I find I do not have a thread on Galileo anywhere on the forum. I want to correct that oversight.
I think it is because the data is formally rated C. It seemed well-documented once upon a time: Arthur Blackwell (American Astrology 7/70) recovered February 16, 1564 OS, 3:00 PM LAT (3:14 PM LMT) from a biography. Galileo evidently rectified his own chart to a later time.
Astro.com now thinks it is less sure. To review the current history of this on Astro.com:
One analyst thinks these charts were for round half-hours of 3:30 PM and 4:00 PM LAT which, allowing for small errors (known to have crept into Galileo's charts), makes much sense. Large copies of Galileo's charts for himself are here: http://www.skyscript.co.uk/galchart.html - Note that they were on the same sheet of paper, suggesting he was unsure of his time of birth when he calculated these (or was trying different times as part of a rectification).Nick Kollerstrom photocopied two charts from the Florence library..., accepted as authentically drawn by Galileo himself. The bottom chart shows an ascendant of [Tropical] 14°33 Leo, which corresponds to 15:41 LMT or 15:27 LAT. The top chart has an ascendant of 21°37 Leo, which corresponds to 16.18 LMT or 16.04 LAT. The date of birth is written as 15 February and most sources give the same. Blackwell gives 16 February.
Conventional scholars give February 15. The gap between this and Blackwell's February 16 seems resolved by notes at the top of the chart. He first writes "1564, February 15, h 22.30," which, in Florentine time, is confusing: It meant 22:30 after Florentine sunset on what we today call February 14 (?!). Immediately under it, though, Galileo wrote, "16 February, h 4 PM h 3.30 pm," following the French post meridian nomenclature and seeming to be 4:00 PM and 3:30 PM LAT on February 16. The reference to 15 February, therefore, seems to be Galileo's error (casually meaning "after sunset on the 15th" even though the timing meant it was the 16th. It looks like he tried to correct this on the paper.
BTW. one source says this piece of paper is the ONLY source of historical information on Galileo's birth date, and it seems it's been interpreted in error.
I think, therefore, from the only documentary evidence available, that we can conclude that (in modern terms) his birth date was February 16, 1564 in Pisa, Italy. One biography Blackwell found gave 3:00 PM, but existing charts in Galileo's hand are for 3:30 PM and 4:00 PM. I suggest we take 3:30 PM either as a mean of these or as Galileo's own starting point. The chart only deserves a C rating if it is a rectification, though it would deserve an A rating if we regard this as from a primary human source (Galileo himself); I shall rate it as such for myself.
