Kepler’s elliptical orbits complicated traditional planetary motion theories.

Evidence

Having discovered almost by accident that elliptical orbits fit the observations well, he could not reconcile them with his idea that the planets were made to orbit the sun by magnetic forces. — Hawking, Stephen, A Brief History of Time, 19

Categorization

• corroborated: 2 independent passages support this claim — Hawking, Stephen, A Brief History of Time, 19
• XREF: connects to: The law of inertia and momentum made outside divine intervention unnecessary for planetary motion. — Bertrand Russell, History of Western Philosophy, 1978