I wonder how to calculate the date of the vernal equinox, summer solstice, autumnal equinox, and winter solstice in Mathematica. I searched Wolfram Community for a post showing how to calculate the solstices and equinoxes but I couldn't a post. Kenneth R. Lang states in his book Essential Astrophysics
As the Sun moves along the ecliptic, it crosses the celestial equator
twice, on its way north at the Vernal Equinox, on about March 20, and
then at the Autumnal Equinox on about September 23. On either equinox,
the Sun lies in the Earth’s equatorial plane, so the twilight zone
that separates night and day then cuts the Earth in equal parts and
the days and nights are equally long. The point at which the Sun is
farthest north, is the Summer Solstice (on about June 21), and its
most southerly point is the Winter Solstice on about December 22. The
days in the northern hemisphere are the longest on the Summer
Solstice, and shortest on the Winter Solstice. So the crossing of the
Sun at the equinoxes and solstices mark the beginning of the seasons
in the Earth’s northern hemisphere, and the location of these points
on the celestial sphere are given in Table 1.1.
I understand autumnal equinox is when the days are the same length.