Last updated July 24, 2025

The Secrets Exchanged by Galileo and Kepler Were Coded

When science hides behind encryption

During the Renaissance, in a Europe still strongly dominated by religious dogmas, expressing ideas contrary to the Aristotelian worldview could lead to censorship or even condemnation. To circumvent this constraint, some scholars, such as Galileo and Kepler, resorted to an intellectual ruse: coded messages. These cryptograms, inserted in their letters or publications, were used to claim authorship of a discovery while postponing its disclosure, in order to secure their position or obtain experimental confirmation.

Examples of coded messages between Galileo and Kepler

Galileo's coded message about Saturn

In July 1610, Galileo observed Saturn through a telescope and noticed a strange shape. He sent Kepler an apparently incomprehensible message: “smaismrmilmepoetaleumibunenugttauiras”. This was actually a Latin anagram, which Galileo later revealed to mean: “Altissimum planetam tergeminum observavi” — “I have observed that the highest planet (Saturn) is triple.” This message was intended to guarantee the priority of his discovery without revealing its content immediately, for fear of appropriation or controversy.

Clearly, Galileo did not want Kepler to decipher the message immediately: he just wanted to be able to prove later that he was the first to know that Saturn had a strange shape.

A rare but significant epistolary exchange

The correspondence between Galileo (1554-1642) and Kepler (1571-1630) was surprisingly brief. Only a few letters exchanged between 1597 and 1600 are known. Kepler, then a young astronomer in Prague, wrote to Galileo to encourage him to publish his Copernican views. In his first letter of 1597, Kepler thanked Galileo for supporting the heliocentric vision and sent him his own essay, Mysterium Cosmographicum, published a year earlier.

Galileo, although touched, remained cautious. He replied to Kepler with a letter in which he confessed his adherence to the Copernican system but expressed his fear of persecution. He wrote to him: “I have not yet dared to publicly proclaim my belief that the Earth moves, for fear of suffering the same fate as Copernicus: mocked by the ignorant.”

In this letter, Galileo did not use cryptographic code, but used a measured tone to implicitly admit his adherence to the heliocentric system, while expressing his fear of being ridiculed or persecuted, as Copernicus had been, whose ideas were largely ignored or ridiculed by ecclesiastical and academic authorities.

Kepler, the enthusiast who read between the lines

Johannes Kepler, imperial mathematician and ardent defender of the heliocentric model, was a fervent admirer of Galileo. In their correspondence, he tried to decipher his codes and guess their meaning, sometimes with famous misunderstandings. Kepler himself sometimes used veiled writing to express ideas still considered heretical, as in his Cosmic Mysteries, where sacred geometry hid astronomical intuitions.

Why encode a scientific message?

The use of coding was not limited to a simple literary maneuver, but responded to strict constraints of a scientific, social, and theological nature. At a time when experimentation alone did not confer immediate legitimacy on a theory, the priority of discovery was a crucial element of scientific authority. The use of anagrams or coded messages, which could only be deciphered later, thus offered the author a formal means of claiming authorship of the idea before its public dissemination, ensuring recognition of its priority in the face of contemporaries and institutions.

Examples of coded messages between Galileo and Kepler

Coded and allusive messages in the Galileo-Kepler correspondence
Author	Message	Form of the code	Meaning / Objective
Galileo	smaismrmilmepoetaleumibunenugttauiras	Latin anagram	“Altissimum planetam tergeminum observavi” = Saturn is triple. Claim of priority without revelation.
Galileo	Haec immatura a me iam frustra leguntur o.y.	Latin anagram	Galileo sends a coded message to the astronomer Johannes Wesler. This sequence is an anagram of: Cynthiae figuras aemulatur mater amorum (“The mother of loves imitates the phases of Cynthia”). Cynthia = the Moon, mother of loves = Venus. ⇒ Allusion to the discovery of the phases of Venus, indirect proof that it revolves around the Sun, hence strong support for the Copernican model.
Kepler	Letter to Galileo (April 1611): “These moons of Jupiter revolve, as does the Earth itself.”	Allusive message	Use of Jupiter's satellites as a natural model of the Copernican system, without naming its core. At the beginning of 1610, Galileo discovered the satellites of Jupiter (Io, Europa, Ganymede, and Callisto) thanks to his improved astronomical telescope. He observed that these stars revolve around Jupiter—a major revolution, as it provided evidence of a celestial center of rotation other than the Earth.
Galileo	Use of the term “System of the World” instead of “Copernican”	Lexical avoidance	Circumvents the prohibition of the term “heliocentrism” by calling it something else in his Dialogues (1632). Before Copernicus: The “system of the world” was that of Ptolemy (geocentrism), based on celestial spheres, epicycles, and an immobile Earth at the center. After Copernicus: The “system of the world” potentially became heliocentric, but the expression continued to be used without necessarily specifying its author, especially to avoid the accusation of heresy.
Kepler	Preface of Harmonices Mundi (1619): praises of celestial music	Pythagorean symbolism	In the preface to his major work Harmonices Mundi (The Harmony of the World), Johannes Kepler enthusiastically celebrates celestial music, the idea that the movements of celestial bodies obey harmonic ratios analogous to those of music. Kepler expresses his deep Platonic conviction that the universe is constructed according to an intrinsic mathematical order, which can be translated into terms of proportion, consonance, and harmony. Kepler circumvents the prohibition of the term “heliocentrism” by calling it something else in his Dialogues (1632). Before Copernicus: The “system of the world” was that of Ptolemy (geocentrism), based on celestial spheres, epicycles, and an immobile Earth at the center. After Copernicus: The “system of the world” potentially became heliocentric, but the expression continued to be used without necessarily specifying its author, especially to avoid the accusation of heresy.
Galileo	Dialogue between Salviati, Sagredo, and Simplicio (1632)	Philosophical fictional dialogue	“Expresses Copernican ideas through a fictional character to avoid a direct attack on the authorities” — which can be linked to Galileo's rhetorical and political strategy in his writings, particularly in the Dialogue Concerning the Two Chief World Systems (1632).
Galileo	“I have not yet dared to publicly proclaim my belief that the Earth moves…”	Prudent and diplomatic language	In a letter addressed to Johannes Kepler, dated August 4, 1597, Galileo hints at his adherence to the heliocentric system in a veiled and cautious manner, revealing the tense intellectual climate of the late 16th century. This correspondence constitutes the first explicit testimony of his Copernican inclination, although expressed in a euphemistic and half-hearted manner.
Kepler	Use of Platonic solids in Mysterium Cosmographicum	Geometric-mystical coding	Metaphor to justify heliocentrism. In his foundational work Mysterium Cosmographicum (1596), Johannes Kepler develops a deeply geometric vision of the universe. He expounds his conviction that the order of the cosmos is inscribed in the mathematical structure of the five Platonic solids, or regular polyhedra. For Kepler, these perfect forms are not the result of chance but of divine intention, translating the profound harmony of Creation.
Galileo	Secret letter to Castelli on Scripture and astronomy (1613)	Indirect theological argumentation	Faced with the theological resistance aroused by the Copernican system, Kepler adopted an approach of reconciliation between the new science and the Christian tradition. Rather than directly opposing heliocentrism to the Scriptures, he endeavored to propose an enlightened and non-literal reading, faithful to the spirit and not to the letter of the biblical text.
Kepler	Letter to Galileo (1605) on the satellites of Jupiter	Tone coded by controlled enthusiasm	In a context where Copernican ideas were increasingly attacked by ecclesiastical and academic authorities, Kepler played a role of intellectual and moral support for Galileo, while remaining cautious in his formulations. Through his allusive and coded style, he adopted a diplomatic posture that allowed him to encourage heliocentrism without ever stating it outright.

References: Stanford Encyclopedia of Philosophy – Galileo, Kepler NASA, Galileo Project – Rice University, Letter to Kepler (August 4, 1597).

Riddles and communication: a secret language to spread knowledge

Even today, these messages remain fascinating symbols of the prudence of the pioneers. They testify not only to their scientific intelligence but also to their political skill in navigating a world where truth was not yet free. The art of coding, far from being an artifice, was a tool to protect innovation in an era hostile to novelty.