<div dir="ltr"><div><div><div><div><div>Following up on yesterday's time series session, I thought it might be<br></div>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></div><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><div>Cheers,<br><br></div><div> - Arnold<br></div><div><br clear="all"></div><div><div><div><div><div><div><div><div><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 href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div>
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