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    Re: Summer Solstice
    From: Stan K
    Date: 2016 Dec 21, 19:33 -0500
    Paul is probably thinking about an AH sight of the sun where the direct and reflected images are overlapped rather than having the limbs just touching.  But if limbs are touching, the Sun Table of the Nautical Almanac is what should be used if the Nautical Almanac is being used at all.  

    However, the Sun Table is not that accurate.  It uses average values over six month periods, allowing a years worth of data to be presented in one-third of a page.  The Sun Table combines refraction, parallax, and semi-diameter.  Refraction is independent of time of year, but parallax and semi-diameter depend on the distance between the Earth and the Sun, with parallax also depending on the altitude of the Sun.  (Horizontal parallax, the maximum value, ranges from about 0.144' to about 0.149').  Semi-diameter, which can be seen at the bottom of the Sun column on the daily pages, ranges from 15.8' to 16.3'.

    For your example, using 21 December and an altitude of 37º, the values (as calculated by Celestial Tools in the "parameters" mode), rounded to the nearest tenth of an arc-minute, are:
    Refraction -1.3' (agreeing with the Stars and Planets column of the Nautical Almanac and the Air Almanac (to whole minutes))
    Parallax +0.1' (my guess is that the Air Almanac ignores this, since it rounds refraction to whole minutes anyway)
    Semi-diameter +16.3' (absolute value agreeing with the Nautical Almanac daily page and the Air Almanac, and plus for the lower limb)

    The total is +15.1', 0.1' more than the +15.0' that the Sun Table indicates.  (In the "SR form" mode, where Celestial Tools tries to calculate the Nautical Almanac value, it gets +15.0'.)  Practically speaking, this difference is insignificant.


    -----Original Message-----
    From: David C <NoReply_DavidC@fer3.com>
    To: slk1000 <slk1000---.com>
    Sent: Wed, Dec 21, 2016 6:15 pm
    Subject: [NavList] Re: Summer Solstice

    You have used the sun altitude correction table.
    With AH-sights you have to use the star and planet altitude correction table instead.
    Which means that the observed altitudes are smaller and the intercepts bigger than in your calculations.
    The difference between an AH sight and a horizon sight is that the former does not require a dip correction. As I am using an AH I am not correcting for dip.
    Consider an altitude of 37 degrees.
    From the Air Almanac 2016 R = 1' and SD = 16.3'. I was using the lower limb so the correction is +15.3'. Note that refraction is given to a precision of 1'.
    From the NA 1973 Sun's Correction Oct-March (scan attached) the correction is +15.0. This is to a precision of 0.1'.
    I believe that I was using the correct value but maybe I am missing something?

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