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    Re: Equinoxes: when Lon=0 (or 180) but not when Dec=0?
    From: Paul Hirose
    Date: 2017 Aug 1, 16:14 -0700

    On 2017-07-31 6:08, Tony Oz wrote:
    > Why the definition of Equinoxes demands the Sun's longitude to be equal to either 0° or 180°?
    > The actual length of the day or night depends directly on the Sun's 
    declination (rather than on its' longitude), so why not Dec == 0° then?
    
    My guess — the time when an equinox occurs is formally defined in terms
    of ecliptic longitude because it's almost a linear function of time and
    its rate of change is fairly high (about 1 °/day). Therefore, it's easy
    to compute the times of the equinoxes and solstices.
    
    You can't say that about declination. Near an equinox it varies almost
    linearly with time, though not as rapidly as longitude. But near a
    solstice it changes so slowly, the time of solstice is difficult to
    identify.
    
    What about right ascension? It's easier to compute than longitude, since
    modern precession / nutation models give a simple transformation from
    the ICRS to the equator and equinox of date. Although it has more
    variation in rate than ecliptic longitude, the Sun's RA is still close
    to a linear function of time. I think RA would be a more convenient
    basis for defining the equinoxes. But I don't write the rules.
    
    (Nevertheless, RA can be used to find the solstices since the 6h/18h
    great circle coincides with the 90°/270° longitude great circle.)
    
    

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