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    Re: Simultaneous fall 2022 equinox and due west sunset
    From: Paul Hirose
    Date: 2022 Sep 24, 15:47 -0700

    For example, let's simulate a computation of the 2023 equinoxes and
    solstices with the tools of 20 years ago. Then compare results with the
    tools of today.
    
    To simulate the old computation use the JPL DE406 ephemeris (1997), the
    IAU 2000 precession model (actually a correction to the 1976 model), and
    2000B nutation, which is a simplified model but accurate to 1 mas (milli
    arc second) from 1950 to 2050.
    
    For the modern computation use the JPL DE441 ephemeris (2020), and the
    current IAU precession / nutation models 2006 / 2006A. The latter is the
    complete model with 1300 terms and includes the small adjustments to
    make it compatible with 2006 precession.
    
    Geocentric apparent longitudes of the Sun at the times given by my site
    for 2023:
    
    359°59'59.98" old
    359°59'59.98" new
    
    90°00'00.01"
    90°00'00.01"
    
    180°00'00.02"
    180°00'00.02"
    
    270°00'00.01"
    270°00'00.00"
    
    where 0.04" of longitude is about 1 second of time. Clearly predictions
    from "20 years ago" are very accurate. However, the phenomena are
    independent of Earth rotation and therefore long term predictions must
    use a time scale which ticks SI seconds with no step adjustments. You
    could use TT, TAI, or even GPS time, but not UTC or UT1.
    
    I think irregularities in the latter two scales are behind some
    discrepancies in equinox and solstice tables. They may have been
    computed years ago with estimated Earth rotation, but Earth did not
    cooperate. A comparison between my table and Wikipedia shows two
    discrepancies (rounded in the wrong direction) in 2023.
    
    http://sofajpl.com/solstice/
    https://en.wikipedia.org/wiki/Solstice
    
    I won't say Wikipedia is wrong, but one of us is a little weak on being
    right. It practical terms it doesn't matter, except that some people,
    like our original poster, may wonder why astronomy cannot be more precise.
    
    --
    Paul Hirose
    sofajpl.com
    
    

       
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