I seem to recall Fred contributing to a similar conversation a few years ago. Speaking from ignorance, I think the sun will not be at exactly 0 declination at 180 longitude because the moon can pull the earth above or below the ecliptic depending on the lunar orbit. I think the effect is tiny, and only affects the timing of the zero degree declination definition of equinox by maybe a minute?
Here is a paste from wikipedia: The modern definition of equinox is the instant when the Sun's apparent geocentric ecliptic longitude is 0° (northward equinox <https://en.wikipedia.org/wiki/March_equinox>) or 180° (southward equinox <https://en.wikipedia.org/wiki/September_equinox>).[34] <https://en.wikipedia.org/wiki/Equinox#cite_note-38>[35] <https://en.wikipedia.org/wiki/Equinox#cite_note-39>[36] <https://en.wikipedia.org/wiki/Equinox#cite_note-40> Note that at that moment, its latitude will not be exactly zero, since Earth is not exactly in the plane of the ecliptic. Its declination will also not be exactly zero, so the scientific definition is slightly different from the traditional one. The *mean* ecliptic is defined by the barycenter <https://en.wikipedia.org/wiki/Barycenter> of Earth and the Moon combined, to minimize the fact that the orbital inclination of the Moon causes the Earth to wander slightly above and below the ecliptic.[38] <https://en.wikipedia.org/wiki/Equinox#cite_note-42> Oops, I did not answer your question, which was about the time intervals, not the cause. Sounds like a job for good astronomy modeling software - maybe Stellarium? -Bill On Mon, Sep 4, 2023 at 7:25 PM Steve Lelievre < steve.lelievre.can...@gmail.com> wrote: > Hello, > > From what I've read recently, there are three variants of an equinox: > > - Modern astronomical definition: apparent geocentric longitude of the > sun is 0 or 180 degrees. > > - The older astronomical definition (often used in dialling) : solar > declination is 0 degrees. > > - 'Temporal equinox': halfway between solstices as measured by passage > of time, which is the lay/folk/traditional understanding > > I'd like to know: > > How big are the time intervals between these three types of equinoxes? > > How much do these intervals change as the years or centuries go by, if > at all? > > Thanks, > > Steve > > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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