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    Re: Figure out LAN?
    From: Paul Hirose
    Date: 2014 Oct 02, 17:34 -0700

    On 2014-10-01 10:01, Samuel L wrote:
    > Is the following example correct in determination of LAN using GHA?
    > Date- October 1, 2014
    > Latitude- N 41d
    > Longitude- W 75d 22.4min
    > LAN= 11:51:06.39  (12:51:06.39 DST)
    
    For a precise computation, I ran my Tinyac program and included Earth
    orientation parameters from IERS Bulletin A. On 2 Sep:
    
    0.1904″ 0.2863″  polar motion x,y
    -0.35270s  UT1-UTC
    
    The values are tabulated at 0h every day. At my precision it's not
    necessary to interpolate to the actual time.
    
    With the JPL DE422 ephemeris and the IAU 2006/00A precession nutation
    model, I find topocentric local apparent noon at 16:51:07.36 UTC. To
    find UT1, simply add the UT1-UTC difference already given, and get
    16:51:07.01 UT1.
    
    I discovered polar motion makes no difference at this accuracy.
    
    For a check, I used the JPL Horizons online ephemeris. It says local
    apparent solar time is 11 59 59.9963 at the UTC I computed.
    
    To attain .01s accuracy it's necessary to correct for the diurnal
    aberration due to the observer's rotation about the geocenter. Neglect
    of this phenomenon changes the calculated apparent solar time by .02s.
    
    Your result is off by about .6s or 1.0s, depending on time scale (not
    stated). If you used the Nautical Almanac, one source of error is that
    GHA is rounded to .1′. Furthermore, Sun GHAs are adjusted up to .15′ to
    eliminate the v correction. Finally, the increments and corrections
    table is also rounded to .1′. Without a current Almanac I don't know how
    those errors actually combine in this case. But one second seems
    reasonable. Of course, that's far in excess of what's needed for a noon
    sight.
    
    

       
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