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<p>Hi Arnold,</p>
<p>Thnaks for that clear synthesis of what is needed to know.<br>
</p>
<br>
<div class="moz-cite-prefix">On 19/05/2017 04:29, Arnold Rots wrote:<br>
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<div>Following up on yesterday's time series session, I
thought it might be<br>
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helpful to summarize the basics on time information.<br>
</div>
It really isn't that complicated.<br>
</div>
For reference and more information than you really want, you
can turn<br>
</div>
to FITS WCS Paper IV: A&A 574, 36 (2015).<br>
<br>
</div>
1. Required:<br>
<div>1.1 Record time stamps in JD, MJD, ISO-8601, or elapsed
time<br>
</div>
<div>(if in elapsed time, you need to give a zero point in a
time stamp that is not<br>
</div>
<div>provided in elapsed time, of course)<br>
</div>
<div>1.2 Provide the time scale used (TT, TDB, TAI, GPS, ET,
UTC, TCG, TCB)<br>
</div>
<div>1.3 Provide the reference position (place where the time is
measured)<br>
<br>
</div>
<div>2. Note the following:<br>
</div>
<div>JD and MJD do not imply a time scale; it needs to be
provided separately.<br>
</div>
<div>JD and MJD are dimensionless, though a unit of "day" is
implied.<br>
</div>
<div>It's a bad idea to mix UTC with JD or MJD, since not all
UTC days are the same length.<br>
</div>
<div>Use the restricted form of ISO-8601:
[[+|-]c]ccyy-mm-dd[Thh[:mm[:ss[.ss...]]]]<br>
</div>
<div>No time zone characters.<br>
</div>
<div>TDB runs on average synchronously with TT, but corrects for
the relativistic<br>
</div>
<div>effects caused by deviations in the earth's orbit from
perfect circularity and<br>
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<div>constant gravitational potential.<br>
</div>
<div><br>
</div>
<div>3. Recommendations:<br>
</div>
<div>Also provide an estimate of the uncertainty in your
timestamps.<br>
</div>
<div>Avoid UTC. It is trivial to convert the times provided by,
e.g., space agencies<br>
</div>
<div>to TT immediately when you get them and it will save
headaches later on.<br>
</div>
<div>We do it for Chandra (and RXTE).<br>
</div>
<div>Use TT: it's the official IAU time scale, continuous with
ET and the one solar<br>
</div>
<div>system ephemerides are based on.<br>
</div>
<div>TAI and GPS are acceptable alternatives, with constant
offsets from TT.<br>
</div>
<div>Use the same reference position for time and space and make
sure it is<br>
</div>
<div>commensurate with your time scale. For instance, when you
convert to<br>
</div>
<div>the barycenter, also convert to TDB.<br>
</div>
<div>Beware that the barycenter is not the heliocenter.<br>
</div>
<div>Be specific in labeling the time axis; e.g.:
JD(TT;GEOCENTER)<br>
</div>
<div>or MJD(TDB;BARYCENTER).<br>
</div>
<div>Use proleptic Gregorian dates for ISO-8601.<br>
</div>
<div><br>
</div>
<div>4. Do never use:<br>
</div>
<div>TJD, HJD, BJD, etc.<br>
</div>
<div>These are not officially recognized and suggest certain
metadata values, but<br>
</div>
<div>leave considerable ambiguity as to what those metadata
values actually are.<br>
</div>
<div>Instead, specify your metadata explicitly. It avoids
confusion later on and isn't<br>
</div>
<div>much more work.<br>
<br>
</div>
<div>5. What if you deal with incomplete data?<br>
</div>
<div>If you don't know the time scale and/or reference position,
you can provide them<br>
</div>
<div>as UNKNOWN and set the systematic error/uncertainty to,
say, 1000 s.<br>
</div>
<div>100 s will do if only the time scale is unknown.<br>
<br>
</div>
<div>6. What else is there to know?<br>
</div>
<div>Quit a lot, especially the so-called coordinate time scales
(TCG and TCB).<br>
</div>
<div>Because TDB runs on average synchronously with TT, but in a
very different potential<br>
</div>
<div>well, you may have realized that this requires different
values for fundamental<br>
</div>
<div>physical constants in the barycenter. That's awkward and
the coordinate time scales<br>
</div>
<div>fix that by running at different rates. Eventually these
may come into more common<br>
</div>
<div>use, but at least for my lifetime I assume we will be
sticking with TT and TDB.<br>
</div>
<div>More in the cited A&A paper.<br>
<br>
</div>
<div>Hope this is helpful,<br>
<br>
</div>
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Definitely<br>
Cheers<br>
François<br>
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<div>Cheers,<br>
<br>
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<div> - Arnold<br>
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<div class="gmail_signature"
data-smartmail="gmail_signature">
<div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>
Arnold H. Rots
Chandra X-ray Science Center<br>
Smithsonian Astrophysical
Observatory tel: +1 617
496 7701<br>
60 Garden Street, MS
67
fax: +1 617 495 7356<br>
Cambridge, MA 02138
<a moz-do-not-send="true"
href="mailto:arots@cfa.harvard.edu"
target="_blank">arots@cfa.harvard.edu</a><br>
USA
<a moz-do-not-send="true"
href="http://hea-www.harvard.edu/%7Earots/"
target="_blank">http://hea-www.harvard.edu/~arots/</a><br>
--------------------------------------------------------------------------------------------------------------<br>
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