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On 9/27/2014 1:06 PM, Samuel L wrote:
> Greg,
>
> Thanks. I usually calculate LAN from 12 UT and subtract the 1 hour for DST.

Personally I don't bother with the meridian passage time posted in the
NA. If I understand it correctly, that is based on the equation of time
when the Sun is on the Greenwich meridian, so not spot on if you are a
goodly distance away from Greenwich.

--Simply find the Sun's GHA nearest to but less than your (west) longitude.
--Subtract that from your AP longitude.
--Convert that difference from arc to time.
--Add that to the whole hour UT used.
--Adjust from UT to your time zone.

NB Your time zone adjustment will be an hour greater when daylight time
ends, assuming you are on DST.


Seat of the pants, being mindful that navigators determined their
latitude from sextant Sun shots using only declination long before the
chronometer was invented. (Also stars, latitude hooks, astrolabes and
other instruments.)

When the shadow of something vertical gets close to true south but a bit
east of south, start observing.

And/or...

--Calculate Hc at LAN for your AP latitude.
--Do a down and dirty Hs calculation from Hc by reversing standard IC,
dip and refraction corrections.
--Make the occasional observation.
--When your Hs gets close to but lower than that, start observing in
earnest. (As Frank implied, the Sun may hang at the same altitude for
minutes given the precision inherent in cel nav.)

   

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