Sun Meridian Passage — Noon Sight

Sextant noon sight in one offline app: Calculator reduces a meridian passage to latitude & longitude with an on-board solar ephemeris (truncated VSOP87D, Meeus Astronomical Algorithms ch. 25 higher-accuracy path, checked against JPL DE421 to ≤0,03′ — no almanac tables, any date works, 2026 included); Worked examples holds the four sights from the course deck, recomputed and corrected; Training is a graded sextant drill against a live GPS referee. Coordinates in Degrees Minutes,Decimal, e.g. 78° 48,2' N.

online offline copy: checking… GPS idle — starts with the Training tab

Part 1  ·  Sextant readouts — find time of true Sun (culmination / LAN)

Enter at least 3 timed sextant altitudes taken around local noon. A parabola is fitted by least squares through all entered points; its vertex gives the time of the true Sun and the corresponding Hs to be used in Part 2. Time = watch time HH:MM:SS, Hs = DD MM,M (or decimal degrees).

— no result yet —
Zjistím v sextantu — read on sextant Vypočtu — computed

Part 2  ·  Meridian passage — reduction to Latitude & Longitude

Initial data

Date:
DR Latitude:N/S
DR Longitude:W/E

Time

Watch time:h:m:s
Zone corr. (zone − UTC):h
GMT (UT):h:m:s

GMT = watch time − zone correction. East zones positive (CET = +1, CEST = +2).

Latitude

Eye height:m
Limb:
Hs (Height Sextant):° '
IE (Index Error):'
Dip −:'
App. Alt:° '
Alt. Corr. + −:'
Ho (Altitude):° '
Declination:N/S
DR Latitude:N/S
Ho (Altitude):° '
ZD = 90° − Altitude:° '
Lat > Dec, same N/SLat = ZD + DEC
Lat, Dec different N/SLat = ZD − DEC
Lat < Dec, same N/SLat = DEC − ZD
LATITUDE
DR / GPS:
Difference:

Longitude

Mer. Pass. Greenwich:GMT
Difference (time)
GMT − Mer.Pass:		h:m:s
Time to Arc:° '

Mer. Pass. Greenwich = 12h − Equation of Time. Time is converted to arc at 15°/h (1h = 15°, 1m = 15', 4s = 1'). Observer west of Greenwich ⇒ transit after Greenwich noon.

LONGITUDE
DR / GPS:
Difference:
Almanac / ephemeris detail for this GMT
—
Ephemeris check table — compare with the printed Nautical Almanac
Date:
—
Vím — I know Zjistím v almanachu — from the Almanac (computed here) Zjistím v sextantu — read on sextant Vypočtu — computed

Worked examples  ·  from the course deck (Festival Bezpečně na vodu)

The four noon sights from the presentation, re-computed here with the on-board ephemeris. Load fills Part 2 of the Calculator tab with the sight; Worked solution shows every step. Where the deck contains a slip, the errata box says exactly what and why — the arithmetic below is the corrected one.

Why the longitudes differ by a few minutes from the deck: the printed Almanac lists Mer. Pass. only to the whole minute (e.g. 11:56), while this app computes it to the second (11:56:20). One minute of time = 15′ of longitude, so the almanac rounding alone can move the longitude by up to ±7,5′. The latitudes agree to ±0,2′ (altitude-correction table rounding).
Deck answer — as printed in the slides Errata — slip in the slides, corrected here

1 · Setup sextant & observer — fill in before the first shot

IE (Index Error):'
Eye height:m
Limb:
Mark delay (sight → entry):s

Mark delay = how long it takes you to lower the sextant and reach the phone. Pressing New reading stamps the sight that many seconds in the past and uses the GPS position from that same moment.

GPS source:
Blind mode:

—

2 · Sextant readings up to 15 — shoot around local noon, log each one

#UT timeHs (DD MM,M)Δ vs GPS truth

Δ = your Hs minus the Hs a perfect sextant would have read at the GPS position at that second (ephemeris altitude, corrections applied in reverse). Grading:  ≤0,3′ perfect ≤0,8′ excellent ≤1,5′ good ≤3′ fair >3′ check that shot

3 · Evaluate fit culmination → reduce to a fix → compare with GPS

LATITUDE
LONGITUDE
GPS at culmination:
Error Lat / Lon:NM
Total error:NM
— no evaluation yet —
Fix grading (course deck): ≤2 NM experienced navigator · ≤5 NM average · ≤15 NM beginner · beyond that, keep practising

4 · Progress saved sessions on this device

Stored only in this browser (localStorage).