[coords] Question - Time domain coordinates and frames

[Arnold Rots]

No, No, No!

JD, MJD, and ISO-8601 have no meaning if the time scale is not specified
(nor the reference position, for that matter).

A time stamp, whether it is expressed in JD, MJD or ISOTime, always HAS to
be referenced to a time scale (any time scale) and a reference position.
Like:
JD 1456344.845 (TT; GEOCENTER)
2018-04-13T17:26:37 (UTC; TOPOCENTER)
MJD 56345.345 (TDB; BARYCENTER)

None of these is identified with any specific time scale.
Though, there is a strong warning that JD and MJD (UTC) are dangerous,
especially on days that contain leap seconds.

  - Arnold

-------------------------------------------------------------------------------------------------------------
Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Fri, Apr 13, 2018 at 8:42 AM, Paul Harrison <
paul.harrison at manchester.ac.uk> wrote:

>
>
> On 2018-04 -13, at 01:10, Arnold Rots <arots at cfa.harvard.edu> wrote:
>
> Since MJD is absolute, the client should ignore the timeOrigin. Actually,
> it should not even look to see whether there is one.
>
> In terms of implementation:
> The common implementation of time is to keep it as either JD or MJD (my
> code uses MJD, the JPL ephemeris uses JD), as either two doubles or an
> integer and a double.
> JD and MJD times would then need either nothing or the JD-MJD fixed offset
> in order to be stored; ISOTime requires a little more; but only when a
> TimeOffset if provided would the code actually look for a timeOrigin value.
> Or, if that feels more comfortable, you can (arbitrarily) assign a default
> value for the timeOrigin; can be zero or 10^10 or your favorite prime
> number.
>
>
> I don’t think that in fact things are as simple as this summary presents,
> both in theory and certainly in practice. I think that it is incorrect to
> say that MJD is absolute -  it is tied to the UTC timescale for working out
> the whole day, but then there is the problem of leap seconds - is the
> fractional part of the MJD for 12 noon on a day with a leap second 1/2 or
> 43200/86401? The Astronomical Almanac in fact discusses MJDs in different
> timescales, where there fractional part of the day is calculated in the
> timescale indicated - so I get the feeling that in general there is not
> even a consensus of definition of MJD that means that a given MJD double
> value is completely unambiguous on days with leap seconds - The real
> problem is the measuring in units of day - only seconds have unambiguous
> definition, and then you do want to specify an origin to be sure what is
> meant.
>
> ISOTime is tied to UTC and does have unambiguous representation of time on
> days with leap seconds, and the textual representation can include any
> timezone offsets in the representation, so need no origin - and for this
> reason is probably superior to MJD for representing instants of time in UTC.
>
> Paul.
>
>
> Dr. Paul Harrison
> JBO, Manchester University
> http://www.manchester.ac.uk/jodrellbank
>
>
>
>
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