The Mystic Who Discovered Planetary Motion: How Kepler's Occult Beliefs Led to Modern Astronomy

Amsterdam — In 1609, Johannes Kepler published Astronomia Nova — the book that established that planets move in ellipses, not circles. It was one of the most important scientific discoveries in history, the foundation of modern astronomy and celestial mechanics.

What most people don't know is why Kepler was looking for ellipses in the first place. He believed the planets were alive.

From Kepler's Astronomia Nova (1609), now fully translated on Source Library in 388 pages of AI-assisted translation from Latin, here is Kepler defining gravity — not as a property of objects falling toward the center of the Earth, but as a mutual attraction between bodies. Eighty years before Newton:

“Gravity is a mutual material tendency between related bodies toward union or conjunction (of the same order of things as the magnetic faculty), such that the Earth attracts a stone much more than the stone seeks the Earth... If two stones were placed in some location in the world near each other, outside the sphere of influence of a third related body, those stones would come together at an intermediate place, much like two magnetic bodies; each would approach the other by a distance proportional to the other's mass.”

And in his summary of the chapters, the kernel of what would become the inverse-square law:

“It is demonstrated that the Moving Power behaves exactly like Light: it occupies space, is weakened over a larger area, and is concentrated in a smaller one.”

Kepler described planets as having both an “animal faculty” — an internal guiding intelligence — and a “magnetic faculty” — a physical interaction with the Sun. His breakthrough was not to abandon the mystical framework but to mathematize it: to ask what shape an orbit would take if a planet were simultaneously drawn toward the Sun and guided by its own internal nature.

The answer was an ellipse. And with that answer, Kepler's mysticism became Newton's physics.

The Scientists Who Believed in Magic

Source Library's collection reveals that the line between “science” and “occultism” is a modern invention. The same thinkers who built the foundations of modern science were deeply embedded in traditions of Hermeticism, alchemy, and natural magic:

• Johannes Kepler (Astronomia Nova, 1609; Dioptrice, 1611) — 514 pages of Kepler's foundational works, fully translated. His optics and astronomy emerged from a Neoplatonic conviction that the universe was structured by divine mathematical harmonies.
• Robert Fludd (Utriusque Cosmi Historia, 1617; Mosaicall Philosophy, 1659) — Fludd's encyclopedic work on the macrocosm and microcosm includes extraordinary engravings showing the musical harmonies of the cosmos. His Mosaicall Philosophy articulates the Anima Mundi — the soul of the world:

“They say that within all things are moved by a soul, by which they believe all things in the world live... there must be an intermediate spirit. This they call neither body nor soul, but a middle substance, participating in both, which brings these two extremes back into one.”

• Athanasius Kircher (Ars Magna Lucis et Umbrae, 1646; Musurgia Universalis, 1650; Mundus Subterraneus, 1665) — Kircher investigated magnetism, optics, acoustics, geology, and Egyptology with equal rigor. In his Musurgia Universalis, he argued that musical harmony was not a human invention but the fundamental structure of nature:

“Trees of different sizes, between which there is a harmonic proportion, emit a harmonious sound when moved by the wind.”

• Giambattista della Porta (Magia Naturalis, 1558) — Della Porta's “natural magic” was essentially experimental physics under another name: optics, magnetism, hydraulics, and chemistry presented as the art of understanding nature's hidden operations.

The Missing Chapter

“Every history of the Scientific Revolution acknowledges that Kepler, Newton, Boyle, and Leibniz were deeply involved in traditions we now call ‘occult,’” says the Source Library editorial team. “But you can't really understand what that means until you read their actual words. These aren't embarrassing footnotes. They're the intellectual engine of modern science.”

Source Library makes these primary sources freely available for the first time — not as curated excerpts in academic publications, but as complete works with AI translations alongside the original Latin, German, and Italian texts.

Open for Discovery

All 1,234 books in the collection are freely available at sourcelibrary.org. The full corpus — translations, original texts, and page images — is released as open data under a Creative Commons license.

“Kepler didn't discover planetary motion despite his mysticism,” says the Source Library team. “He discovered it because of his mysticism. That's a fact that should inform how we think about the relationship between science and other ways of knowing — including the ones we're building into AI.”