Kepler Publishes His Three Laws of Planetary Motion
Working from Tycho's data on Mars, Kepler discovers that orbits are ellipses, not circles
Quick facts
- First and Second Laws published
- Astronomia Nova, Heidelberg, 1609
- Third Law published
- Harmonices Mundi, Linz, 1619
- Data source
- Tycho Brahe's planetary observations
- Key discovery
- Planetary orbits are ellipses, not circles
What happened
Using Tycho Brahe's precise observational data on the planet Mars, Johannes Kepler discovered that its orbit was an ellipse with the Sun at one focus, a result he later extended to all the planets as what is now called Kepler's First Law, and that a line joining a planet to the Sun sweeps out equal areas in equal times as the planet moves along its orbit, known as Kepler's Second Law. Kepler published both laws in Astronomia Nova, printed in Heidelberg in 1609, after an intensive multi-year study of Mars's orbit. A decade later, in Harmonices Mundi, published in Linz in 1619, Kepler added a third relationship: for any two planets, the ratio of the squares of their orbital periods equals the ratio of the cubes of the mean radii of their orbits, now known as Kepler's Third Law.
Why it matters
Kepler's laws replaced the assumption, held since antiquity, that celestial motion had to be built from combinations of perfect circles, with a mathematically precise description of elliptical orbits that actually matched observation. This gave heliocentric astronomy the predictive accuracy it had lacked against Ptolemy's geocentric system, and it supplied the empirical foundation that Newton would later explain physically through universal gravitation.
How we know
Kepler's Astronomia Nova and Harmonices Mundi both survive as printed books from 1609 and 1619 respectively, letting historians trace his derivation of each law directly from his stated calculations using Brahe's Mars data.
Sources
- MacTutor History of Mathematics, University of St Andrews. Johannes Kepler · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- MacTutor History of Mathematics, University of St Andrews. Tycho Brahe · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
See something wrong? . Corrections with a source get fixed fastest.
Related timelines
- The Scientific Revolution → · Kepler's laws are a core episode in the Scientific Revolution's shift toward mathematical, predictive natural science; see that timeline for the wider context.