[fitswcs] [fitsbits] Draft WCS Paper V: Time

[Steve Allen]

On Tue 2009-04-07T19:05:14 +0200, Lucio Chiappetti hath writ:
> I would definitely exclude the usage of ISO format for dates before
> october 1582.  It is the first time I hear of a gregorian proleptic
> calendar

The Gregorian proleptic calendar is already mentioned in various IVOA
documents.  Given the use of ISO 8601 we are already constrained to
the use of Gregorian for all values which it represents.

>       Or leave it at discretion of the few archeo-astronomers which would
>       ever write a FITS file with some ancient phenomena.

If we allow proleptic usage at all, we want to be sure everyone knows
what it means.

>    where does the difference UTC 00-60 vs all other 00-59 come from ?

CCIR Recommendation 460, now known as
ITU-R TF.460-6

This is the document which defines the nature of the international
broadcast time scale, which created leap seconds, and which assigned
the name UTC to that time scale.

>    REAL*8 (eventually with a specified weight (TIMEDEL ?) on the LSB,
>    expressed as elapsed time from an offset (I called it offset, but it is
>    sort of DATEREF, the beginning of the day of the observation start time
>    in "Unix 1970" seconds).

I personally warn against use of "Unix 1970" seconds unless accuracy is
only relevant to around one minute.  Your jurisdiction may vary.  See
http://www.ucolick.org/~sla/leapsecs/epochtime.html

> - sect. 3.1.4 doublet vectors

We should probably make it explicit by pointing out that the JPL
ephemerides have used such a convention for decades.  Without such a
mechanism observations such as pulsar timing could not be reduced to
the relevant accuracy.

>    Also GPS was deprecated in App. B and now it is no longer. Correct ?

The subject of GPS time deserves a separate question to the community.

>    I do not like the truncated name XPOSURE (if not a typo)

This remains an issue of community discussion.
We have to balance the need for a precise definition in this standard
against all of the existing and variant forms of exposure keywords
which are already in use.

If we use EXPOSURE or EXPTIME we almost certainly create ambiguities
in the existing archives of FITS files.  Future researchers will have
to be very diligent to ascertain whether any given data acquisition
system produced those keywords with values which are consistent with
the new standard.  Operators of existing systems will face a dilemma
of finding an opportune moment of changing those systems and a way
of documenting when that change happened.  It seems safer to define
something new and likely never used than to cloud the entire history
of FITS with ambiguity.

>    Instead of XPOSURE one could have EXPOKWD with a string value, pointing
>    to the name of the keyword (file dependent according to pre-existing
>    conventions) containing the "main" exposure.

That's an interesting form of indirection in FITS.

>    also is it necessary to quote Herschel, or cannot we find the same thing
>    in some recent issue of the Astronomical Almanac ?

It was Herschel who brought the use of JD into astronomy, and his work
on calendars is relevant to any DATExxxx keywords with values prior
to the Russian revolution (the October revolution which happened in
November for most of the rest of the world).

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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