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Appendix 1 There has been widespread uncertainty over the date of Galileo's birth, as to whether it was the 15th or 16th of February 'Old Style' (i.e. Julian calendar), 1564. The evidence for his birthdate comes from the horoscopes which he drew up for himself, which rather sunk into oblivion after his death, as he became an icon of the new science and people wanted to bury this side of him. Two different time-conventions then current, the earlier one used by Galileo's parents counting hours from sunset1, in terms of which he was born on the 22nd hour of February the 15th; while on the more modern convention of hours post meridiem, after noon, he was born at three p.m. GMT on the 16th. Galileo drew up two charts for himself, here shown, and these differ by half an hour. They were not published in Favaro's twenty-volume Opere of Galileo in 1929 and so remained hardly known. His astrological manuscripts are kept at Florence's National Library, and in a 1980 exhibition these were displayed2. Above them Galileo wrote, 1564, 15 February, h 22.30. lat 42.30 16 February, h 4 pm h 3.30 pm The first two lines describe the same moment, using the two different time-conventions: the first line gives it as twenty-two and a half hours as from sunset of the previous day, while the second gives the hours as measured from noon. His latitude for Pisa is out by a degree, as gives us an idea of the accuracy involved.
The two charts shown are both for the 16th of February, and have the Moon in Taurus at 4° 40' and 4° 47' respectively. The charts differ by merely half an hour in time, shown by the way their house divisions differ by some ten degrees. To the left are the latitude and longitude values for the planets, as he needed for drawing up 'primary directions,' a technique for plotting concerning the course of his life. At the bottom of these charts he has put in Fortuna, the Path of Fortune, an Arabic concept found by adding the Sun-Moon angle to the Ascendent. To check Galileo's timing we start with sunset that day at Pisa, which the computer gives as 17h 38 minutes local mean time. Next we require the 'Equation of Time:' to find this, adjust your program to have the Sun at MC for that day, then the time-difference between that and noon is the 'Equation of Time:' it was fifteen minutes for that day; adding this gives their time for sunset as it were, i.e. sunset in 'local apparent time:' 17h 53 minutes. Then, adding on the 22 hours 30 minutes after sunset gives us his time of ten past four in the afternoon of the 16th: that is his birth-data in LAT. He used the house system called 'Regiomontanus,' which has equal thirty-degree angles around the celestial equator. Reconstructing his charts using a modern program gives us about 4 p.m. LMT as their average, and adding the fifteen minutes gives more or less the same time as before, which is reassuring. If the GMT time is required, one subtracts 42 minutes for Pisa (one hour per fifteen degrees of longitude) to get 3.30 GMT (Compare these with the modern horoscope given in Chapter 4, which has 14° of Leo rising, the same as the lower of these two charts). Two other charts were published in Favaro's 'Opera', where one is not sure who made them or when. The first is for 3 pm on February 15th or 16th, with the Moon in Aries and not Taurus3, while the second is for the 18th. These posthumous charts became better known by posterity than the authentic charts drawn up by Galileo. References
1) For sunset time, see Michael Talbot, 'Ore Italiane' p.52 Italian Studies, 1985, Vol.L. | |||||||||
