Galileo di Vincenzo Bonaiuti de' Galilei (1564- 1642), most well-known as simply Galileo, perhaps needs no introduction. The Italian astronomer, physicist and engineer is often called the father of observational astronomy, modern physics, the scientific method, and modern science, and with good reason. Besides his various other scientific pursuits, such as the development of pendulums, throughout his career he pushed the boundaries on what we thought we knew about the universe, making countless discoveries about the nature of our solar system. Some of his many contributions in the field of astronomy include the movement of planets, the phases of Venus, discovering the four largest satellites of Jupiter, observation of Saturn's rings, and analysis of sunspots, as well as acting as a champion of Copernican heliocentrism, or the idea of Earth rotating daily and revolving around the sun back in a time when this wasn’t widely accepted, laying the building blocks of modern astronomy even as it got him in trouble with the Catholic Church. His scientific achievements and contributions are vast, reverberating through astronomy, physics and beyond to this day, which is why it might surprise many to hear that he was also very into astrology and horoscopes.
It may seem pretty strange that the father of modern astronomy and such a groundbreaking frontiersman of science as Galileo should be into horoscopes, but back in Galileo’s time it actually was not all that strange that mathematicians and astronomers should believe in astrology. For centuries astrology was a discipline intrinsically tied to the studies of mathematics and astronomy, being taught right alongside them at the finest Italian universities. Indeed, Galileo, like Ptolemy and Kepler before him, held a position known as a mathematicus, a term which had a threefold meaning as referring to mathematics, astronomy, and astrology. Indeed, at the time one of the reasons for studying the movement of planets relative to the stars to begin with was for the purpose of predicting their positions and pinpointing their relation to each other in order to make oracular predictions or fashion horoscopes. At the time it was not uncommon for medical students to be taught astrology in order to cast horoscopes to predict the onset of a disease and indicate the appropriate remedy, and this kind of medical astrology was not all that unusual for the era.
Galileo was caught up in this astrology trend as well, and indeed he seems to have spent much of his time studying the stars and planets to make horoscopes. He made extremely detailed horoscopes for his family, including his two daughters, as well as for friends and students of his, and was known for having tremendous talent in the art. He was supposedly so good, in fact, that he was soon casting horoscopes for influential people for money, and as word got out about his supposed talent for astrology, Galileo would get into trouble with the Church for the first time of many.
At the time, the Catholic Church was not against astrology per se, but rather against the idea of deterministic interpretations of astrological predictions, or basically the idea that the stars, planets and other celestial influences were able to determine the course of worldly events, something which they believed only God had the power to do. This fatalistic or deterministic approach to astrology was highly frowned upon by the Church, with such interpretations of horoscopes being seen as a grave offense. In 1604, Galileo got into hot water over this, when a servant of his, a Silvestro Pagnoni, went to the Church with accusations that he was telling the household that his predictions were certain. He told the church of one of these instances, saying, "a man who would live, he said, for another twenty years, and he maintained that his prediction was certain and would inevitably come to pass." Pagnoni also accused Galileo of never going to mass, and none of this was a good look for him in the eyes of the Inquisition.
It is unknown just how true these accusations were, but they were enough to get Galileo summoned by the Inquisition and brought before them on possible charges of fatalistic astrology, and therefore heresy. These would not have been light charges at the time, holding the very real possibility of a death sentence, but luckily Galileo had a few points in his favor. One was that Pagnoni, while still insisting that Galileo was taking an overly deterministic approach to his astrology, also admitted that he had never seen him engage in any actually heretical behavior or demonstrate an unbelief in God. Perhaps more persuasive for Galileo’s cause was that at the time he held the chair of mathematics at the prestigious Padua University, and the Church was hesitant to start any trouble with the university. Due to these reasons, the charges against Galileo were dropped, and were not passed to the Holy Office in Rome.
None of this seems to have deterred Galileo from his astrological pursuits. In 1610, he wrote what would become perhaps one of his best known works, Sidereus Nuncius, or “The Message of the Stars.” Amid all of the amazing scientific discoveries chronicled within its pages, such as the discovery of four new planets, the moons of Jupiter, and the new observation that the Milky Way was actually made up of tiny stars, he also found the time to go on about astrology. With his study of Jupiter, his approach to astrology had changed somewhat, with the planet and its moons, which he called the Medici sidera, after one of his patrons Cosimo de Medici, the Duke of Tuscany, holding a prominent and powerful role in his views towards horoscopes. In his view, Jupiter and its moons held a more pronounced influence over us with what he called “superior” causes, than other planets and stars, which had “inferior” causes. Galileo would write of Jupiter’s influence:
If, therefore, of the inferior causes, those which arouse boldness of heart are diametrically contrary to those which inspire intellectual speculation, it is also most reasonable that the superior causes (if indeed they operate on us) be utterly different from those on which courage and the speculative faculty depend; and if the stars do operate and influence principally by their light, perchance it might be possible with some probable conjecture to deduce courage and boldness of heart from very large and vehement stars, and acuteness and perspicacity of wit from the thinnest and almost invisible lights.
Jupiter and its moons became the cornerstone of his practice of astrology, the celestial objects that dominated all others, and his regular horoscope readings for Cosimo de Medici were heavily based on Jupiter’s influence. De Medici was apparently so impressed with Galileo’s predictions that he made him court mathematician and astrologer, and Galileo’s list of clients continued to grow, including royalty, aristocrats, and society’s elite. Not everyone was on his side, with some of his enemies trying to use his practice of astrology to get him in trouble, such as when his nemesis the Jesuits accused him of astrologically predicting the death of the Pope in 1630. However, although he would face more persecution from the Church due to his championing the idea that the Earth went around the sun, with Galileo forced to denounce his beliefs in a Copernican heliocentric universe on several occasions, they never again bothered him about his astrological pursuits.
Galileo practiced astrology all the way up until his death in 1642, and during his lifetime it was never seen as a particularly anomalous thing, or something that clashed with his status as a respected mathematician and astronomer. It would not be until the latter part of the 17th century after his death that skepticism in astrology would grow and it would slowly be pushed to the fringes of science and then out altogether. After that, modern historians did their best to gloss over Galileo’s involvement with astrology at best, and completely omit it at worst. For a long time, it was treated as sort of like his dirty little secret by those in the modern age of scientific rationalism who wanted to distance the great Galileo from the world of astrology and horoscopes, which was seen as more in the realm of the occult. In the end, it is a relatively obscure aspect to this great scientist and astronomer that many may not be aware of, and a curious historical oddity that shows that the line between science and mysticism was not always so clear-cut.