Galileo's Horoscopes

Originally Parte VI, Tomo 1, fasc. 1 of the Galileo manuscripts, called “Astrologica nonulla”; partial contents are listed by Antonio Favaro in Le opere di Galileo Galilei (Florence, 1890–1909; henceforth Opere), xix, 205–6. The manuscript was brought to my attention by Darrel Rutkin , who kindly provided me with a photocopy, and later Paolo Galluzzi sent me very fine digital images on CD-ROM. I am very grateful to both and also to Nicholas Kollerstrom for his comments on an earlier version of this note and in particular for catching my persistent error in the computed longitude of Mercury. Of the literature on the horoscopes, I have seen Ernst G. , “Aspetti dell'astrologia e della profezia in Galileo e Campanella”, Novità celesti e crisi del sapere , ed. by Galluzzi P. (Florence, 1984), 255–66, which has references to earlier literature; Kollerstrom N. , “Galileo's astrology”, Largo campo di filosofare, Eurosymposium Galileo 2001 , ed. by Montesinos J. Solís C. (Puerto de la Cruz, 2001), 421–31. Galileo's baptismal record is reproduced in Opere , xix, 25.

Ephemerides are computed for mean time, and thus Galileo's interpolation for 3;30^h is for a mean time, not the apparent time of the horoscope. For the sun at Pisces 7°, the equation of time for converting apparent to mean time is +0;7^h or +0;4° in the motion of the moon, that is, the mean time corresponding to the apparent time of 3:30 p.m. is 3:37 p.m. and the longitude of the moon should be increased, not decreased, by +0;4° from Taurus 4;52° to Taurus 4;56°. Errors of this kind, and omissions of the equation of time, which is required only for the moon, are very common in horoscopes and other computations. The standard table of the equation of time in ephemerides of the later sixteenth century is from the Prutenic tables..

By interpolation for 4^h after noon, the longitude of the moon would be Taurus 5;11°, but since 4 p.m. apparent time corresponds to 4:07 p.m. mean time, the longitude is correctly Taurus 5;15°. (I assume it is a coincidence that the exact apparent time of the horoscope is 4:08 p.m.) The moon at Taurus 4;40° would be correct at about 3:10 p.m. mean time, which corresponds to an apparent time of 3:03 p.m. I go through these details only because they are confusing; they are not important.

Darrel Rutkin has informed me that he has also found that the longitudes in the horoscopes are from Stadius's Ephemerides; I claim no priority for this identification, which I willingly grant to anyone who wants it. Galileo makes no adjustment for the meridian of Pisa, correctly about 6° = 0;24^h east of Antwerp, which can affect the longitude of the moon by about −0;13°. Stadius gives Florence as 0;32^h east of Antwerp, but this does not explain the difference of 0;30^h in the times of the horoscopes since these are changes of apparent local time, not a change of meridian. In La libreria di Galileo Galilei (Rome, 1887), 39, Antonio Favaro lists among Galileo's books ephemerides by Carelli Battista Giovanni for 1563–80 (Venice, 1563) for the meridian of Venice, which can be ruled out for these horoscopes.