### A Day in the Life of a Navigator » Observing LAN

There are two methods for observing LAN. We can observe the Sun with a sextant, making constant adjustments as needed, until the Sun begins to descend to a lower altitude. Or we can record sights at intervals before and after LAN, then approximate when LAN occurred. Because we don’t want to miss this once-per-day event, both methods require a prediction of the time of meridian passage of the Sun at our DR position.

For the purpose of this module, we will assume we have access to the internet. The U.S. Naval Observatory has a free “Rise/Set/Transit calculator” at http://aa.usno.navy.mil/data/docs/mrst.php. One can also use the UNSO Complete Sun and Moon Data for One Day site, at http://aa.usno.navy.mil/data/docs/RS_OneDay.php

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Again, if doing this by hand, strip forms and The Nautical Almanac are used.

At our DR position, we estimate LAN to occur at 15:45.

Note that this is only an estimate because our DR is only an estimate. Because of this, we need to begin our observations several minutes prior to this time in order to be sure we don't miss it. For our purposes, we will observe the Sun with a sextant, making constant adjustments as needed, until the Sun begins to descend. We note the largest sextant reading. This is our LAN measurement.

Chief Sheedy: At time 15:45 and 05 seconds, we observe local apparent noon. This is when the Sun is at its highest point for the day. The sextant altitude at that time was 79 degrees 50 minutes decimal 0. We need a different strip form from the ones used before to complete LAN, and it’s labeled LAN. So we pull the LAN strip form and complete it. After completing it, the latitude is 32 degrees 5 decimal 3 North.