Hi Sara, thank you for your comments. 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. ... 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. I agree that his astrologers would not have counted minutes by the clock. Venus was indeed far enough from the Sun to be clear at dawn, presuming it were a clear day.
Is the astrologer using unequal hours which were still more common in these early days of clocks? I can't answer that with certainty. My guess would be equal hours, since astronomers had no practical use to distinguish between daylight and nighttime hours, but would want a consistent system throughout for their calculations. I am sure I can find the answer, though. Do we have a clue what table the astrologer was using? We know the textbooks used at Pavia, where the first Lectureship of Astrology was established, so it should be easy to find out. Monica Azzolini, who has written on astrology in the time of Visconti and Sforza rule of Milan, compares modern values to the Alfonsine Tables when she discusses Galeazzo Maria Sforza's chart, so I assume that those tables were standard, and I won't go wrong using them. A Swedish physicist, Lars Gislén, has produced spreadsheets to calculate various pre-modern astronomical data, including by the Alfonsine Tables . His homepage http://home.thep.lu.se/~larsg/Site/Welcome.html At "Download my applications," http://home.thep.lu.se/~larsg/Site/download.html , scroll down to "Astromodels" to get all of the Excel spreadsheets if you want to try them out. He has Ptolemy Almagest, two Arab astronomers, Toledan, and Alfsono. Thanks for your interest. I'll share my final result with you before daring to publish it. I would be interested to know more about the methods of astrologers in the 14th-15th centuries in Milan, including astrolabes, which is John Davis' specialty. Best regards, Ross Caldwell 43.349399 3.22422981 Béziers, France ________________________________ De : Schechner, Sara <[email protected]> Envoyé : mardi 30 juin 2020 22:20 À : Ross Sinclair Caldwell <[email protected]> Cc : 'sundial list sundials' <[email protected]> Objet : 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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