"Julian" ambiguity (was Re: [Python-Dev] strptime recapped)

John Machin sjmachin@lexicon.net
Sun, 23 Jun 2002 12:15:22 +1000

21/06/2002 10:27:22 PM, Steven Lott <s_lott@yahoo.com> wrote:

>Generally, "Julian" dates are really just the day number within
>a given year; this is a simple special case of the more general
>(and more useful) approach that R-D use.
>for more information.

AFAICT from perusing their book, R-D use the term "julian-date" to mean a tuple (year, month, day) in the Julian calendar.
The International Astro. Union uses "Julian date" to mean an instant in time measured in days (and fraction therof) since noon on 1 January -4712 (Julian ("proleptic") calendar). See for example 

A "Julian day number" (or "JDN") is generally used to mean an ordinal day number counting day 0 as Julian_calendar(-4712, 1, 1) as above. Some folks use JDN to include the IAU's instant-in-time.

Some folks use "julian day" to mean a day within a year (range 0-365 *or* 1-366 (all inclusive)). This terminology IMO should be severely deprecated. The concept is best described as something like "day of year", with a 
specification of the origin (0 or 1) when appropriate. 

It is not clear from the first of your sentences quoted above exactly what you are calling a "Julian date": (a) the tuple (given_year, day_of_year) with calendar not specified or (b) just day_of_year. However either answer seems 
IMO to be an inappropriate addition to the terminology confusion.