NauticTools2Celestial navigation
Sun-run-noon fix – morning sun line plus noon latitude
The classic daily fix on an ocean passage: a sun line in the morning, the noon latitude at midday. NauticTools2 runs the morning line on with COG and SOG to meridian passage, works out the noon DR position from it and gives the observed position, the fix run and a plotting sheet.
- Morning sun line
- Noon latitude
- Run with COG/SOG
- Meridian passage
- Plotting sheet
- Fix run
What is a sun-run-noon fix?
During the day the sun is often the only body you can shoot. A single sun line only tells you which line you are on. About three hours later, at meridian passage, the sun bears due south (or north) – its altitude gives the latitude directly, an east-west parallel. Run the morning line on by the distance sailed in between, and it crosses the noon latitude at the vessel's position at noon.
When do you need it?
- Daily routine offshore: morning sun and noon latitude give a fix every day without waiting for twilight and stars.
- Day's run: from noon position to noon position you get the distance made good in 24 hours.
- Little arithmetic, few errors: the noon latitude needs no hour angle, and the time hardly matters for it.
- Checking the GPS: pick up the sextant once a day and cross-check the satellite position.
Formulas and working
Morning position line
Like any position line: ephemeris, sextant corrections, computed altitude Hc and azimuth, giving the intercept Δh = Ho − Hc (see fix from altitudes).
Noon DR position
Morning DR position plus SOG · Δt along COG up to meridian passage (mid-latitude). The time of meridian passage depends on the longitude and is found together with it.
Noon latitude
Sun's altitude at meridian passage, corrected to the observed altitude Ho, declination at meridian passage, giving the latitude. As a position line: Hc = 90° − (Lat − Dec) at the noon DR position, azimuth 180°.
Run and intersection
The morning line is moved by SOG · Δt along COG and crossed with the noon latitude – that is the observed position.
Fix run
Direction and distance from the noon DR position to the observed position.
Worked example
22 October 2026, a yacht approaching Martinique from the east. At 13:05:20 UT (09:05 ship's time) the sun's lower limb is observed from the DR position 14° 30.6′ N, 059° 56.7′ W: Hs 42°52.9′ (index error −1.0′, height of eye 3 m). Until noon the yacht makes good 260° at 6 kn. At meridian passage the sun's lower limb is observed at Hs 64°10.6′. Where is the yacht at noon?
Morning line: intercept +4.4′, azimuth 121.0°, run 16.0 nm on 260°
Noon DR position: 14° 27.8′ N, 060° 13.0′ W · meridian passage 15:45 UT
Noon latitude: Ho 64°22.1′ → 14° 25.3′ N
Angle of cut 59° · fix run 124.9° / 4.4 nm
Observed position: 14° 25.3′ N, 060° 09.3′ W
Things to watch out for
- Interval: take the morning sight about three hours before meridian passage. The morning line and the noon latitude then cross at a useful angle – 59° in the example.
- Catching meridian passage: shortly before noon the sun rises only slowly. Start about 15 minutes early and take the highest altitude. The meridian passage tool tells you when.
- Run over the ground: COG and SOG, not heading and speed through the water – current and leeway add up over three hours.
- Sun near the zenith: in the tropics the sun can stand almost overhead at noon. The altitude is then hard to measure, and you should choose deliberately whether it bears south or north.
How it works in NauticTools2
Morning line of position
Sun with limb, date and time, DR position, sextant reading, index correction and height of eye, plus COG and SOG until noon.
Noon latitude
Date and DR position are filled in by the app – from the morning, run on to meridian passage. You only enter the sun's limb, sextant reading, index correction and height of eye; south or north is determined automatically.
Read off the fix
Plotting sheet, observed position, fix run and working appear straight away.
Print and save
Both position lines with their working on one page, the plotting sheet on the second. Exercises can be saved and loaded again.
Printable record
The sun-run-noon fix can be printed as a record or saved as a PDF: both position lines side by side with their working, the noon DR position, the result and the working on one page, the plotting sheet on the second.
View the example record as PDF ›
Frequently asked questions
What is a sun-run-noon fix?
A fix from two sun sights: a position line in the morning and the noon latitude. The morning line is run on to noon and crossed with the parallel of latitude.
How much time should pass between the two sights?
About three hours. The shorter the interval, the flatter the morning line crosses the noon latitude.
How is the morning line run on to noon?
With course and speed over ground: by SOG times the time to meridian passage, along COG. The noon DR position follows in the same way.
Can the noon latitude be combined with another body?
Yes – Fix from 2 or 3 altitudes offers the noon latitude as its own type of position line, for example together with Jupiter in the evening.
Related tools
- Fix from 2 or 3 altitudes – position lines from any body, including the noon latitude
- Noon latitude – latitude from the meridian altitude alone
- Meridian passage – when the sun crosses the meridian
- Sextant altitude corrections – from the reading Hs to the observed altitude Ho
NauticTools2 is free and runs on Windows 10 and 11. An Android version is in preparation.
Get it from the Microsoft Store