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    Re: Low Altitude Sun Artificial Horizon Exercise
    From: Bill Ritchie
    Date: 2020 Dec 4, 23:21 +0000
    Hi Greg,

    Thanks for adding the benign Florida temperature and pressure information. That makes my median result 245°/0.7nm Away.

    This example highlights the fact that, at very low altitudes, variations from 'standard'  T & P have a much larger effect. For example, Table A4 Zone K gives a correction of +1.5' at 1° 30' apparent altitude, but only +0.3' at 10° apparent altitude.

    To change the subject slightly, why are 'standard' MSL conditions 10°C and 1010 hPa when both the International Standard Atmosphere and the US Standard Atmosphere use 15°C and 1013.25hPa? I agree it feels colder at sea and that multiples of 10 makes for an easier to use Nautical Almanac table, but does anyone know the real reason?

    Bill Ritchie
     50N 003W 6°C 980 hPa.

    On Thu, Dec 3, 2020 at 10:16 PM Richard MacCormac <NoReply_MacCormac@fer3.com> wrote:

    Hi Greg

    Thanks! I'm using a different set of correction tables to you, and with the additional info I'm now up at 1° 06.2', which is just over 1/2 mile off your result. That's plenty good enough for me:) (My second ever attempt at a sight reduction, btw).



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