[time-nuts] Calculating sidereal time

[Didier Juges]

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 <sla at ucolick.org 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                    <sla at ucolick.org>              WGS-84 (GPS)
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