Kepler States His Laws of Planetary Motion
A German astronomer replaces perfect circles with ellipses and finds an exact rule linking every planet's orbit to its distance from the Sun
Quick facts
- First two laws published
- 1609, Astronomia Nova
- Third law published
- 1619, Harmonices Mundi
- First law
- Planetary orbits are ellipses, Sun at one focus
- Later derivation
- Deduced from Newton's law of gravitation, 1687
What happened
Working from Tycho Brahe's precise decades of naked-eye observations of Mars, Johannes Kepler published two laws of planetary motion in 1609 and a third a decade later. MacTutor's history of the laws describes the first as the Ellipse Law, that the curve or path of a planet is an ellipse whose radius vector is measured from the Sun which is fixed at one focus, discarding the circular orbits that Aristotle, Ptolemy, and even Copernicus had all assumed. The second, the Area Law, states that the time taken by a planet to reach a particular position is represented by the area swept out by the radius vector drawn from the fixed Sun, meaning planets move faster when nearer the Sun and slower when farther away. A third law, published in 1619, related a planet's orbital period to its average distance from the Sun by a fixed mathematical ratio, valid for every planet then known.
Why it matters
Kepler's laws were the first exact, testable mathematical description of how planets actually move, replacing millennia of geometric guesswork built on the assumption that heavenly motion had to be circular. MacTutor calls the discovery of the laws of planetary motion Kepler's greatest achievement; six decades later Newton would derive all three laws directly from his law of universal gravitation, turning Kepler's empirical rules into a consequence of a deeper physical principle.
How we know
Kepler published his first two laws in Astronomia Nova in 1609 and the third in Harmonices Mundi in 1619, both surviving as complete printed texts; his underlying data came from Tycho Brahe's observational records, which Kepler inherited and which modern historians can still check his calculations against.
Sources
- MacTutor History of Mathematics, University of St Andrews. Kepler's Planetary Laws · Primary source (author-declared)mathshistory.st-andrews.ac.uk · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
- MacTutor History of Mathematics, University of St Andrews. Gravitation · Primary source (author-declared)mathshistory.st-andrews.ac.uk · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- The Scientific Revolution → · Kepler's laws were built on Tycho Brahe's observations and fed directly into Newton's Principia; see the Scientific Revolution timeline for the astronomers around him.