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Omar Reis quoted a message he received, as follows-

>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 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 arc-minute
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.

George.

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