Hi Steve, Thanks a lot for the references and background information. Looks like I have more reading to do... I was not planning to for this project but it is interesting.
I have seen references to using half days in some code samples. I suppose this is what Brooke referred to when he said astronomers work at night :) I need to look into that. That could cause a ~2 minutes error in my code (difference between sidereal day and calendar day is ~4 minutes). I would still be off by a minute, but in the other direction... I have not looked at the new reference you suggested (Capitaine) yet but I wonder why the previous formula was replaced. It's hard to imagine that suddenly calculations would be off. When looking at seconds, I am not sure what's wrong with setting equivalence between earth rotation and time. Certainly it seems to meet my present purpose. In the more general sense, I appreciate that it may not be absolutely correct, or that it would not work at the microsecond level because the earth wobbles. If the earth does not rotate at a constant rate, how can we know where an observer on earth is in space at any time? I am not sure how the current GPS system deals with it but that's another subject. On Fri, Jan 18, 2019, 11:00 PM Steve Allen <[email protected] wrote: > On Fri 2019-01-18T15:18:08-0600 Didier Juges hath writ: > > //----------------------------------------------------------------------- > > // Craig Haley 20/09/01 > > // 30-06-04 CSH made small changes > > // 05-01-05 CSH modified to use mjd rather than converting to jd > > // Didier Juges > > // 18Jan19 converted to C. > > //----------------------------------------------------------------------- > > /* original Mathlab code: > > * mjd2000 = 51544.5; %Modified Julian Date of Epoch J2000.0 > > * int_mjd = floor(mjd); > > * frac_mjd = mjd-int_mjd; > > * Tu = (int_mjd-mjd2000)/36525.0; > > * gmst = 24110.54841+Tu.*(8640184.812866+Tu.*(0.093104-Tu*6.2e-6)); > > * %add the mean sidereal time interval from midnight to time > > * gmst = mod(gmst+frac_mjd*86400*1.00273790934,86400); > > * %convert to hours > > * gmst = gmst/3600; > > */ > > This is the expression for use with FK5 that was codified > by Aoki et al. in > > http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1982A%26A...105..359A > > It is required that this expression be evaluated at 0h UT, > which means that the argument in days since epoch must be half integral > +/-0.5, +/-1.5 etc. > > This expression is no longer in use. It was superseded Capitaine et al. > > http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003A%26A...406.1135C > in which formalism it is explicitly disavowed that earth rotation is time. > This is also far afield from the kind of time usually in this group. > > -- > Steve Allen <[email protected]> WGS-84 (GPS) > UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat > +36.99855 > 1156 High Street Voice: +1 831 459 3046 Lng > -122.06015 > Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
