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The Wikipedia page on Gallileo Galilei mentions, among other things:

His multiple interests included the study of astrology, which at the time was a discipline tied to the studies of mathematics and astronomy.

My question is - did he actually believe that astrology worked? Is there any evidence on his attitude towards astrology?

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    $\begingroup$ A comment, because I have no source. When Gallileo discovered the moons of Jupiter, the question arose: Should these new heavenly bodies be used in horoscopes? The conclusion was: No, they are so close in the sky to Jupiter that we can safely lump together their effects on us here with the effects of Jupiter. $\endgroup$ – Gerald Edgar Apr 13 at 11:35
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    $\begingroup$ Maybe "believe" is misleading today... At that time, astrology was a mathematical discipline and Galileo In 1589 was appointed to the chair of mathematics in Pisa. In 1592, he moved to the University of Padua where he taught geometry, mechanics, and astronomy until 1610. $\endgroup$ – Mauro ALLEGRANZA Apr 13 at 14:35
  • $\begingroup$ Related article on medieval weather prediction and its connection to astrology in Physics Today (the magazine of the American Physical Society): physicstoday.scitation.org/doi/10.1063/PT.3.4724 $\endgroup$ – 2ndQuantized Apr 14 at 16:22
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    $\begingroup$ Just wait 500 years... "They believed WHAT in the 21st century?!" $\endgroup$ – corsiKa Apr 15 at 0:24
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Galileo not only believed but taught and actively practiced astrology, like Ptolemy and Kepler before him. His primary source might have been Porphyry’s commentary on Ptolemy's Tetrabiblos. In particular, he constructed and interpreted horoscopes, including ones for himself and his daughters that are extant. His discovery of the moons of Jupiter was followed by making a 1610 horoscope of Cosimo de Medici, where Jupiter is prominently featured. Astrology was taken as a matter of course at the time, so believing it carried no more weight than believing that the Sun goes around the Earth by someone never forced to confront contrary evidence or consider alternatives, and going about their everyday business on that assumption.

The subject was intertwined with what we now call astronomy, and taught at top universities by mathematici like Galileo. Ironically, the Church was on the opposite side on this one, as it opposed "fatalistic" interpretations of astrological predictions. Galileo was investigated for practicing "fatalistic astrology" by the Inquisition in 1604, but the charges were dropped. The seeds of modern skepticism about astrology only emerged in the later part of 17th century, after Galileo's death.

Galileo's practice was long overlooked and/or misinterpreted by earlier historians, which is also reflected in the more literary works like Brecht's play Galileo and Koestler's Sleepwalkers, which anticipated some themes from Kuhn's Scientific Revolutions. For a comprehensive review of primary sources and commentary see Galileo’s Astrology volume (2003). Here is from Rutkin's review Galileo as Practising Astrologer of Ernst's critical edition (2017) of Galileo's astrological manuscript no.81, that was only recently made available to scholars in full:

"It contains horoscopes for 19 people (some with two slightly different figures) in Galileo’s own hand and was gathered together into one manuscript from originally separated folia. Although it has been known for over a century (since 1881 when Antonio Favaro wrote about it in a very brief article) and although its attribution has never been seriously challenged, MS Gal. 81 and the essentially normal astrological practices that it reveals have not yet been taken fully into account in the literature seeking to understand and assess Galileo’s life and works, and it has never been published in its entirety...

While teaching at the University of Padua, Galileo practised astrology by constructing and interpreting horoscopes for a range of patrons, students and family members, seeking, among other things, to raise extra money as we can see in his contemporary account books for payments made to him “per sortem.” This evidence dates some of these horoscopes to 1603, that is, soon after the death of one of his early patrons, Gian Vincenzo Pinelli (1535–1601)...

We possess other evidence for Galileo’s astrological practice around this time, including his first investigation by the Venetian Inquisition in 1604, following the official denunciation made by his former amenuensis, Silvestro Pagnoni from Pesaro, who had accused Galileo of practising a deterministic astrology. We know about this revealing incident in some detail from Antonino Poppi’s valuable researches, including that the case was dropped because Galileo was practising normal, not deterministic astrology. As we now know without any doubt, throughout the 16th and well into the 17th century astrology was still taught at the finest Italian universities, including at both Padua and Bologna, and was a normal part of the study, practice and teaching of an early modern “mathematicus.”...

In addition to setting out all of the astronomical and astrological data as well as the numerical calculations used in MS 81 for erecting and interpreting these horoscopes, perhaps the most valuable feature of Ernst’s edition is the transcription of Galileo’s astrological interpretations, which were all written in Latin in the five horoscopes that include them, some of which are very difficult to read in the manuscript. These interpretations are very interesting, in that we can thereby catch a glimpse of Galileo doing the normal work of both casting and interpreting horoscopes... The construction of his own horoscope (in two variant versions, but without an interpretation) and those for his two daughters (with their interpretations), as well as for other family members, belie assertions claiming that Galileo himself personally rejected astrology or that he only used it cynically for patronage purposes."

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    $\begingroup$ I've read Brechts play on Galileo, I don't recall there being any reference to astrology in it. $\endgroup$ – Mozibur Ullah Apr 15 at 0:34
  • $\begingroup$ Brecht followed historians, it was thought as "embarrassing" to cover the issue. $\endgroup$ – Conifold Apr 15 at 12:21
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    $\begingroup$ Brecht took the view of dialectical materialism and was more interested as taking Galileo as the spirit of a new type of thinking in the world - robust, energetic and revolutionary - and this is why he described Galileo as such. The point is, you're saying "Galileo's practise [of astrology] was long overlooked ... and reflected in more literary works like Brechts play Galileo". Like I said, there is no description of astrology in Brecht, it's not 'reflected' there. You might want to edit your answer. $\endgroup$ – Mozibur Ullah Apr 15 at 14:06
  • $\begingroup$ Overlooking usually is reflected by absence of looking. $\endgroup$ – Conifold Apr 15 at 23:17
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    $\begingroup$ You're being cryptic to no end. Brecht and Galileo were not known for being cryptic. This is why Brecht used Galileo as his foil for the modern outlook on the world and on life - to be clear and clarifying. $\endgroup$ – Mozibur Ullah Apr 16 at 6:32
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The Galileo's astrology activities are well summarized in Conifold's answer. But, in relevance to the question at hand (did he believe it worked) I'd like to make a couple of points:

  • The fact that Galileo was making horoscopes of people of power and wealth doesn't really mean he believed that they actually meant anything. He could have been motivated by money and whatever political favors he could have gained a the time.

  • Making a horoscope of oneself and the people you know best is the way a skeptical person would go about if he wanted to check the validity of astrology.

  • Galileo's views on astrology could have evolved with time.

As the evidence for Galileo's later disdain for astrology I have two things of notice.

  1. One is the quote from Dialogo sopra i due massimi sistemi del mondo

Ma tra tutti gli uomini grandi che sopra tal mirabile effetto di natura hanno filosofato, piú mi meraviglio del Keplero che di altri, il quale, d’ingegno libero ed acuto, e che aveva in mano i moti attribuiti alla Terra, abbia poi dato orecchio ed assenso a predominii della Luna sopra l’acqua, ed a proprietà occulte, e simili fanciullezze.

Galileo essentially states that taking about "occult properties" and actions of planets (the Moon in this case) - "fanciullezze" - a childish and unserious endeavors.

  1. Second is the letter from Ascanio Piccolomini, in whose house Galileo was hosted after his arrest in 1633. Claim is that Ascanio stated that Galileo derided astrology and made fun of it as a "profession founded on the most uncertain, if not false, foundations". I can only provide this source for that quote although I'd love to see the precise text of the letter someday...
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    $\begingroup$ One more point: Galileo cast horoscopes for his daughters (I read somewhere, checkable)... $\endgroup$ – sand1 Apr 14 at 19:42
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    $\begingroup$ The "occult properties" of the moon that Galileo is referring to in that passage, is Kepler's bizarre belief that the moon influenced the tides... :) $\endgroup$ – seumasmac Apr 16 at 7:26
  • $\begingroup$ @seumasmac Agree, but that'd still mean that Galileo didn't believe that celestial bodies affect life on Earth. $\endgroup$ – Tziolkovski May 7 at 17:44
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Rutkin D., Galileo Astrologer, Galileana II 2005 p 107-143

"Favaro (in vol.19 of the Ed.N) transcribed and published the astrological judgments written in Latin of Galileo's two daughters which along with the construction of his own horoscope undercuts the argument that Galileo only practiced astrology toward patronage ends. In addition to those of his family and himself there are 20 additional horoscopes.." P.117

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Nick Kollerstrom in his online Galileo's Astrology points out:

In 1609 Galileo moved to Florence. In 1610, his revolutionary best-seller, Sidereus Nuncius (The Message of the Stars), appearing in March 1610, opened with an eloquent account of the traditional qualities assigned to Jupiter and he states:

"Who, I say, does not know that these qualities, according to the providence of God, from whom all good things come, emanate from the benign star of Jupiter?"

This shows that he believed in astrology as he believed in the Christianity of his time which also put credence in astrology. The roots of this is intermingled with the emanationist philosophy of Plato.

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What we call now astronomy and what we call astrology have been one and the same thing for millennia, and it was so at Galileo's time. The reason for studying the motion of planets relative to the stars was to predict their position for oracular responses, or to pinpoint their past position at birth's time for compiling horoscopes.

The fact that (as stated in other responses) Galileo made horoscopes for his own family members is proof enough that he attributed at least some value to them.

The fact that (Tziolkovski's answer) he was skeptical about some specific topics does not necessarily mean that he rejected astrology. I'd say that is is analogous to being skeptical today about string theory, which does not mean that you don't believe in modern physics.

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    $\begingroup$ Astronomy and astrology are two different words that have remained in use ever since ancient Greeks started using them. This was enough for scholastics (e.g. Isidore of Seville) to note a difference. Ptolemy has kept separated astronomy in the Almagest from astrology in the Tetrabiblos. Copernic, as Westman explained, intended to follow this model but did not live long enough to compose his own specifically astrological treatise. Kepler already used astronomy and astrology with our contemporary sense. $\endgroup$ – sand1 Apr 15 at 8:52
  • $\begingroup$ Thank you. I was clearly wrong. Maybe I should remove my answer altogether? $\endgroup$ – Francesco Potortì Apr 15 at 10:14
  • $\begingroup$ +1: I think it's fine, there is another tradition that links them as I noted in my answer. The harmony of the spheres comes from a harmony between astronomy and astrology. $\endgroup$ – Mozibur Ullah Apr 15 at 14:10

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