Every sight is a distance from a point on Earth.
At any instant, the Sun, Moon, planet, or star is directly above one geographic position (GP). The Nautical Almanac gives that GP as declination and Greenwich hour angle. Your sextant altitude tells you how far you are from the GP: zenith distance is 90° - altitude, and each minute of arc equals 1 nautical mile.
One sight gives a circle of equal altitude, which is drawn locally as a line of position (LOP). Two LOPs, or one LOP advanced from dead reckoning, gives a fix. Everything else is bookkeeping: time, corrections, lookup tables, and plotting.
The cheat within the cheat.
Noon latitude
Purpose: get latitude from the Sun's maximum altitude at local apparent noon.
ZD = 90° - Ho
Lat = Dec ± ZD
Gotcha: decide whether the Sun bore south or north at meridian passage before choosing the sign.
Correction order
- Read Hs.
- Apply index correction.
- Subtract dip.
- Apply refraction.
- Add or subtract semidiameter by limb.
- Apply parallax.
Almanac terms
GHA is longitude west from Greenwich to the body's GP. Declination is celestial latitude. LHA is hour angle from your meridian. Hs/Ha/Ho are sextant, apparent, and observed altitudes. Zn is true azimuth.
Accuracy to expect
A practiced small-boat navigator with a real horizon should treat a good noon latitude as ±1-2 nm, a plotted two-LOP fix as ±2-5 nm, and no-instrument methods as ±30-60+ nm.
| Method | What it gives | Typical error | Use it when | Do not trust it when |
|---|---|---|---|---|
| Noon Sun sight | Latitude; rough longitude if timed | ±1-2 nm latitude; longitude depends on clock and LAN pick | You have a clear horizon around meridian passage. | Clouds hide the maximum or you use civil zone time as if it were UTC. |
| Two or more timed sights | Full fix from intersecting LOPs | ±2-5 nm with careful plotting | Morning/evening Sun, planets, or stars are available. | DR is stale and you advance lines without current correction. |
| Polaris shortcut | Northern latitude | Usually within about 1° without table correction | You are in the Northern Hemisphere with a visible Polaris. | You are low-latitude, hazy, or mistaking another star. |
| Dead reckoning only | Estimated position between fixes | Error grows every hour with speed/current error | You need a continuous plot between celestial fixes. | Current, leeway, or steering error is unknown. |
| No-sextant fallbacks | Direction, rough latitude, landfall strategy | ±30-60+ nm | Survival navigation and keeping a deliberate course. | You need harbor entrance precision. |
The sextant measures angle, not position.
Parts that matter: frame and arc, index arm, micrometer drum, index mirror, horizon mirror, shades, telescope, and clamp. A 7-inch plastic training sextant can read to 0.2′; a metal sextant holds adjustment better and is easier to trust offshore.
How to shoot the Sun: set shades before looking near the Sun, bring the reflected lower limb down to the visible horizon, rock the sextant slightly so the limb kisses the horizon at the bottom of the arc, then call the UTC second and read the arc plus micrometer.
Buying note as of Jul 2026: Davis lists the Mark 15 at $399.99; Celestaire lists the Astra IIIB Deluxe at $999.00. Older "cheap plastic sextant" advice is still pedagogically useful, but current retail pricing is not the old $60 anchor.
Sextant correction stack, with real numbers
| Height of eye | Dip correction | Example use |
|---|---|---|
| 1 m | -1.8′ | Low dinghy seat. |
| 2 m | -2.5′ | Small cockpit. |
| 3 m | -3.0′ to -3.1′ | Typical yacht deck. |
| 4 m | -3.5′ | Raised deckhouse. |
| 5 m | -3.9′ | Small bridge deck. |
| 6 m | -4.3′ | Fishing vessel bridge. |
| 8 m | -5.0′ | Tall cockpit or low ship deck. |
| 10 m | -5.6′ | Ship bridge wing. |
| 15 m | -6.8′ | Large ship. |
| 20 m | -7.9′ | High bridge. |
Dip values use the standard approximation 1.76 × sqrt(height in meters), rounded to 0.1′. Always subtract dip for a sea horizon.
Noon sight: latitude the easy way.
Clock error to position error
3.0′ of longitude error
2.4 nm east-west error at this latitude.
Time sources
- WWV: 2.5, 5, 10, 15, and 20 MHz from Fort Collins, Colorado, operating continuously as of Jul 2026.
- WWVH: same NIST service family from Hawaii; useful in the Pacific.
- CHU: 3330, 7850, and 14670 kHz from Canada as of Jul 2026.
- Quartz watch: log daily drift before departure. A watch gaining 2 s/day creates 15′ of longitude error after 30 days if uncorrected.
Intercept method: compare the sky you saw with the sky tables predict.
Marcq St. Hilaire sight reduction is not magic trigonometry in practice. Choose an assumed position near your DR, use the Nautical Almanac plus Pub. 229, Pub. 249, or the Nautical Almanac sight-reduction tables to compute altitude Hc and azimuth Zn, then compare observed altitude Ho with Hc.
| Recipe step | Concrete example | Gotcha |
|---|---|---|
| 1. Assume a position | AP 38°00.0′ N, 70°00.0′ W near the DR plot. | For table work, round AP to make LHA and latitude integral as the table requires. |
| 2. Get GHA and declination | At 16:44:38 UTC, Sun GHA 69°59.7′, Dec 22°44.1′ N. | Use UTC/UT1, not local watch time or time zone labels. |
| 3. Compute LHA | GHA + east longitude = about 359°59.7′, effectively meridian passage. | East/west sign errors move the body to the wrong side of the sky. |
| 4. Look up Hc and Zn | USNO computes Hc 74°44.1′, Zn 180° for the example AP. | Tables give Hc; your sextant gives Ho. Keep names distinct. |
| 5. Intercept | If Ho were 74°46.1′ and Hc 74°44.1′, intercept = 2 nm toward Zn 180°. | "Ho Mo To": Ho More, plot Toward; Ho Less, plot Away. |
| 6. Plot LOP | From AP move 2 nm toward 180°, draw a line perpendicular to azimuth. | Advance old LOPs by course and distance before crossing them with new ones. |
Free tables: Bowditch explains the method; Pub. 229 is the marine sight-reduction table set; Pub. 249 is compact and common for air navigation and selected stars; the Nautical Almanac includes its own concise sight-reduction tables and forms.
Workhorse stars: learn Sirius, Canopus, Arcturus, Vega, Capella, Rigel, Procyon, Altair, and Polaris first. Pub. 249 Volume 1 selects seven stars for a given latitude and LHA of Aries; Polaris gives a quick latitude in the Northern Hemisphere when corrected from the Polaris tables.
Celestial fixes correct DR; they do not replace it.
D = S × T
Purpose: advance your position by speed and elapsed time. Example: 5.8 knots for 3 h 20 m is 5.8 × 3.333 = 19.3 nm. Gotcha: knots already mean nautical miles per hour; do not convert to statute miles.
Six-minute rule
Purpose: quick distance mental math. In 6 minutes, distance run is speed divided by 10. Example: at 6.4 kt, every 6-minute tick is 0.64 nm. Gotcha: it assumes steady speed over ground, not just engine RPM.
Set and drift
Purpose: represent current as a vector. Example: intended 090°T at 5 kt plus current setting 140°T at 1 kt produces a track south of east. Gotcha: current "set" is the direction it flows toward; wind is named from where it comes.
Leeway
Purpose: correct for sideways wind drift. Example: a sailboat steering 045°T with 6° starboard leeway may be tracking 051°T. Gotcha: leeway grows fast with sail balance and sea state; log observed wake angle.
Advance an LOP
Purpose: cross an old sight with a newer one. Example: advance a 09:10 Sun LOP by course 080°T and distance 14 nm before crossing it with a 12:00 meridian LOP. Gotcha: advance the line, not just the plotted point.
TVMDC bearings
Purpose: move between true, variation, magnetic, deviation, and compass. Example: True 100°, variation 12°W gives Magnetic 112° by "west is best." Gotcha: celestial azimuths are true; your steering compass may not be.
Crude methods are for landfall strategy, not harbor piloting.
| Method | Definition and example | Honest error bar | When not to use |
|---|---|---|---|
| Kamal | A card and knotted string measure star altitude. Make a 5 × 8 cm card, hold the string in teeth, and knot the line so Polaris spans horizon-to-card-top at known home latitude. | ±1-3° after calibration. | Do not assume another person's knot spacing fits your arm length. |
| Fist and fingers | At arm's length, calibrate your fist near 10° and one finger near 2°. Example: Polaris four fists up is roughly 40°N. | ±3-5° uncalibrated. | Do not use generic body-angle rules without checking against a known horizon. |
| Polaris by eye | Polaris altitude is approximately northern latitude. Example: Polaris 32° above the horizon means near 32°N. | ±1° with careful sighting; worse by eye. | Not available in the Southern Hemisphere. |
| Southern Cross pointer | Extend the long axis of Crux about 4.5 times to estimate the south celestial pole, then drop to horizon for south. | Direction within ~5-10°. | Fails if Crux is confused with the False Cross. |
| Shadow stick | Mark a vertical stick's shadow tip, wait 15-30 minutes, mark again; the line from first to second mark points roughly west-to-east. | Direction within ~10° with care. | Poor near noon, high latitudes, or moving platforms. |
| Analog watch Sun method | In the Northern Hemisphere, point hour hand at Sun; halfway to 12 approximates south. Adjust for daylight time and longitude within zone. | Often ±15-30°. | High latitudes and wrong daylight-saving correction make it bad. |
| Star-rise bearings | A star rises and sets at repeatable bearings for your latitude. Example: tracking the same bright star over nights preserves a course. | Direction trend, not a fix. | Do not treat unknown stars as compass points. |
| Day length latitude | At solstice seasons, day length varies with latitude. Example: very long summer twilight implies high latitude. | ±5-10° at best. | Nearly useless near equinox or without a date. |
| Ocean swell | Long-period swell often persists from a stable weather system. Example: keep swell two points on the port bow to hold a rough heading. | Course-keeping cue only. | Local wind seas, reflections, and crossing swells mislead. |
| Birds, clouds, and water color | Seabirds at dawn/dusk, fixed cumulus over islands, and shoaling water can suggest land direction. | Local clue, not a position. | Never override charted hazards or DR with a single natural sign. |
These methods have real precedent: Polynesian wayfinding used stars, swell, birds, and cloud signs; Frank Worsley navigated Shackleton's lifeboat voyage with tiny sight opportunities; WWII life-raft kits included improvised celestial aids. The modern lesson is humility: rough direction is valuable, but error bars are large.
The errors that ruin otherwise good sights.
True vs magnetic vs compass
Celestial azimuths are true. Example: plotting Zn 180°T on a chart is not steering 180°C if variation is 12°W and deviation is 3°E. Use TVMDC deliberately.
Wrong index sign
If the horizon image is off the arc, add; on the arc, subtract. Example: 1.2′ off the arc changes 74°30.5′ to 74°31.7′ before dip.
Zone time in almanac work
The almanac uses UTC/UT1. Example: 12:44 local EDT is 16:44 UTC; using 12:44 as UTC puts GHA about 60° wrong.
Upper/lower limb mix-up
Lower limb Sun usually adds semidiameter; upper limb subtracts. Mixing them creates about 32′ of error, roughly 32 nm.
Bad assumed-position rounding
Pub. 229/249 table entries assume specific integer latitude/LHA setup. Rounding the wrong direction can force interpolation you did not account for.
No equation-of-time awareness
Clock noon is not apparent noon. Use observed maximum altitude or almanac GHA; do not infer longitude from 12:00 zone time.
Trusting a single sight too much
A single LOP constrains position in one dimension. Example: a morning Sun line plus stale DR is not equivalent to a two-star fix.
Recording time after the sight
Call the time at the instant of contact. Four seconds equals 1′ of longitude, so fumbling the watch can dominate a good altitude.
Primary sources used for volatile and specific facts.
- Bowditch, The American Practical Navigator - celestial geometry, corrections, time, almanacs, and sight reduction.
- U.S. Naval Observatory, The Nautical Almanac - almanac contents, hourly GHA/declination precision, sight-reduction forms, and correction tables.
- USNO Celestial Navigation Data - worked 2026-07-05 Sun GHA, declination, altitude corrections, Hc, and Zn.
- NGA Sight Reduction Tables for Marine Navigation, Pub. 229 - intercept-method table structure and interpolation accuracy notes.
- NIST Radio Station WWV - WWV broadcast status, frequencies, and transmitter powers.
- NRC CHU shortwave broadcasts - Canadian CHU frequencies and station details.
- Davis Instruments Mark 15 and Celestaire Astra IIIB - current sextant prices and product specs as of Jul 2026.
Related field references
Emergency Radio Card - print-first radio procedures.
Ham Radio Technician - HF/VHF fundamentals and radio vocabulary.
Baofeng UV-5R Quick Ref - small-radio emergency operations.