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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:
    http://asa.usno.navy.mil/SecM/Section_M.html
    
    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.
    
    -- 
    I filter out messages with attachments or HTML.
    
    
    
    
    

       
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