My thanks go to Bill and Hervé for responding to my equinox question.
The information they provided indicates that the change in definition of
astronomical equinox, from delta = 0 to lambda = 0/180, had no practical
impact as far as sundialing is concerned (only a few seconds). For the
difference between astronomical equinox and temporal equinox, the French
article recommended by Hervé explains the long-term drift of the
solstices and astronomical equinoxes, and I now see the direct effect
that phenomenon would have on temporal equinoxes (the midpoint between
solstices.)
As well, I took Bill's hint and found a table of solstice and
(astronomical) equinox dates and times for years 2001 to 2099. I used
http://www.russellcottrell.com/blog/solarEvents.htm. From the data I
produced the enclosed graph of the time delay between the astronomical
and the temporal equinox. Assuming I did the calculations right, then in
the present day there are about 1.87 to 1.9 days between the two events.
If we think in terms of whole days, those differences will correspond to
one or two calendar days depending on when exactly each instant occurs
within its day.
Cheers,
Steve
On 2023-09-05 2:23 a.m., Hervé Guillemet wrote:
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
------------------------------------------------------------------------
*De: *"Steve Lelievre" <[email protected]>
*À: *"Sundial List" <[email protected]>
*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
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