S. Caro wrote:
Sam Wormley wrote:



Good thing I looked this up, as I would have been totally wrong!

Explanatory Supplement - Astronomical Almanac

9.211 Equinoxes and Solstices (pg 477)

The times of the equinoxes and solstices are *defined* when the Sun's
*apparent ecliptic longitude* lambda_s is a multiple of 90°; i.e.,
it is calculated from f(t) = 0, where f(t) = lambda_s -0°, 90°, 180°,
or 270°. Thus in the northern hemisphere, for the spring equinox
f(t) = lambda_s, for the summer solstice, f(t) = lambda_s - 90°, for
the autumn equinox f(t) = lambda_s - 180° and for the winter solstice
f(t) = lambda_s - 270°. At the equinoxes the Sun crosses the equator
when the length of the day exceeds the length of the night due to
refraction, semidiameter, and parallax of the Sun. At that time the
lengths of the day and night are approximately equal everywhere.


Does the analemma effect not factor into this ? (Perhaps the math
above explains it :-)



The analema is a figure 8-shaped plot of the apparent Sun relative to
the mean Sun. This curve is sometimes seen on globes, maps and the
photography of Dennis di Cicco and most notably, Anthony Ayiomamitis!
And it does change slightly over the millennia.