A day's work with the sun
- Take a morning sun sight for a position line and run it up along the boat's track.
- At meridian passage (local noon) measure the maximum altitude, which gives latitude directly.
- Cross an afternoon sight with the run-up morning line for a running fix.
- Use the sun's azimuth, or its bearing at rising or setting, to check compass error.
The noon latitude
At meridian passage the sun crosses the observer's meridian, bearing due south in the northern hemisphere at its greatest altitude for the day. Latitude then follows directly from the true altitude and the declination, needing no assumed position and no complex reduction — which is why the noon latitude is the ocean navigator's most reliable daily fix component. The time of meridian passage can also yield longitude. A morning sight run up to the noon latitude, and an afternoon sight run back to it, together give running fixes around midday.
TIP — Noon needs no assumed position Unlike an ordinary sight, the meridian-passage latitude falls straight out of the maximum altitude and the declination, so it is the simplest and most robust fix of the ocean day and is worked whenever the sun shows.
NOTE — Check the compass with the sun Far from land there is no transit to check the compass against, so the sun's azimuth at a sight, or its amplitude at sunrise and sunset, is used to find and monitor the compass error.