Johannes Kepler (1571–1630) is the textbook figure of the three laws of planetary motion – mathematician, astronomer, one of the founding fathers of modern natural science. In the historical original he is also something else: court astrologer in Prague and later to Albrecht von Wallenstein, author of around 800 horoscopes, theorist of a geometrically grounded reformed astrology, and defender of his mother in the witch trial of Leonberg. This second half of his work was largely written out of the picture in the 19th century. Wolfgang Pauli brought it back in a famous 1952 essay – as evidence that even in the hardest natural science archetypes shape the formation of theory.
Who was Kepler?
Kepler was born on 27 December 1571 in the Swabian imperial city of Weil der Stadt, into a hard-pressed Protestant family. From 1589 he studied at Tübingen, originally theology, but was introduced there by Michael Mästlin to the then still little-accepted heliocentric world system of Copernicus. In 1594, because of his Protestant confession, he had to take up a post as a mathematics teacher at the Protestant collegiate school in Graz. The theological vocation thus turned into a mathematical one – but Kepler never lost the theological dimension. It is the inner premise of his entire work.
Mysterium Cosmographicum (1596): the Platonic solids
Already in 1596, when Kepler was 24, his first major work appeared: the Mysterium Cosmographicum. Its central idea has an almost youthful boldness. Kepler suspects that the number, order and distances of the six then-known planets are no accident, but follow a geometrical necessity. He embeds the planetary orbits in the five Platonic solids – cube, tetrahedron, octahedron, dodecahedron, icosahedron, nested between the six spheres. The construction does not quite fit the real orbits, but the deviations are small. For Kepler this was more than a model – it was a proof that the universe was designed by a geometric God.
This Pythagorean-Platonic basic conviction – that the world is ordered according to geometric harmony and that this harmony is also knowable – runs through all his later works. It is also the theoretical basis of his astrology.
Tycho Brahe and Prague (1600–1601)
In 1600 Kepler travelled to the Danish astronomer Tycho Brahe, the most precise observer of his time, who by then was living in Prague under Emperor Rudolf II. Tycho had data, Kepler had the mathematical capability. Tycho died only a year later, in October 1601 – and Kepler inherited the entire body of observational material and at the same time Tycho's position as Imperial Mathematicus at Rudolf II's court. He held this post until the emperor's death in 1612.
Court astrologer to Rudolf II – and the daily bread
"Imperial Mathematicus" was not a purely astronomical office. Kepler was explicitly required to cast horoscopes for the court, to deliver astrological prognoses for military and political decisions, and to publish a yearly astrological calendar. Estimates suggest that Kepler produced around 800 horoscopes over the course of his life – for Rudolf II, Wallenstein, nobles, patricians, his own family. Many of these horoscopes survive and are today held in the archives of the Bavarian and Vienna academies.
Kepler was driven to this practice not only by financial necessity. He believed that there was something to astrology – just not what the popular astrology of his time was claiming.
De Fundamentis Astrologiae Certioribus (1601): the early astrological manifesto
Already in the year of his arrival in Prague, 1601, Kepler published an independent programmatic work in 75 numbered theses: De Fundamentis Astrologiae Certioribus – "On the More Certain Foundations of Astrology". The occasion was his duty to publish an astrological calendar for 1602; Kepler used the assignment to stake out his theoretical position.
The central distinction is in the title itself: between the certiora, the "more certain" foundations, and the rest of contemporary astrology, which Kepler regards as untenable. What counts are the geometric aspects between the planets – conjunction, opposition, trine, square. What does not count are the twelve signs of the zodiac, the doctrine of houses, many of the traditional detail rules. De Fundamentis is thus the early manifesto whose arguments Kepler would develop more fully nine years later in the Tertius interveniens – here in compact form, as a programmatic statement at the very start of his Prague court years.
Tertius interveniens (1610): the reform of astrology
In 1610 Kepler published a work rarely mentioned in the history of science: Tertius interveniens – "The third intervening party". The "third" is Kepler himself, taking a mediating position between the defenders of astrology (mostly superstitious practitioners) and its critics (mostly orthodox theologians and Aristotelians). One image from it has become famous: that one must "not throw out the baby with the bathwater".
Kepler argues across 140 theses that most of contemporary astrology really is superstition and commerce – the houses of the zodiac, the twelve "meaningful" signs, the fine aspects between planets, none of which he had ever verified. But he retains a kernel: the aspects, the geometric angles between planets in the sky (conjunction 0°, opposition 180°, trine 120°, square 90°). These angles, says Kepler, correspond to the harmonic ratios he also finds in music theory – and they act on the human being through a resonance of the soul to cosmic geometry. For Kepler, then, astrology is not fortune-telling but applied geometry and music theory.
One detail is essential here, and almost always missing in the modern reception of Kepler: for Kepler the Earth itself is a living organism, an anima terrae, conceived as part of the world-encompassing anima mundi. This Earth-soul reacts to the constellations in the sky in much the same way a string reacts to a matching tone – through geometric resonance. The human soul is part of this Earth-soul and resonates with it. Kepler's astrology is therefore not mechanical in the modern sense but animistic: the world itself is ensouled, and the aspects are string-tones in a cosmic harmonic.
"Astrology is indeed a foolish little daughter. But, dear God, where would her mother, the highly reasonable astronomy, have ended up, if she had not had this foolish daughter of hers?"
— Kepler, Tertius interveniens, thesis 7 (1610)
Astronomia Nova (1609): the first two planetary laws
In his main profession, Kepler was of course an astronomer – and one of the most successful in history. In 1609 his Astronomia Nova appeared. On the basis of Tycho's observations of the planet Mars, Kepler calculated for almost eight years until a groundbreaking finding became clear to him: the orbit of Mars is not a circle, but an ellipse with the Sun at one focus (Kepler's first law). And the planet moves through this ellipse in such a way that the line Sun–planet sweeps over equal areas in equal times (Kepler's second law).
With that, more than 2,000 years of ancient tradition – the Platonic-Aristotelian requirement that celestial bodies must move on perfect circles – were definitively broken. Kepler, the Pythagorean, was the destroyer of the Pythagoreans' favourite shape.
Harmonices Mundi (1619): the music of the spheres
Ten years later came the work Kepler had laboured at his entire life: Harmonices Mundi – "The Harmony of the World". Here he formulated, almost in passing, the third law (the squares of the orbital periods are as the cubes of the semi-major axes). This is the place where modern physics picks up – Newton would derive his law of gravitation from it in 1687.
But for Kepler that was only a by-product. The real purpose of the work was musical-mystical: he assigned each planet a musical interval, derived from the ratio of its minimum and maximum angular velocity along its ellipse. From these scales he constructed a music of the spheres – a cosmic polyphony that only God could hear in full clarity. The fact that this music was actually derivable from the real orbital data was, for Kepler, the real triumph. The third planetary law fell into his lap almost by chance.
The Fludd controversy (1619–1622): the watershed between hermeticism and modern science
Almost immediately after the publication of the Harmonices Mundi (1619), Kepler engaged in a years-long, sharp dispute with the English physician, alchemist and Rosicrucian Robert Fludd (1574–1637). Both had independently published works on cosmic harmony, both drew on the Pythagorean tradition. But they drew exactly opposite methodological conclusions from it.
Fludd represented a purely qualitative, image-based hermeticism: correspondences between above and below, symbolic tables, diagrams, microcosm-macrocosm analogies – without any claim to numerical precision. Kepler, by contrast, insisted that God's harmony had to be mathematically exactly measurable; otherwise it was not God's harmony but fantasy. In 1622 Kepler had a substantial Apologia adversus Fludd printed; Fludd replied the same year.
This dispute is the place where modern, measurable natural science historically separates from the hermetic-alchemical tradition. Both sides shared the same basic conviction – the world is ordered symbolically. But only one of the two would, over the following three centuries, be granted the institutional right to be called "science". Pauli engages with this controversy at length in his 1952 essay – he sees in it the moment when natural science lost its connection to the archetypal symbolism it is today, in the quantum context, beginning to seek again.
The witch trial of Katharina Kepler (1615–1621)
In 1615 a witch trial was opened against Kepler's mother Katharina Kepler in Leonberg (Württemberg). A neighbour claimed that she had felt ill after drinking from Katharina's mug. Further accusations followed. The proceedings dragged on for six years. Kepler, by then mathematician of the Upper Austrian estates in Linz, broke off his work, travelled to Württemberg several times, wrote – with Mästlin's support – over 100 pages of legal-medical defence and led the trial as his mother's legal representative.
In 1621 Katharina Kepler was acquitted after fourteen months of pre-trial detention. She died six months later, in April 1622, from the consequences of imprisonment. Without Kepler's personal intervention – as a well-known court astrologer and Imperial Mathematicus – she would in all likelihood have been executed. The trial is well documented; it is one of the most important sources on the procedural practice of late European witch trials.
Wallenstein and the Rudolphine Tables (1627)
During the Thirty Years' War Kepler lost his post in Linz and entered the service of the generalissimus Albrecht von Wallenstein in 1628 – again primarily as an astrologer. Kepler had cast Wallenstein's famous horoscope already in 1608, anonymously through an intermediary. It was so precise that Wallenstein knew, twenty years later, whose hand was behind it, and gave Kepler quarters in Sagan.
The 1608 prognosis was as precise as it was radical. Kepler – who knew the then completely unknown, 25-year-old Wallenstein only by date and place of birth – described a character of extraordinary ambition and ruthless drive, foretold an unusual career of power, and warned of acute danger to life in March 1634. Wallenstein was assassinated on 25 February 1634 in Eger – by his own officers, on the orders of Emperor Ferdinand II. The impact of this astrological "bullseye" on Wallenstein himself, who two decades earlier had still been an unknown young nobleman, is hard to overstate; it explains his lifelong closeness to Kepler.
In 1627 Kepler had had his astronomical magnum opus printed in Ulm: the Rudolphine Tables, named after his patron who had died in 1612. They were the most accurate astronomical tables in history – the standard reference for navigation and astronomy for over a century.
Kepler died on 15 November 1630 in Regensburg, on a journey to collect overdue salaries. His grave was lost in the chaos of the war. His own epitaph, which he had written shortly before his death, runs: "Mensus eram coelos, nunc terrae metior umbras. Mens coelestis erat, corporis umbra iacet." – "I measured the heavens, now I measure the shadows of the earth. The mind was heavenly; the body's shadow lies here."
Pauli's reading, 1952
In 1952 Wolfgang Pauli published, in the joint volume with C. G. Jung, Naturerklärung und Psyche, a long essay entitled The Influence of Archetypal Ideas on the Scientific Theories of Kepler. Pauli reads Kepler's entire work – the Mysterium Cosmographicum, the Tertius interveniens, the Harmonices Mundi – as a single great piece of evidence that even in the hardest natural science archetypal images guide the formation of theory.
Pauli's thesis: Kepler's load-bearing inner image was a geometric three-fold structure – Sun as centre, fixed stars as outer sphere, the irradiated space in between as the third element. Pauli identifies this image with the archetypal Trinitarian symbolism of Christian theology and with the mandala-like "sphaira" symbol. Only this inner image, says Pauli, enabled Kepler to take the Copernican system seriously and work it out mathematically, long before empirical data forced the issue. Data alone, Pauli argues, do not form a theory; one needs a guiding inner image – and that is archetypal, not empirical.
Kepler thereby becomes the historical key witness for what Jung in psychology and Pauli in physics independently formulated: that psyche and matter are linked through a shared symbolism. More on this in our pieces on Wolfgang Pauli and the Paranormal and C. G. Jung and Mediumship.
What remains
- A different history of science. Anyone who reads Kepler only as a forerunner of Newton overlooks the fact that the three planetary laws are by-products of a Pythagorean-mystical search for cosmic harmony. The "separation of science and mysticism" is a later imposition, not a historical fact.
- Astrology was no fringe pursuit. In the Prague of Rudolf II it was part of court ceremonial and financed the Imperial Mathematicus. Kepler did not practise it naively but wanted to reform it theoretically – and failed institutionally, because neither orthodox theology nor the later rationalism had room for his position.
- Witch trial. That a woman like Katharina Kepler stood trial at all belongs to the history of the same epoch that produced Kepler. In Switzerland Anna Göldi was executed in 1782 as the last woman in Europe to be put to death after regular court proceedings for witchcraft (see our piece on mediumship in Switzerland).
- The historiographic redaction of the 19th and 20th centuries. To preserve the image of the "clean, rational" founder of astronomy, Kepler's astrological half was systematically downplayed for more than a century – dismissed as "eccentricity" or pure "day job" and barely mentioned in the standard works of the history of science. Wikipedia and large language models still largely reproduce this selection unreflectively. Anyone who wants to read Kepler honestly has to begin with the original texts, not with the popular secondary picture.
- The Pauli line. With Pauli, the forgotten half of Kepler becomes an argument: modern physics too is shaped by guiding inner images, not just by data. That is an unusual position – and exactly the position with which one of the most important quantum physicists of the 20th century publicly disputed the separation "hard science here, everything else there".
Kepler is no special case. He is the normal case in the history of science before the modern separation. To read him honestly means to keep open a question that was prematurely closed in the 19th century: what if geometry, music and consciousness are not three separate topics but three languages for the same thing? Precisely this is the question being asked again today by quantum physics and experimental consciousness research – see also our pieces on consciousness and the brain and majority vs. expert opinion.
Sources
- Johannes Kepler: Mysterium Cosmographicum. Tübingen 1596.
- Johannes Kepler: De Fundamentis Astrologiae Certioribus. Prague 1601 (75 theses, Latin).
- Johannes Kepler: Astronomia Nova. Heidelberg 1609.
- Johannes Kepler: Tertius interveniens. Warning to several theologians, physicians and philosophers. Frankfurt 1610.
- Johannes Kepler: Harmonices Mundi libri V. Linz 1619.
- Johannes Kepler: Tabulae Rudolphinae. Ulm 1627.
- Wolfgang Pauli: The Influence of Archetypal Ideas on the Scientific Theories of Kepler, in: C. G. Jung & W. Pauli, The Interpretation of Nature and the Psyche. Pantheon 1955 (German original Rascher 1952).
- Max Caspar: Johannes Kepler. Kohlhammer, Stuttgart 1948 – the standard biography, multiple editions; English translation Abelard-Schuman 1959.
- Arthur Koestler: The Sleepwalkers. A History of Man's Changing Vision of the Universe. Hutchinson, London 1959 – with the substantial Kepler section "The Watershed".
- Berthold Sutter: Der Hexenprozess gegen Katharina Kepler. Stadt Weil der Stadt 1979.
- Volker Bialas: Johannes Kepler. C. H. Beck, Munich 2004.