Hi Steve,
I think that some answers to your questions can easily be found on the following link of the French " Institut de Mécanique Céleste et de Calcul des Éphémérides" ( IMCCE) : https://www.imcce.fr/newsletter/html/newsletter.html#current-article2 They publish (in French) a free information letter every month and in March and September it contains the timing of the equinox with a picture, easy to understand even if you do not speak French. They remind that in the northern hemisphere the Autumn equinox is when the geocentric longitude of the Sun is exactly equal to 180° (and 0° for the Spring). As indicated there is a difference of a few seconds with the time when its declination is equal to 0° and when its right ascension is equal to 12h. The previous information letters can be accessed via : https://www.imcce.fr/lettre-information/ and the data can be retrieved each March and September month Best regards Hervé ----- Mail original ----- De: "Steve Lelievre" <steve.lelievre.can...@gmail.com> À: "Sundial List" <sundial@uni-koeln.de> Envoyé: Mardi 5 Septembre 2023 01:23:15 Objet: Difference between types of equinox 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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