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    Re: DeltaT (was Tycho Brahe Mars oppositions)
    From: Omar Reis
    Date: 2004 Dec 3, 08:32 -0200

    ----- Original Message -----
    >George Huxtable wrote
    > >P.S. Message from the amateur astronomy newsgroup:
    > >~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
    > >Pannekoek's table appears to employ the Julian calendar. The first
    > >opposition listed, for the year 1580, was the only one that occurred
    > >before the Gregorian reform was implemented in Catholic countries.
    > >
    > >The times shown are for a time zone 10 hours *ahead* of UT (GMT).
    > >Subtracting 10 hours from each entry (being sure to roll back the
    > >date by one if the result is less than zero) will give you the UT
    > >Julian calendar dates/times.
    > >
    > >Incidentally, Jean Meeus lists every Mars opposition for the years
    > >0 - 3000 in his book _Astronomical Tables of the Sun, Moon, and Planets_
    > >(second edition), 1995, pp. 62 - 96.
    > >
    > >Mark Gingrich - San Leandro, California
    > >~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
    > The "time-zone" references of this message surprise me somewhat.
    > Time zones were NOT in use in that period. Instead, observations will
    > presumably have been related to the apparent local time of the observer.
    > There's no need for a time-difference from UT to be a whole number of
    > hours.
    > From the longitudes of Tycho's observatories at Hveen and near Prague, I
    > would expect times there to be roughly 45-50 minutes ahead of time at
    > Greenwich. Then, presumably, you have to account for the use of the
    > Astronomical Day, which at Greenwich was 12 hours later than Greenwich
    > Civil Time (effectively modern UT). Can anyone explain the reference in
    > that message to "a time zone 10 hours ahead of UT"? What's it based on? I
    > suggest that Omar takes it with some caution.
    I don't know how he got this 10 timezone.
    Now that you mentioned, It looks strange to me too.
    I just copied the message.
    > I am rather surprised that problems with the value of delta-T are worrying
    > Omar. Sure, Omar can calculate the moment of opposition with great
    > accuracy. But could Tycho observe it to any great accuracy? It would
    > involve measuring RA's and decs, of Mars, on successive nights, and of the
    > Sun in the days, and then converting to ecliptic coordinates and
    > interpolating. I doubt if he could measure to much better than 1
    > or so. Anyone know different?
    > If Omar is trying to check the precision of his predictions for early
    > dates, I wonder if the timing of oppositions is the best way to go about
    > it, because these are not sudden events identifiable to an exact time. Are
    > eclipse timings available from that era, for testing? Of course, at that
    > date, before pendulums, clocks were untrustworthy, which bedevils almost
    > everything.
    It is not related to Tycho measures. I agree with you that, considering
    the primitive instruments Tycho used, the DeltaT would be irrelevant.
    I mentioned the DeltaT because of  M. Dorl calculation below:
    > So, I get a time of 11:44:00 TDT dT = 95.4 for the
    > 11/18/1580 opposition TDT JDate 2298474.98889.
    I had previous problems with the DeltaT.
    For the date 1580, using J. Meeus AA formula:
      DeltaT=50.6+T*(67.5+T*22.5)      (T in centuries since J2000)
    I got DeltaT=164, which differs from M. Dorl dT=95.4.
    As F. Reed said in another message, the DeltaT is a difficult value.
    I'm not sure how the formula for early DeltaT values in
    Meeus book was constructed. Methods for the DeltaT
    calculation from the same book are already diverging just 20
    years from publication. How someone could estimate what
    the value of DeltaT was 500 years ago scapes me.

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