Johannes Kepler

About this figure

Johannes Kepler (1571-1630) was a German mathematician and astronomer who found the actual shape of the solar system: not the circles that Copernicus and every astronomer before him had assumed, but ellipses. Working from decades of naked-eye observations of Mars compiled by his employer and rival Tycho Brahe, Kepler discovered that planets sweep out equal areas in equal times as they orbit the sun, moving faster when near it and slower when far away. He published this and the elliptical-orbit law in the Astronomia Nova (1609), then added a third law relating a planet's orbital period to its distance from the sun in the Harmonices Mundi (1619). These three laws of planetary motion, arrived at through years of laborious calculation rather than a single insight, gave Isaac Newton the description of planetary motion his theory of gravity would later explain.

Kepler was also an early and vocal supporter of Copernican astronomy at a time when doing so carried real professional and religious risk, and he corresponded with Galileo about the telescope, writing a preface praising the instrument and what it had revealed. His life was frequently difficult: he worked as an imperial mathematician under patrons whose support was unreliable, and he spent years defending his mother against a formal accusation of witchcraft.

Almost none of Kepler's major work survives in a complete public-domain English translation; the standard modern translations of the Astronomia Nova and the Harmonices Mundi are still under copyright. What can be offered here in his own words is therefore a set of excerpts rather than the finished books: passages from those two works, his shorter Epitome of Copernican Astronomy, and his preface responding to Galileo's telescope. Read together they show the outline of his method and his conviction that the universe was built on discoverable mathematical harmony, but they are excerpts of a much larger body of work, and there are questions about the full argument of either book that this smaller selection cannot answer.

Written by Epoch Voices. Not the figure’s own words.