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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Ex Meridians in practice
From: Jeremy C
Date: 2010 Dec 29, 09:34 EST
From: Jeremy C
Date: 2010 Dec 29, 09:34 EST
Some of the older navigators can probably add to this, but this is how I'd
use Ex-meridians of the sun in practice without GPS help.
Starting with the morning star fix I'd advance a DR to the previous day's
calculated LAN (compensating for any time changes) and get a DR position.
I'd then calculate LAN and if it were more than a few minutes away from my 1st
estimate, I'd do another sailing to that time and re-calculated LAN (I use the
GHA method over the EoT).
If it were coming close to calculated LAN time and the sun was in and out
of the clouds, I'd take a series of shots near the time of LAN. If the sun
was obscured at my calculated LAN, I'd just use the shot closest at that time
for the ex-meridian.
You still need a position to enter the ex-meridian tables. In
the case of Lat, you just use your DR latitude. If you have questions
because of a shaky DR, you can get pretty darn close by calculation.
Just reduce your observation as if it were made at LAN and you will
certainly be within a degree. I tend to use the time away from calculated
LAN for the meridian angle in the second table or formula.
I'd then get my correction applied to Ha for a latitude line and cross it
with an AM sunline for my noon running fix.
To be honest, I don't bother with ex-meridians of the sun; not much point
in my opinion. When I use them most out here is for stars. This
typically happens when we have a star that crosses my meridian just outside
twilight. I can shoot it when I have a good horizon and do an
ex-meridian calculation to get a latitude line. This makes the most sense
in the Southern Hemisphere since in the north you can use Polaris for the same
purpose. It is still a waste of time, but I find stars more interesting
when I want to practice my ex-meridian techniques.
Jeremy