(12C) Gregorian Date to Julian Day Number and vice versa

[Dieter]

(01-14-2019 05:30 PM)Albert Chan Wrote:  I was using the abbreviation from https://www.hermetic.ch/cal_stud/jdn.htm, section 6:

"Given a Julian day number JD, the modified Julian day number MJD is defined as MJD = JD - 2,400,000.5"

But, now that you mentioned it ... what is the difference ?

My understanding is that Julian dates may include the time as fractional part, i.e. JD 2444059 is 4 Jul 1979 at noon and 2444059,6 is 5 Jul 1979 at 2:24 h. One the other hand the Julian day number (sic!) is an integer indicating the ordinal number of that day (from 0:00 to 23:59'59,999...."), starting at a certain point in the past (here 1 Jan 4713 BC). This causes an offset of 0,5 between both definitions, apart from the one being a real number and the other an integer.

Please correct me if I'm wrong here.

Addendum: The German Wikipedia article on Julian Dates includes two good and concise commented algorithms for the conversion between dates in the Julian and Gregorian calendar and the Julian date / day number. There also is a detailled and comprehensive explanation how the formula works. So at least if you read German you should take a look at that.

Dieter