A double heliocentrism anniversary?

Georg Joachim Rheticus was born 500 years ago today in Feldkirch, now in Austria, on 16^th February 1514, as Dennis Danielson put it in the title of his excellent Rheticus biography, he was the first Copernican. [1] It was Rheticus who travelled to Frauenburg in 1539 and over many months persuaded Copernicus to publish his De revolutionibus, publishing his own Narratio prima, to test the water for the heliocentric hypothesis in 1540.

Rheticus’ Horoscope taken from Danielson The First Copernican p. 16

Rheticus’ name will always be associated with that of Copernicus and heliocentrism.  Another who is inseparably bound up with Copernicus and heliocentrism is Galileo Galilei who celebrated his 450^th birthday yesterday, or did he? All over the Internet people were celebrating Galileo’s 450^th birthday yesterday, the 15^th February, Both the MacTutor History of Mathematics article and the English Wikipedia give his birthdate as 15^th February 1564, the latter citing Galileo at Work by Stillman Drake, the Grand Seignior of Galileo studies, but was it? Already on Thursday last week as people were gearing up for the great day Lorenzo Smerillo of Montclair State University pointed out on the History of Astronomy Mailing List that Galileo was actually born on the 16^th February and not the 15^th.  Somewhat confused, I turned to the two most recent   scholarly biographies of Galileo, David Wootton’s Galileo Watcher of the Skies[2] and John Heilbron’s Galileo[3]; the former says the 15^th, the latter the 16^th. Interestingly both refer to Galileo’s horoscope, which he cast himself. Another Galileo expert William Shea gives the birthdate in his Galileo Selected Writings, as the 16^th and in a footnote explains the reason for the confusion, the same one that Smerillo had already given and which I will now explain.

Galileo’s Horoscope

As you can see Galileo’s horoscope gives the date and time of birth twice:

1564. 15. febr. h. 22.30. lat. 42. 30

16. febr. h. 4. pm

3.30

The first set of figures are given in Italian hours, which follow the Islamic and Jewish custom of numbering the hours of the day from sunset. Sunset on the 15^th February in Northern Italy was at 5:30 pm so the 22.30 hour on the 15^th would be 4 pm on the 16^th. This is of course the second set of figures giving the date and time of birth in French hours. This shows clearly that Galileo was born on the 16^th not the 15^th.  The 3.30 is probably an error estimate based on uncertainty as to the time of sunset.

Confusingly Wootton argues against this interpretation insisting on the 15^th in a complex discussion of the subject.[4]

Our double anniversary is however somewhat confused by another piece of calendrical confusion. Both of our heliocentric astronomers were born before the calendar reform so their birthdates are given according to the Julian Calendar, this of course means that our double anniversary is not actually today but first on the 26^th February.

Just to throw another spanner into the works, both of the birthdates are taken from Renaissance horoscopes making them instantly suspect, why? It was a common practice in the Renaissance for astrologers to rectify horoscopes. This was the practice of adjusting times of birth by hours and even sometimes a whole day to make the resulting horoscope more harmonious with the real life of the horoscope’s subject. Rheticus for example was known to be an adherent of this practice.

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[1] Dennis Danielson, The First Copernican, Georg Joachim Rheticus and the Rise of the Copernican Revolution, Walker and co., New York, 2006

[2] David Wootton, Galileo Watcher of the Skies, Yale University Press, New Haven and London, 2010

[3] J. L. Heilbron, Galileo, Oxford University Press, Oxford, 2010

[4] David Wootton, Accuracy and Galileo: A Case Study in Quantification and the Scientific Revolution, The Journal of the Historical Society, Vol.10, 2010 pp. 43-55