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    Re: Skyfield
    From: Paul Hirose
    Date: 2018 Oct 25, 12:58 -0700

    On 2018-10-23 17:15, Josh Paterson wrote:
    > Using the method in skyfield.almanac I get:
    > 2018-Dec-21 22:22:44.1118 UT
    
    Solstice at 22:22:44.1117 UTC per the SofaJpl demonstration program, JPL
    DE431 ephemeris, IAU 2006 / 2000B precession nutation model.
    
    The Sun moves about 1°/day in ecliptic longitude, or about 1e-5 °/sec.
    Thus, 1e-8 degree precision in longitude should allow single
    milliseconds of time to be resolved. I set that as the program's
    precision. It's phony precision, since IAU 2000B nutation is accurate to
    only about a millisecond of arc, or 3e-7 degree.
    
    Therefore, I believe my 3rd decimal place is valid within a few units,
    and the 4th is noise. The excellent agreement with Josh is probably
    coincidence. I'm not sure what I ought to expect, since Josh has not
    stated which ephemeris and precession / nutation model he's using. I
    assume his "UT" is UTC. (At the time of the solstice, UT1 and UTC will
    be only a few hundredths of a second different.)
    
    JPL Horizons puts the solstice at 22:22:45.246 UTC, about 1.1 second
    later than Josh and I got. It uses the same JPL ephemeris that I used,
    so I suspect the difference is due to precession and nutation.
    

       
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