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    Ex Meridians in practice
    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
       
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