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Omar Reis asked-

>Hello,
>
>First, I know that this message may be out
>of scope on a navigation list, but this one
>seems to be very ecletic. So here is the
>problem:
>
>I'm trying to validate the results of
>a planet position algorithm for early
>dates. So I searched for the oldest
>Mars observations I could find.
>
>In the book "History of Astronomy",
>by Anton Pannekoek, I found a little
>table of times of Mars oppositions
>in the period between 1580 and 1604,
>which is reproduced below.
>
>The book is not very detailed about that
>table (which is quoted below).
>It is based in the observations by Tycho Brahe,
>and is very important to the history of astronomy,
>since it was used by Kepler to discover his
>fundamental planet laws.
>
>- Are the dates in Gregorian calendar ?
>( I know protestant countries were slow
>  to adopt the new calendar, by as much
>  as 100 years ).
>
>- What is the time zone he used
>( or how can I convert the times to UT )
>Any hint will be appreciated.
>
>Omar
>
>P.S. Mars Oppositions by T. Brahe
>18/11/1580 01:31
>28/12/1582 03:58
>30/01/1585 19:14
>06/03/1587 07:23
>14/05/1589 06:23
>08/06/1591 07:43
>25/08/1593 17:27
>31/10/1595 00:39
>13/12/1597 15:44
>18/01/1600 14:02
>20/02/1602 14:13
>28/03/1604 16:23

==========================

Response from George-

I doubt if there's a simple answer to Omar's interesting question. Here is
some further information, which may only succeed in muddying the waters
further.

Most of Tycho's observations were made from his castle of Uraniborg, on the
island of Hveen (now Ven) in the Oresund, between present-day Denmark and
Sweden (and part of Denmark). The longitude was about 12deg 40' East of
Greenwich.

The Explanatory Supplement to the Ephemeris tells me that the Gregorian
calendar was introduced in 1582 but was not adopted in Denmark until Feb
19/March 1 of 1700. In Sweden, not until 1753. So presumably, Hveen dates
were Julian ones.

My guess is that Tycho would have used Astronomical Time, which starts a
particular day at 0 hours at local apparent noon (at Hveen) of the civil
day with the same date. At that date there was no mean-time, and no
time-zones.

Many of Tycho's observations were published later by Kepler, who had become
his assistant, and Kepler came from a protestant region of Germany: these,
mostly, held out against the new calendar until 1700.

So I suggest it's most likely that Tycho's and Kepler's dates related to
the Julian calendar.

However, just to complicate matters, Tycho left Hveen in 1597, and set up
an observatory near to Prague (long. about East 14deg 33') in 1599, when
his collaboration with Kepler started. Tycho died in 1601. This observatory
was in Bohemia, which had adopted the new calendar on January 7/17, 1584.
Which makes it quite possible that Tycho's Mars observations were published
according to the Gregorian calendar.

For a more detailed and authoritative answer, I suggest Omar asks Robert
van Gent, "R.H. van Gent" , from Utrecht in
Holland, who at one time was a contributor to Nav-l and who is a real
polymath in history-of-astronomy, and helpful with it. If he can't answer
Omar's question directly, I'm sure he knows a man who can. There's also a
History of Astronomy mailing list.

Finally, here are some relevant dates and times taken from Jean Meeus'
"Astronomical tables of the Sun, Moon, and planets" (Willman-Bell 1995).
These are from Table 2, "Oppositions of Mars", on page 81. Meeus tells us
that he uses the Julian Calendar until Oct 4 1582, then the Gregorian from
15 Oct 1582 (the next day). The times are in TD, and are geometrical, i.e.
at the moment the opposition occurred; not the moment it was seen, which
would be about 7 min later. That distinction was presumably irrelevant to
Tycho.

18/11/1580 11:41
07/01/1583 16:11
10/02/1585 07:27
16/03/1587 21:38
24/04/1589 19:55
18/06/1591 18:21
05/09/1593 02:11
10/11/1595 10:40
24/12/1597 02:41
29/01/1600 03:14
03/03/1602 04:28
08/04/1604 05:53

George.



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