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Thanks for the replies to my posting and note on this subject.

Richard pointed out a couple of good parameterizations of EoT and Andres has an accurate tabulation of EoT in his almanac, but what I had in mind was to develop an equation for EoT where I understood how it can be derived from basic physics and geometry. Parameterizations and tabulations are very useful for calculating various complicated processes, but they don't give you any insight into what is happening.

Also Richard and Robert pointed out the extensive articles on EoT provided by Wikipedia and USNO. Both of these offer a great deal of background information and also explain the origin of a non-zero EoT, but neither of them go on to actually derive it. Actually there is a "derivation" given in Wikipedia, but I've scanned it a couple of times and I confess I don't understand what they are doing!

Having come up with what I consider to be a good explanation of EoT in terms of the basic parameters of the earths motion, a much more basic question arises: which came first? By understanding the earth's motion I can calculate EoT, but say that I had first observed and recorded EoT over the course of a year. How much about the motion of the earth would I be able to surmise from my EoT observations? Couldn't I fit my EoT data to the equation I give at the end of my note and thus determine everything: the timing of the winter solstice and orbit perihelion, the orbit eccentricity, and the rotation axis tilt?

In case anyone missed my first posting I've attached the note below.

George B
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