VM44,
Please do not ask me to comment or explain. I am certain your skills far exceed mine.
I am simply providing this information as background, given your interest in Julian dating. From my simple reading of the following, it seems possible that caution needs to be exercised when plotting Julian dates into a system, since these guys seem to have a Julian day number (JDN) as well as a Julian ephemeris date (JED). Whether that has any significance for you, I cannot even begin to guess.
Here then is the piece that speaks about converting from either the Julian or Gregorian calendar to the JDN (if that matters to you)
Doug
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On a systematic solar calendar - such as the Julian or Gregorian system - it is a relatively straightforward matter to calculate the exact number of days between any two selected epochs. Most computations of this sort make use of the Julian day number (JDN), devised by Joseph Justus Scaliger (AD 1540-1609). This scheme is based on a cycle of 7980 Julian years, commencing in 4713 BC. The precise epoch from which Julian days are numbered is Greenwich noon on Jan 1 (Julian calendar) in that year. A major reference epoch is 1900.0 (i.e. Greenwich noon on 1899 Dec 31), for which the JDN is 2415020.0. Since most modern astronomical computations make use of terrestrial time, it has become customary to use the Julian ephemeris date (JED) where necessary; this begins at 12h TT.
Various algorithms have been developed to convert from either the Julian or Gregorian calendar to the JDN, among the simplest being those deduced by Muller (1975), which make full use of the rules of FORTRAN. These are given in equations (1.20) and (1.21). Here J is the JDN, Y the year, M the month and D the day of the month; all variables are treated as integer.
For any date AD on the Gregorian calendar:
J=367*Y - 7*(Y+(M+Y)/12)/4 - 3*((Y+(M-9)/7)/100+l)/4+275*M/9+D+1721029. (1.20)
For any date on the Julian calendar the following simpler formula applies:
J=367*Y-7(Y+5001+(M-9)/7)/4+275*M/9+D+1729777. (1.21)
I have employed these formulae in many of the computer programs which I have developed.
(Historical Eclipses and Earth’s Rotation, pages 31-32, F Richard Stephenson)