Historians have greatly ignored the practice, by Galileo, of astrology. The recent Cambridge Companion to Galileo for example makes no mention of the matter. It was claimed by Eugenio Garin that he only practiced the art in his youth. This paper will argue that the historical record does not support these views. Antonio Favaro, before becoming editor-in-chief of the massive twenty-volume Opera of Galileo (1890-1909), had earlier in 1881 composed his essay, Galileo Astrologo. This concluded:

"It seems to me impossible to have the slightest doubt that Galileo was involved with astrology, indeed, that he was famous for his great ability in that art, so that distinguished people consulted him with complete confidence, in many cases asking for horoscopes and predictions."

There is some exaggeration here, as we have evidence for Galileo being unsure over aspects of chart interpretations, where he would seek advice from colleagues, but fundamentally our inquiry supports this view. The letters by Galileo to these persons have been lost and we have only the replies, as likewise the most famous charts composed by him have lost. However some twenty-five charts drawn up by him do remain, plus four or five instances of his chart analyses. The book from which he learnt his astrology while at Pisa may have been "Porphir’s Introductio in Ptolemaei opus de effectibus astrorum…" of which a copy annotated in his hand remains in Florence.