I think Andrew James' explanation about the sum of two symmetrical curves (inclination and orbital eccentricity) may be exactly how the zero point for the equation of time was determined. There is a good article on this in Sky and Telescope, July 1972, pages 20-3 by Bernard Oliver. He breaks down the analemma into these two curves, each of which is appears to be symmetrically referenced around the zero point (EoT=0). He then demonstrates, as Luke Coletti points out, that these curves change over time, changing the overall shape of the analemma. He calculates that in 1246AD the analemma was a symmetric figure 8, with its intersecting point (where the 8 crosses itself) at EoT=0.
Bill Gottesman Burlington, VT
