[coords] Question - Time domain coordinates and frames

[Arnold Rots]

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.

  - 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 Thu, Apr 12, 2018 at 6:23 PM, CresitelloDittmar, Mark <
mdittmar at cfa.harvard.edu> wrote:

>
>
> On Thu, Apr 12, 2018 at 5:18 PM, Arnold Rots <arots at cfa.harvard.edu>
> wrote:
>
>> No, the timeOrigin is only REQUIRED in a TimeFrame if it is referenced by
>> a TimeOffset.
>> So, both the TimeOffset and the timeOrigin can (and should!) refer to the
>> same TimeFrame.
>> (they should, because specifying the origin in a different TimeFrame is
>> asking for trouble)
>>
>>
> So, when a client reads a TimeOffset with timeOrigin, loads that (MJD) and
> sees it references the same TimeFrame with a timeOrigin (itself), what is
> the client supposed to do?
> Since timeOrigin is only REQUIRED for TimeOffset, can I have a data column
> that is Time in MJD, and have it refer to a TimeFrame with a TimeOrigin?
> If so, what does the client do with that since MJD is absolute?
>
>
>> It does mean that TimeOffsets that use different timeOrigins need to
>> reference different TimeFrames.
>> That is not a problem, since most TimeOffset-based time series will use
>> the same timeOrigin.
>>
>>   - Arnold
>>
>
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