[fitswcs] Draft WCS Paper V: Time

[William Thompson]

Steve Allen wrote:

> Section 4.1.2.1 of the ISO 8601 standard allows years 0000 through
> 1582 by "mutual agreement", so the language of the current draft uses
> "proleptic Gregorian" and Herschel to make that usage explicit.

I agree that the proleptic Gregorian calendar is the correct approach to take.

Our task is not to try to reconcile the various civilian calendars as they are 
used by different nations at different points in history.  Instead, our task is 
to outline a specific and unambiguous way of encoding date/time values in FITS 
files.  In my opinion, this is best accomplished using the proleptic Gregorian 
calendar, and as far as I know, the most common approach taken within astronomy.

> Section 3.5 of the ISO 8601 standard allows years prior to 0000 or
> after 9999, again by "mutual agreement".  The current draft does not
> attempt to define these.  Any such dates prior to 1583 could not have
> been in contemporary use, and it is pure hubris to specify 8 millennia
> into the future.

It's also my understanding that the most common approach within astronomy is to 
associate the year 0 with the year 1 B.C.E., -1 with 2 B.C.E., and so on.

> Should the paper attempt to require that parsers of DATExxxx keywords
> be prepared to accept formats where the year is either negative or
> 8 millennia into the future?

I personally don't have a strong opinion on whether years outside of 0000-9999 
should be handled in the ISO-8601 format or not, but I can offer one real-world 
example of the usage of astronomical data prior to the year 1 C.E.  In Marsden 
(1989), A.J. 98, 2306-2321, there is a discussion of the observations of a 
sungrazing comet by Aristotle in the year -371, and speculation that it could 
have been a parent body for the Kreutz group.

-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov

(Note changed email address)