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    Re: When is the Autumnal Equinox?
    From: Paul Hirose
    Date: 2011 Sep 04, 13:56 -0700

    The equinoxes and solstices are the moments when the geocentric apparent 
    Sun reaches 0, 90, 180, and 270 degrees of ecliptical longitude. See The 
    Astronomical Almanac glossary:
    In general, declination isn't zero at an equinox. That would require the 
    Sun to be exactly on the ecliptic. But in reality its geocentric 
    coordinates can deviate from the ecliptic by a several tenths of an arc 
    second. This happens because the ecliptic is defined by the motion of 
    the Earth - Moon barycenter, which is far enough from the geocenter to 
    cause noticeable parallax on an approximate monthly cycle.
    According to my program, the Sun will have 180°00'00.00" geocentric 
    apparent true longitude on 2011 September 23 at 09:04:38 UTC. At that 
    time its ecliptic latitude is -.37" (IAU 2006 precession and 2001A 
    nutation, JPL DE422 ephemeris).
    Declination is zero at 09:04:17 UTC, 21 seconds earlier.
    JPL HORIZONS and MICA have different ephemerides and precession - 
    nutation models, but agree with those figures within one second of time. 
    With MICA, the TT time scale should be used, keeping in mind that TT is 
    presently 34 + 32.184 = 66.184 s ahead of UTC.
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