Ron Anthony wrote:
>
> Prof Gregorio,
>
> >> As far as the Equation of Time is concerned, the last
> edition of the Explanatory Supplement to the Astronomical
> Almanac reports (pag. 484) the following algorithm:<<
>
> Thank you for the algorithm. No matter how hard I try I cannot
> make the algorithm yield the correct results. I attribute this
> to my incorrect math assumptions. Could you or someone else
> walk thru the algorithm with a real date and time, (for
> example 12:00 July 27, 1980) to help me find my wrong thinking?
>
> Does "remove multiples of 3600" mean "reduce to the range of
> 0 to 3600 by adding/subtracting multiples of 3600"? Or is the
> range -3600 to 3600?
>
> What is ET expressed in? decimal hours?
>
> ++ron
Dear Ron,
in the algorithms suggested by the Explanatory
Supplement, the solar mean longitude and the mean
anomaly must be reduced in the interval 0-360 degrees
(and NOT 0-3600 degrees); it was probably a misunderstanding,
due to e-mail transmission.
In order to convert a Gregorian calendar date into a
Julian date (at Greenwich mean noon), you can use the
form (valid for calendar dates since March, 1900, personal
communication of T.C. Van Flandern & K.F.Pulkkinen, US Naval
Observatory, Washington)
JD = 367*Y-7*(Y+(M+9)/12)/4+275*M/9+D+1721014
where Y=year, M=month number, D=day (all integers).
In the above formula, division by integers implies truncation
of the quotients to integers; no decimals are carried. You can also
use the method (less suitable for computers) suggested by Peter
Duffet-Smith in his excellent book "Practical Astronomy with
Your Calculator", Cambridge University Press, 2nd edition, 1981.
Finally, the EOT of the Expl.Supplement algorithm is expressed
in degrees; you may convert it in hours (or primes or seconds)
remenbering that
24 hours = 360 degrees (1 hour = 15 degrees)
or
1 degree = 1/15 hour = 4 primes = 240 seconds
I hope that these specifications are sufficient for you; if
something is not completely clear, don't fear, e-mail...
With my best regards,
Paolo GREGORIO
