I calculate the Equation of Time as: Sun's Mean Longitude - Sun's Apparent Right Ascension.
This is equivalent to: Right Ascension of the Fictitious Mean Sun - Right Ascension of the Sun (which is the definition of the Equation of Time) even though the Sun's mean longitude is measured on the ecliptic because the fictitious mean sun used to define civil time keeping is defined is such a way that its mean right ascension is always equal to the Sun's mean longitude. It has never been totally clear to me whether it is more accurate to use the Sun's mean right ascension or the apparent right ascension in calculating the equation of time. In the Electric Astrolabe, I used the Sun's apparent right ascension (calculated using VSOP87 corrected for light time and the true nutation of date and optionally corrected for dynamical time). This calculation might more correctly be called the 'apparent equation of time'. I don't think it makes much difference. I suspect the reason that the Dialist Companion gives a value for when the Equation of Time is 0 that is quite a bit different than Luke and I got is because it uses a series expansion for the calculation. Perhaps someone involved in Dialist Companion can tell us. The general series expansion gives a pretty good value, but is not accurate, in the astronomical sense, for any specific day or year (but is good enough for sundials). The US Naval Observatory used to publish the series expansion for calculating EQT for a given year, but I don't know if they still do. Best regards, Jim James E. Morrison Astrolabe web pages at: http://myhouse.com/mc/planet/astrodir/astrolab.htm ----- Original Message ----- From: Luke Coletti <[EMAIL PROTECTED]> To: Phil Pappas <[EMAIL PROTECTED]> Cc: <sundial@rrz.uni-koeln.de> Sent: Friday, April 16, 1999 7:18 PM Subject: Re: EOT=0 > Hello JOHN, > > I think the difference between my reported time and that given by Jim > Morrison converges very closely the same value, if as I suspect, Jim has > incorporated DeltaT in his calculation i.e., the difference between > Dynamical Time and Universal Time. I know that I have not, still on the > punch sheet though! I don't have the exact current value handy but it is > on the order of a minute of time. Therefore, I would say you have two > values that are quite close to one another. The interesting thing to me > is that like Jean-Paul I too use the VSOP87 periodic terms to calculate > solar position however the two EoT zero times based upon it are quite > different, I guess I would suspect a different computation method of the > EoT itself. > > > Regards, > > Luke Coletti > > > John Carmichael wrote: > > > > Several people wrote me with their calculations of when EOT=0. I hope they > > don't mind if I sumerize their answers for you here. > > > > Jim Cobb: 4/16/1999 3:04:36 UTC (xephem version 3.0) > > Luke Coletti: 4/16/1999 0:40:00 UT (Solar Calculator) > > Jean-Paul Cornec: 4/16/1999 1:06:04 UT (VSOP87) > > James Morrison: 4/16/1999 0:40:57 UT (?) > > > > As you can see, all the calculations are different. I assume this is due to > > the different calculating methods that were used. Surely there can only be > > one correct answer.(Or should I use the average time of all the answers?) > > > > Thanks again to Jim, Luke, Jean-Paul, and James for taking the time to do > > the calculations. >
