The algorithm used in my JavaScript is
from the book of Meeus. It is valid for the years
1000 to 3000, and for several centuries after 3000
with errors still quite small.
Spring equinox:
is the beginning of spring (vernal equinox). It
occurs on March 19, 20 or 21 in the Northern
Hemisphere.
For the years from 1583 to 3082 there
are 100 Spring equinoxes on March 19, 1032 on
March 20, and 368 on on March 21.
Summer solstice:
is the beginning of summer. It occurs on June 21
or 22 in the Northern Hemisphere.
Autumnal equinox:
is the beginning of autumn. It occurs on Sept. 22
or 23 in the Northern Hemisphere.
Winter solstice:
is the beginning of winter. It occurs on on Dec.
21 or 22 in the Northern Hemisphere.
Visit my Seasons Applet
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