). Correct to Ho, then: your latitude = 90° − Ho ± declination (from the almanac). Longitude ≈ your time of LAN converted to arc (1 hr = 15°). One sight, one position line, but a clean latitude.
Morning/Afternoon Sun Line
A single sun sight away from noon gives one position line (an LOP) running roughly NE–SW or NW–SE. Cross it with a second sight 2–4 hours later (the sun has moved ~30–60°) for a running fix — advance the first line by your course and distance run.
Star Sights at Twilight
At dawn and dusk twilight you can see both stars and the horizon — the ideal window for a 3-star fix. Take 3–4 stars spread roughly 120° apart in azimuth, plot each LOP, and the cocked hat (small triangle where they cross) is your position.
Polaris (Latitude Only)
In the Northern Hemisphere, Polaris gives a direct latitude: latitude ≈ Ho of Polaris (with a small correction from the almanac for its tiny offset from the pole). No longitude from it.