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    Re: Equinox, 2019 September
    From: Paul Hirose
    Date: 2019 Oct 1, 14:25 -0700

    On 2019-09-27 9:46, I wrote:
    >
    > 07:50:11 ecliptic longitude = 0
    
    Correction: it's 180° at that time. Zero would be the March equinox.
    Equinoxes and solstices occur when the Sun is at multiples of 90°
    ecliptic longitude.
    
    The 6h/18h right ascension great circle coincides with the 90°/270°
    ecliptic longitude circle, so either coordinate is suitable to calculate
    solstice times.
    
    Sun longitude increases 1 degree per day, so an equinox or solstice
    prediction accurate to one minute (= 1/1440 day) requires longitude
    accurate to about 1/1440 degree, or 2.5 arc seconds.
    
    For predictions accurate to one second, ecliptic longitude must be good
    to 40 mas (milli arc second). I think that's attainable even for
    predictions 25 years in the future. My equinox and solstice predictions
    for 2045:
    
    March 20 05:08:19 TAI
    June 20 22:34:37 TAI
    September 22 14:33:38 TAI
    December 21 11:35:50 TAI
    
    Computed with the JPL DE431 ephemeris (2013), IAU 2006 precession model,
    and 2000A nutation.
    
    For a check I used the DE406 ephemeris (1997), IAU 2000 precession, and
    2000B nutation models. Rounded to the nearest second, the check times
    were identical.
    
    Time scale is TAI because delta T and leap seconds that far in the
    future are too unpredictable to use UT1 or UTC. The latter is always an
    integer number of seconds from TAI, so you could convert the times to
    UTC about a year in advance (when the next leap second becomes predictable).
    

       
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