At 19:36 29/12/96 +0100, Fran├žois BLATEYRON wrote:
>Hi dear gnomonists...
>
>Can someone help me with the calculation of the equation of time ?
>

Curiously enough it was "The Equation of Time" and its derivation that first
got me interested in Sundials. I thought - Oh yes, a piece of cake to apply
some simple Newtonian Mechanics and come up with an answer.

Well some considerable time later I concluded that it was not as easy as I
had at first thought! However, I have two references which may be of
interest to you or others on the mailing list:

1) "Practical Astronomy" by Robert H. Mills published by Albion (ISBN
1-898563-02-0 for hardback or 1-898563-00-4 for paperback) gives an analysis
of The Equation of Time.

2) There is a paper "The Equation of Time" in the Monhly Notices of the
Royal Astronomical Society" (1989) 238 1529-1535 submitted by David W.
Hughes, B. D. Yallop and C. Y. Hohenkerk. David Hughes is listed as being
from Sheffield University and the other two authors are from the Royal
Greenwich Observatory.

The Summary of this notice reads

"An Equation is developed which gives the Equation of Time as a function of
Universal Time. This enables it to be calculated for any epoch within 30
centuries of the present day, to a precision of about 3 s of time. We also
give several expressions for the Equation of Ephemeris Time which ignores
the distincion between the time-scale of the Ephemeris and Universal Time,
and so may be compared with expresions given in old text books."

I hope that these two references help - though I note that other side issues
such as the inner workings of Excel date functions may also cause some
anomolies!!!!

Best of luck

-----------------------------------------Andrew
Pettit-----------------------------------------

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