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    Equinox, 2019 September
    From: Paul Hirose
    Date: 2019 Sep 26, 20:26 -0700

    The four different "equinox" times (the third one is the formally
    correct equinox):
    07:49:53 Sun declination = 0
    07:50:04 ITRS latitude = 0
    07:50:11 ecliptic longitude = 0
    07:50:14 right ascension = 12 h
    Times are UTC on 2019-09-23.
    computational basis:
    JPL DE431 ephemeris
    IAU 2006 precession
    IAU 2000B nutation
    0.2029 0.3185 polar motion x, y (arc sec)
    If the Sun were exactly on the ecliptic, the first, third, and fourth
    events would be simultaneous.
    If not for polar motion, declination and ITRS latitude would be zero at
    the same time. But Earth's axis of rotation (the celestial pole), which
    is the basis of declination, is offset a few tenths of an arc second
    from the geodetic (ITRS / WGS84) north pole.
    The polar motion parameters I used are from IERS Bulletin A. The numbers
    constantly change since the celestial pole follows a quasi circular path
    with respect to the crust. One circle takes about 400 days and the
    circle is about 20 feet in diameter. It is not centered on the geodetic
    north pole, which is well outside the circle.

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