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    Re: A Sun-compass landfall?
    From: David Pike
    Date: 2021 Apr 12, 00:56 -0700

    Geoffrey Kolbe you wrote: David Pike said "Yes, if you increased the Sun compass azimuth reading by one degree every four minutes making a further allowance for your rate of change in longitude, you would maintain the same heading."

    But that is what you do with a sun compass. You are effectively saying that a sun compass is useless as a navigational aid...!

    Geoffrey

    We're of to ///backyards.calculates.dates or thereabouts shortly where there’s little light pollution but no signal, so just a quick reply before going ex-communicado.  My answer was to the original question as I read it, i.e. would the Sun’s relative bearing remain constant.

    A Sun compass is a fine navigational aid in areas of low horizontal magnetism or uncertain variation, or as a compass check at any time.  We flicked the azimuth ring lock from ‘azimuth’ to ‘heading’ at mid-time to get a heading check every time we took an astro observation.  In the Varsity, we set up the astro compass in the astrodome to take occasional checks, but it’s horses for courses.  I don’t think using a Sun compass is the best solution for the situation in the question even if you'd made a little azimuth table before beforehand and could guarantee Sun on the compass at all times.

    As for time between one degree tweaks, please check my calculation that time between tweaks = 60/(15-(Vgcos(trk-270)/coslat))minutes where Vg = groundspeed.  As an occasional heading check, we simply took the ten minute GHA Sun from the Air Almanac; subtracted longitude west; and made sure we took the check on the ten minute time.  DaveP

       
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