A final(?) note on this topic. By drawing little pictures of ellipses and
crosses and triangles and applying Kepler's Second Law, I was able to come
up with an expression for the difference between the length of summer and
the length of spring to lowest order in the orbital eccentricity
(epsilon=0.017) and the difference between the winter solstice and the
perihelion (delta=13 days):
4 * epsilon * delta = 21 hrs
This is in good agreement within the assumptions I made, so I put to rest my
remaining doubts that this is the correct answer to Willy's question.
While I was at it, I found the corresponding formula for the difference
between the spring/summer half year and the fall/winter half-year:
4 * epsilon * 365 dys / pi = 8 days
Also in good agreement with the information from
http://aa.usno.navy.mil/data/.
--Art Carlson
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