In the winter of 1615, Johannes Kepler received a letter from his sister informing him that their mother had been accused of witchcraft.
Katharina Kepler was sixty-eight years old, a herbalist and healer in the small Württemberg town of Leonberg, and someone who had made enough enemies over the years that when the local administrator decided to pursue a witchcraft accusation, there were witnesses willing to testify. The charges were serious — sorcery, causing illness in neighbors, bewitching the local barber-surgeon’s daughter — and the punishment for a convicted witch in seventeenth-century Württemberg was death.
Kepler dropped everything and went home.
What followed was six years of legal proceedings in which Kepler, one of the most prominent natural philosophers in Europe, applied himself to defending his mother with the same methodical thoroughness he had applied to the problem of Mars’s orbit. He interviewed witnesses, identified inconsistencies in testimony, prepared legal arguments, and ultimately appeared in person at the proceedings. In 1621, Katharina Kepler was released without conviction after fourteen months of imprisonment. She died six months later.
During those six years, Kepler continued his scientific work — publishing Harmonices Mundi in 1619, which contained his third law of planetary motion, while his mother was in legal jeopardy. He also continued casting horoscopes.
This is the Kepler that the popular history of science prefers to elide: not only the discoverer of the laws of planetary motion but a man who practiced astrology throughout his career, who believed in a universe animated by geometric harmony and spiritual force, and who never fully separated the celestial mechanics he was developing from the astrological framework within which those mechanics were embedded.
The Career of a Court Astrologer
Kepler spent his life in financial precarity. The positions he held — as a mathematics teacher in Graz, as Imperial Mathematician in Prague under Rudolf II, later under Matthias and Ferdinand II — paid irregularly and rarely in full. His primary instrument of supplementary income was astrology.
He cast horoscopes for clients ranging from wealthy merchants to the Emperor himself. He published annual almanacs containing astrological predictions that sold well and for which he was paid. He calculated astrologically significant dates for patrons who needed to know when to begin ventures, when to travel, when to expect danger.
He was, throughout his career, ambivalent about this work in ways he documented extensively. He wrote that astrology was “the foolish daughter of a wise mother” — astronomy being the wise mother — and that most of the specific claims astrologers made were unfounded or contradictory. He was particularly contemptuous of sun-sign astrology, of the crude assignment of personality traits to zodiacal divisions, and of the mechanical application of astrological rules without physical reasoning to support them.
But he did not conclude from this critique that astrology was simply false. He concluded that it was partly false — that many of its specific claims were unfounded — and partly pointing at something real that a properly reformed astrology could better capture. His Tertius Interveniens (1610), a response to a physician who had attacked astrology and a theologian who had defended it, is a nuanced document arguing that both sides were wrong: the physician was wrong to dismiss all celestial influence on earthly life, and the theologian was wrong to accept the specific claims of popular astrology without critical examination.
Kepler’s position was that the heavens genuinely influenced the Earth — this was, after all, the conclusion of his physical reasoning about the solar system — and that understanding how required better physics, not the abandonment of the question. The specific mechanism he proposed, drawing on his concept of a “moving soul” (anima motrix) in the Sun that drove planetary motion, was not ultimately correct. But the instinct that led him to it — that the relationship between celestial and terrestrial phenomena was a genuinely physical question, not merely a symbolic or divinatory one — pointed toward the gravitational theory that Newton would develop half a century later.
Tycho, Kepler, and the Data That Changed Everything
The intellectual lineage that produced Kepler’s laws of planetary motion runs through one of the stranger relationships in the history of science: his collaboration with Tycho Brahe.
Tycho Brahe was the greatest naked-eye astronomical observer in history. Working from his observatory at Uraniborg on the island of Hven, later from Prague, he accumulated twenty years of planetary observations with a precision that no one before him had approached. His measurements of Mars’s position over time were accurate to within two arc-minutes — an extraordinary achievement before the telescope.
Tycho was also an astrologer. Not ambivalently, like Kepler, but with genuine conviction. He maintained elaborate astrological predictions, cast horoscopes for the Danish royal family, and structured his astronomical research partly around the production of better astrological data. The need for accurate planetary positions was, for Tycho, inseparable from the need for accurate astrological predictions — the two projects motivated each other.
When Tycho died in 1601, he left his data to Kepler, with whom he had had an extraordinarily tense and productive eighteen-month collaboration in Prague. Kepler spent the next decade wrestling with that data, particularly the Mars observations, trying to find the orbital geometry that fit it. The result — announced in Astronomia Nova in 1609 — was that the planets moved in ellipses, not circles, with the Sun at one focus, and that a line from the Sun to a planet swept equal areas in equal times.
The data that produced these laws of planetary motion — laws that Newton would eventually explain through universal gravitation, and that underlie every calculation of rocket trajectories and satellite orbits in the modern world — was collected by a man who needed accurate planetary positions to cast better horoscopes.
The history of astronomy is entangled with the history of astrology in ways that the standard narrative of scientific progress prefers to ignore. The telescopes, the observations, the mathematical models — much of the apparatus that produced the revolution in our understanding of the cosmos was developed partly in service of a divinatory practice that the subsequent revolution declared obsolete. The parent fed the child that eventually replaced it.
What Kepler Actually Believed
Kepler’s actual cosmological beliefs were stranger and more interesting than either the scientific hagiography or the occult appropriation of him typically acknowledges.
He was a committed Christian who believed that the mathematical structure of the cosmos reflected the rational order of God’s creation — that the geometric harmonies he was discovering were not merely computational conveniences but the actual language in which the universe was written. His Harmonices Mundi — the work containing his third law, but also the work he considered his greatest achievement — argued that the planetary orbits produced a kind of celestial music, with each planet assigned a range of musical tones corresponding to its orbital velocity. This was not metaphor. He believed it was literal.
He also believed that the soul of the Earth (anima telluris) was a genuine entity — not a metaphor for natural processes, but an actual animating intelligence that responded to the angles formed between the planets as they appeared from Earth. This was his explanation for why astrological aspects — the angles of 0°, 60°, 90°, 120°, and 180° — had effects: the Earth’s soul perceived them as a kind of sensory input.
Whether this constitutes proto-physics (a genuine attempt to explain celestial-terrestrial interaction through a physical mechanism) or proto-vitalism (an animistic framework dressed in geometric language) has been debated. Probably both. Kepler’s thought was sufficiently hybrid — genuinely rigorous in its mathematics, genuinely animist in its cosmology — that it resists easy categorization.
What it was not was simple rationalism. The man who wrote the first modern work of astronomical physics was not a proto-materialist who happened to make astrological predictions reluctantly for the money. He was a thinker who genuinely believed in a cosmos animated by mathematical-spiritual forces, and who was simultaneously developing the mathematical tools that would eventually undermine that cosmological framework.
The Witchcraft Trial and What It Reveals
The defense of his mother against witchcraft accusations is one of the most humanly revealing episodes in Kepler’s life — and one that complicates the easy narrative of a scientific rationalist surrounded by superstitious contemporaries.
The accusation against Katharina Kepler arose partly from social conflict — she had made enemies in a small town — and partly from the specific anxieties of early seventeenth-century Germany, where the witch trials were at their peak and where the categories of natural and supernatural were genuinely uncertain in ways that even educated people didn’t fully resolve. Kepler’s sister Margaretha, who reported the situation to him, believed the accusations were malicious but was frightened by them. Some of the witnesses who testified against Katharina appear to have believed what they were saying.
Kepler’s defense was entirely rationalist: he systematically dismantled the witnesses’ testimony on legal and evidentiary grounds, identified inconsistencies, documented the accusers’ motives. He did not argue that witchcraft was impossible. He argued that this specific accusation was not proven.
This is not the response of a man who has cleanly separated himself from the worldview within which the accusation made sense. It is the response of a man who is operating in that worldview with more methodological rigor than most of his contemporaries — applying the standards of evidence he had learned from astronomical observation to a legal question in which those standards were not typically applied.
The same instinct — more rigor, better evidence, careful reasoning from observation — is what produced both Kepler’s laws of planetary motion and his mother’s legal defense. It was not an instinct toward demystification. It was an instinct toward precision about what the evidence actually showed.
The Separation That Wasn’t Clean
The standard history of science tells a story of clean separation: astronomy and astrology were once confused, scientists gradually distinguished them, and the separation produced modern astronomy while leaving astrology behind as a discarded relic.
This story is wrong in its timeline and misleading in its characterization. The separation took centuries and was never fully achieved by the people who were doing the work. Newton was deeply interested in alchemy. Kepler cast horoscopes and believed in the Earth’s soul. Tycho Brahe built his observatory partly to produce better astrological data. Galileo calculated horoscopes for clients throughout his career.
What happened was not that scientists decided astrology was false and moved on. What happened was that the new physics — Newtonian mechanics, universal gravitation, a universe governed by mathematical laws without animating souls — made the specific mechanism that astrology had always assumed (the direct influence of celestial bodies on terrestrial life through some non-gravitational, possibly spiritual channel) increasingly hard to locate in the physical picture that was emerging. The question of how the heavens influenced the Earth, which Kepler had tried to answer through his concept of aspects perceived by the Earth’s soul, became harder and harder to answer as the Earth’s soul stopped appearing anywhere in the equations.
But the question itself — whether and how celestial patterns relate to terrestrial events — was never definitively answered. It was abandoned, not refuted. The evidence that it was unanswerable was primarily the failure to find a plausible mechanism, not the demonstration that the correlations were absent. Given what we now know about the difficulty of demonstrating the absence of correlations, and given what we know about the history of mechanisms being found for phenomena that had previously seemed inexplicable, the abandonment was reasonable but not final.
Kepler, who was more honest about what he knew and didn’t know than most of the scientists who came after him, never claimed it was final. He died in 1630 on his way to collect an overdue salary payment, leaving behind the mathematical laws that would transform humanity’s relationship to the cosmos, a collection of unpublished manuscripts including a fantasy of space travel, a legal record documenting his mother’s defense, and the horoscope he had cast for himself at birth.
The horoscope he kept. The laws he gave to everyone else.
Whether the horoscope told him anything useful, only he knew. He never said.