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