A day's work with the sun
- Morning sun sight for a position line, run up along the ship's track.
- Meridian passage (local noon): the maximum altitude gives latitude directly, with no assumed position needed.
- Afternoon sight crossed with the run-up morning line for a running fix.
- The sun's azimuth, or amplitude at rising and setting, to check the compass error.
Meridian passage and the running fix
At meridian passage the sun crosses the observer's meridian, bearing due south (in the northern hemisphere) at its greatest altitude, and latitude follows directly from the true altitude and the declination. Because it needs no assumed position or complex reduction, the noon latitude is the most robust fix component of the celestial day. A morning line run up to the noon latitude, and an afternoon line run back, give running fixes, and the whole day's work is cross-checked against the DR and any electronic position.
TIP — Noon needs no assumed position The meridian-passage latitude falls straight out of the maximum altitude and the declination, so it is the simplest and most reliable fix of the celestial day and is worked whenever the sun shows.
NOTE — Check the compass with the sun at sea Far from land there is no transit to check the compass against, so the sun's azimuth at a sight, or its amplitude at rising and setting, is used to find and monitor the compass error.