American writer Dan Brown released The Da Vinci Code in 2003. A mystery novel that tracks the secret surrounding the Holy Grail of Jesus through ciphers hidden in the works of Leonardo S.p.A., a Renaissance artist and scientist, it not only became a bestseller but was also made into a Hollywood film that gained great popularity.
A "Galileo code" containing clues to track the greatest event in the history of science, as if out of a novel, has been discovered. Galileo (1564–1642) is the figure who changed the geocentric cosmology that lasted 1,400 years to a heliocentric one. It has been confirmed that he left notes in the margins of a Greek astronomer's classic that contains geocentrism, recording his own thoughts. The Galileo code is expected to help concretize the intellectual debates of the cosmological transition period.
◇ Galileo's handwriting found in a 16th-century edition of Almagest
Ivan Malara, a postdoctoral researcher in the philosophy department at the University of Milan, said on the 18th that he "found marginal notes in a 1551 printed edition of Almagest at the National Central Library of Florence in Italy that are presumed to be in the handwriting of Galileo Galilei," in the Italian newspaper Il Sole 24 Ore.
Almagest is a work in which Claudius Ptolemy, a Greek astronomer of the 2nd century, systematized the geocentric theory (Ptolemaic theory). In this book, he explained that Earth is fixed at the center of the universe and that the sun, moon and planets revolve around it. The Ptolemaic theory, combined with the Christian worldview until Copernicus advocated heliocentrism, dominated philosophy and science for 1,400 years.
Malara said he found an awkward part while turning the pages of Almagest at the library in January. Someone had written notes in the margins, and, remarkably, the handwriting closely resembled Galileo's. Finding writing left by Galileo, who supported the heliocentric theory that Earth orbits the sun, in the most authoritative book for scholars on the other side was an enormous discovery in the history of science.
Malara, too excited by the great discovery to sleep, sent an email at 3 a.m. the next day to two Italian Galileo scholars. As Malara hoped, the reply said the notes were Galileo's. Michele Camerota, a professor in the philosophy department at the University of Cagliari in Italy who received the email that day, told the international journal Science, "I am completely convinced that the marginal notes are Galileo's."
◇ Raising issues with respect rather than political provocation
Galileo openly advocated Copernicus' heliocentric theory and was convicted in the papal Inquisition in 1633, spending nine years under house arrest. People have thought of Galileo as a reformer who stood up to a dogmatic Church. But based on the notes discovered this time, Malara suggested that Galileo's deep understanding of Ptolemy instead became the foundation for his cosmological revolt.
The Ptolemaic theory was systematized by Aristotle, the pinnacle of ancient Greek scholarship. He argued that with spherical Earth at the center of the universe, the sun and other celestial bodies each move in circular motion—the most perfect motion by nature—on transparent rotating spheres. However, the geocentric theory could not explain the "retrograde motion," in which planets appear to temporarily move backward toward Earth.
Ptolemy recognized this problem through astronomical observations and found a solution. He explained it with a complex theory that planets rotate while drawing small circles (epicycles) as they revolve around large circles (deferents) near Earth. After mastering Ptolemy's explanation, Galileo solved this problem with heliocentrism. A planet's retrograde motion is an apparent motion that occurs when Earth, orbiting the sun at a faster speed, overtakes that planet—an optical illusion. It is like how a slower car seems to move backward when a faster car passes it on a highway.
The Galileo code proves this. The marginal notes in book 1, chapter 3 of Almagest used the same expressions as in On Motion, which Galileo released in 1590. The way proper nouns were written and underlined was also the same as Galileo's. Rather than belittling the other side as ignorant, he carefully explained his thoughts after mastering their claims. It is the very image of a courteous junior scholar.
◇ A devout believer also revealed in the notes
The notes discovered this time provide clues to how Galileo moved away from Ptolemy's astronomy after seeing On the Revolutions of the Heavenly Spheres, which Copernicus released in 1543. It can be seen that instead of immediately switching to heliocentrism, he chose a method of pointing out problems one by one based on Ptolemy's geocentric theory.
Galileo later found decisive evidence supporting heliocentrism in observations of Venus. According to Ptolemy's geocentric theory, Venus should appear only as a crescent, but actual observations showed it periodically changing into crescent, half and full phases, and its apparent size also varied with shape. This was a phenomenon possible only if Venus, like Earth, orbited the sun.
Clues also emerged showing Galileo's religious perspective. Interestingly, Galileo also left Psalm 145 from the Old Testament in the margins of Almagest. This means that, contrary to today's fixed idea that Galileo ignored religious authority, he maintained devout faith even in scientific research.
In fact, Malara found in another 16th-century printed edition of Almagest a note by an anonymous author claiming that "before studying Ptolemy, Galileo prayed to God." Mathematician Alessandro Marchetti also wrote in a 1673 letter that Galileo prayed every time he opened Almagest. Malara plans to publish the results of his analysis of the Galileo code soon in the international journal Journal for the History of Astronomy.
References
Il Sole 24 ORE (2026), https://www.ilsole24ore.com/art/galileo-appunti-cambiare-cosmo-AI6NJkTB
Science (2026), DOI: https://doi.org/10.1126/science.zrozy54