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    Re: Skyfield
    From: Peter Monta
    Date: 2018 Oct 23, 23:19 -0700

    Using the method in skyfield.almanac I get:

    2018-Dec-21 22:22:44.1118 UT


    I used a generic solver and got a similar result:

    $ python solstice.py
    2018-12-21T22:22:44.1336Z
    $

    I wasn't aware of the almanac functions, so those may come in handy.  Thanks.  Here's my quick hack for comparison:

    from skyfield.api import load
    from scipy.optimize import fsolve

    planets = load('de421.bsp')
    earth,sun = planets['earth'],planets['sun']
    ts = load.timescale()

    def sun_ra_resid(dt):
      astrometric = earth.at(ts.utc(2018,12,21,0,0,dt[0])).observe(sun)
      ra,dec,dist = astrometric.apparent().radec(epoch='date')
      return ra.hours - 18

    dt_solstice = fsolve(sun_ra_resid,0)
    t_solstice = ts.utc(2018,12,21,0,0,dt_solstice[0])
    print(t_solstice.utc_iso(places=4))

    The small disparity in time, 0.022 second, might be due to the specific ephemeris in the DE4xx family, or the nutation model if that's separate.  I'm not 100% sure my code is set up correctly for the conventional definition of solstice.

    Cheers,
    Peter

       
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