Hi John, Sara et al.,
My understanding is that a seasonal (or unequal) hour is a period of time ('in
the first hour' etc) and not an instant. It is never divided up into minutes
and so the time of 6 minutes after dawn must be referring to a time in equal
hours, most probably measured with an astrolabe as you suggest.
I agree. All that remains unknowable is the visibilty at the time. If it were
overcast at dawn, they must have calculated rather than observed. But I tend to
think it was observed, and determined with an astrolabe.
Ross
________________________________
De : John Davis <[email protected]>
Envoyé : mercredi 1 juillet 2020 10:10
À : Ross Sinclair Caldwell <[email protected]>; Schechner, Sara
<[email protected]>
Cc : 'sundial list sundials' <[email protected]>
Objet : RE: Time problem
Hi Sara, Ross et al,
My understanding is that a seasonal (or unequal) hour is a period of time ('in
the first hour' etc) and not an instant. It is never divided up into minutes
and so the time of 6 minutes after dawn must be referring to a time in equal
hours, most probably measured with an astrolabe as you suggest.
Regards,
John
-----------------------
------ Original Message ------
From: "Schechner, Sara" <[email protected]>
To: "Ross Sinclair Caldwell" <[email protected]>
Cc: "'sundial list sundials'" <[email protected]>
Sent: Tuesday, 30 Jun, 20 At 21:20
Subject: RE: Time problem
>>> In short, I am researching the biography of Filippo Maria Visconti
>>> (1392-1447), duke of Milan, and you probably know that these Italian
>>> princes relied heavily on astrology. So, Visconti's time of birth is known
>>> precisely - "six minutes after sunrise," Monday, 23 September, 1392. His
>>> natal chart was of course produced and interpreted, but it has been lost. I
>>> am trying to recreate it as it might have been done by a court astrologer
>>> of the time.<<<
I have some thoughts about ascertaining the time of “6 minutes after sunrise”
in 1392 in Milan.
First of all, Milan is one of the earliest towns to have a public tower clock
in the 14th century, but it would only strike and show hours according to local
solar time. It would not be divided into minutes. It was not reliable enough
for such a horological chart.
Sundials would be the more commonly used timepiece, but the six-minutes is an
unusual amount of precision. My guess is that the court astronomer was using an
astrolabe, which can be divided into units in the range of 4-6 minutes. Many
also had arcs for the astrological houses and for both equal and unequal hours.
The actual time might have been taken from a bright star still visible in the
dawn.
It is also worth considering what this 6-minutes after dawn really means. Is
the astrologer using unequal hours which were still more common in these early
days of clocks? If so, then six minutes would be equal to 1/10 of the first
hour on that day of the year—i.e., 1/10 of 1/12 of the length of daylight.
Lastly, in reconstructing a horoscope, one needs to know the position of the
planets to place them on the chart. Some might be observed, but mostly they are
taken from a table. These varied in different manuscript traditions. Do we have
a clue what table the astrologer was using?
Good luck with your project.
Sara
Sara J. Schechner, Ph.D.
David P. Wheatland Curator of the Collection of Historical Scientific
Instruments
Lecturer on the History of Science
Department of the History of Science, Harvard University
Science Center 251c, 1 Oxford Street, Cambridge, MA 02138
Tel: 617-496-9542 | Fax: 617-495-3344
[email protected]<mailto:[email protected]> | @SaraSchechner
http://scholar.harvard.edu/saraschechner
http://chsi.harvard.edu/
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