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    Re: Sun near zenith sight
    From: Francis Upchurch
    Date: 2016 Sep 22, 20:55 +0100

    Thanks Peter, Greg and Hewitt.

    I think by Jove , I’ve got it! (sometimes takes a while these days I find!).

    I’ll have to dig out my homemade Azimuth compass ( adjustable mirror brings flash image of sun down from high alts), but not planning on going to tropics in near future.( I wish!) Doesn’t look as if I’ll ever see these sort of very small Zenith distances and close GPs at latitude 50, or am I still confused? Unchain me from my frustrating ignorance on this.



    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Peter Hakel
    Sent: 22 September 2016 19:49
    To: francisupchurch@gmail.com
    Subject: [NavList] Re: Sun near zenith sight


    Mathematically this is a solved problem for any combination of inputs,


    but practically you’d have to know the azimuth to an unrealistic precision, if the GP is too far. Therefore, I agree with Hewitt that your zenith distance should be quite small.

    Peter Hakel


    From: Hewitt Schlereth <NoReply_Schlereth@fer3.com>
    To: pmh099---.com
    Sent: Thursday, September 22, 2016 10:28 AM
    Subject: [NavList] Re: Sun near zenith sight


    Francis, you can do this sight whenever the GP is within the limits of your plotting sheet - about 120nm on the VPOS.


    Once on a delivery trip from Bermuda to the BVI, I did a high altitude noon sight. (It's on pages 71-72 in Celestial Navigation in a Nutshell.) Back then, the DR was my only crosscheck and I was content the circle of position was about 5 miles from my DR.



    On Sep 22, 2016, at 2:23 AM, Francis Upchurch <NoReply_Upchurch@fer3.com> wrote:

    Thanks Frank.

    I got the same as Greg, but previously thought you could not get a fix from a single sight  (well + Az). Do you have any ground rules to cover this situation? Is it only when very close to equator or what? In what situations can you do this kind of fix?



    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
    Sent: 22 September 2016 01:14
    To: francisupchurch---.com
    Subject: [NavList] Sun near zenith sight


    This is a screen cap from my gps anti-spoof app taken today, live, by Bob Miorelli from a location where, as you can see, the Sun was nearly at the zenith. There is enough information here to figure out his position. By luck the volume controls on his phone momentarily covered the displayed GPS latitude and longitude. You have the calculated Sun LL altitude (assume height of eye is zero [edited after follow-up from Bob] and zero index error) as well as the azimuth and the date and time. Solve!!

    Frank Reed
    Conanicut Island USA

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