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    What's "under the hood" of the Raft Book?
    From: Tony Oz
    Date: 2017 Oct 21, 04:26 -0700

    Hello!

    As Nick Mason (Pink Floyd percussionist) said in a song (on his 1980 "Fictitious Sports" album) - "looking under the hood never did any good".

    Despite the abovementioned warning - I did try and now I'm confused.

    The "Raft Book" has a table of times for sunrise and sunset on different dates and latitudes intended for latitude determination by a duration of day. One should notice the time of the sunrise, wait till the sunset thus measuring the day duration. Then by looking-up the table and some interpolations one can evaluate his position latitude-wise.

    Mr Harold Gatty specifically mentions to watch for the upper limb of the Sun to touch the horizon while timing the sunrise/sunset events. Because his Book was written for a laymen (like cooks or cabin stewards :) ) he never mentions any details like refraction of dip. A raft is low enough to garantee the HoE (height of the eye) was small so that the dip correction was not nesessary - especially considering the overall precision of the method discussed.

    I tried to re-construct some table entries for the refraction correction used by Mr Gatty.

    Many authors (for example - Mr David Burch in his "Emergency Navigation" and Mr John H. Karl in his "Celestial Navigation in the GPS Age") suggest to use the refraction correction value in tune of −0°50' (give or take a couple of arc-minutes) when using the same upper limb as Mr Gatty prescribes.

    However when I visited the USNOA Application Department Celestial Navigation Data page and entered the 1944-Apr-01T05:18 for the 66°N;0°W tabulated position - I got the Hc equal to −0°31.8'. Checking some other tabulated entries I consistently was getting the same Hc of −0°31' (give or take an arc-minute). This approx. 0°20' consistent difference makes me puzzled - why Mr Gatty used −0°16' instead of −0°50' as the upper limb refraction correction?

    Or am I wrong somewhere?

    Please comment.

    Thank you in advance.

    Warm regards,
    Tony

       
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