GPS fails. Batteries die. Satellites get jammed. Celestial navigation works because the sky is a clock and a map that no one can turn off. The noon sight (local apparent noon) and the intercept method (Marcq Saint-Hilaire) are the two pillars. Here is the math, the tools, and the procedure.

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The Core Idea: Circle of Position

A sextant measures altitude (Hs) of a celestial body above the horizon. Corrected for index error, dip, refraction, parallax → Ho (observed altitude).

From the Nautical Almanac, you get the body’s GP (Geographical Position) — the point on Earth directly beneath it at that exact UT.

You are somewhere on a circle centered on the GP with radius = 90° – Ho (in nautical miles, 1° = 60 NM). This is your Circle of Position (COP).

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Method 1: Noon Sight (Latitude Only)

At Local Apparent Noon (LAN), the sun crosses your meridian. Its altitude is maximum.

Step	Action
1	Estimate LAN from Almanac (meridian passage + longitude correction)
2	Start shooting 15 min before; track altitude rising
3	At max altitude, record Hs and exact UT
4	Correct Hs → Ho
5	From Almanac: Sun’s Declination (Dec) at that UT
6	Latitude = 90° – Ho + Dec (same name: add; contrary: subtract)

Accuracy: ±1–2 NM with careful work. No longitude.

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Method 2: Intercept Method (Fix: Lat + Lon)

For any body, any time (not just noon):

Step	Action
1	Assume a DR (Dead Reckoning) position
2	From Almanac: body’s GHA, Dec at sight UT
3	Compute LHA = GHA ± DR Lon
4	Solve navigational triangle → Hc (calculated altitude), Zn (azimuth)
5	Intercept = Ho – Hc (Toward if Ho > Hc; Away if Ho < Hc)
6	Plot DR → advance/retard along Zn by intercept → LOP (Line of Position)
7	Repeat for 2+ bodies → Fix = intersection of LOPs

Tools: Nautical Almanac, sight reduction tables (HO 249 / HO 229), plotting sheet, parallel rules, dividers.

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The Tools (Minimal Kit)

Item	Purpose	Backup
Sextant (metal, 7× scope)	Measure Hs	Plastic Davis Mark 3 (emergency)
Nautical Almanac (yearly)	GHA, Dec, LAN, corrections	PDF on phone (battery risk)
Sight Reduction Tables (HO 249 Vol 1–3)	Solve triangle fast	Calculator + formulas
Plotting Sheet (universal)	Plot LOPs	Paper + compass rose
Watch (rated, set to UT)	Exact time	GPS time sync daily

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Corrections (Hs → Ho)

Correction	Typical Magnitude	Direction
Index Error	±0.5’ to ±5’	Subtract if on the arc
Dip (height of eye)	–1.7’ × √(height in m)	Always subtract
Refraction	–1’ to –30’ (low alt)	Subtract (always)
Parallax (Sun/Moon)	+0.1’ to +60’	Add (Moon large)
Semi-diameter (Sun/Moon)	±16’	Add (lower limb) / Subtract (upper)

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Practical Accuracy

Method	Typical Error	Best Conditions
Noon sight (lat)	1–2 NM	Clear horizon, steady hand
Intercept (fix)	2–5 NM	3+ bodies, good spread (60–120° azimuth)
Running fix (advancing LOPs)	3–8 NM	DR accurate, short advance

GPS: ±3 m. Celestial: ±2–5 NM. Different leagues. But celestial doesn’t need satellites.

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Why Learn It (Now)

• Grid-down resilience: No infrastructure dependency
• Cross-check: Verifies GPS, catches spoofing
• Skill depth: Forces you to understand time, coordinates, spherical trig
• Tradition: Same math that carried Slocum, Heyerdahl, Chichester across oceans

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The Takeaway

Celestial navigation is spherical trigonometry with a clock and a protractor. The noon sight gives latitude. The intercept method gives a fix. Both work because the sky is a deterministic, unjammable reference frame.

Carry a sextant. Know the math. The sky doesn’t need a subscription.