Johannes Kepler

Kepler was born in 1571 in the free imperial city of Weil der Stadt in southern Germany. His father Heinrich made a precarious living as a mercenary and left the family when Johannes was five. His mother Katharina, an innkeeper's daughter, worked as a herbal healer. In 1577 the family moved to Leonberg in the Lutheran duchy of Württemberg.

Childhood smallpox left him with weak eyesight and damaged hands, which made observing the sky difficult for him. Through monastic schools at Adelberg and Maulbronn he reached the Stift at Tübingen in 1589. There he learned both the Ptolemaic and the Copernican systems from Michael Maestlin.

In 1594 Kepler took a post as teacher of mathematics and astronomy in Graz. His duties included preparing calendars with astrological predictions. In his Mysterium Cosmographicum (1596) he explained the distances between the six planets then known with nested regular polyhedra. That model is obsolete, but it shows his search for a geometrical order.

In Prague, from 1600, he analysed Tycho Brahe's observations of Mars. After years of calculation he found that planets move in ellipses with the Sun at one focus, and that the line between planet and Sun sweeps out equal areas in equal times. He published this in 1609 in the Astronomia nova, subtitled 'celestial physics'.

In Linz he published the Harmonices Mundi in 1619, which contains the third law: the square of a planet's orbital period is proportional to the cube of its mean distance from the Sun. In his Epitome Astronomiae Copernicanae (1618–1621) he applied the laws to all the planets. The Rudolphine Tables with calculated planetary positions appeared in 1627.

Kepler lived in the Holy Roman Empire during the Reformation and Counter-Reformation. In Graz a Catholic Habsburg ruler governed a predominantly Lutheran region, and in 1600 he had to leave the city when Protestant teachers were sent away. Later he also ran into difficulties within Lutheranism: in Linz the pastor refused him communion because he did not fully endorse the Formula of Concord.

At the court of Emperor Rudolf II in Prague, mathematics, astronomy, astrology and alchemy came together. Astrology still belonged to the mathematician's profession: Kepler wrote calendars and hundreds of horoscopes, including for Wallenstein, and tried to place astrology on a new foundation. This provided part of his income.

Witch persecution was common in German-speaking lands around 1600. In 1615 Kepler's mother Katharina was accused of witchcraft in Leonberg. In 1620 she was arrested and held for over a year; Kepler defended her through letters and in person. She was threatened with torture but did not confess, and was released on 4 October 1621; she died about six months later.

Kepler's laws broke with the assumption that heavenly bodies move in circles, an idea present since antiquity, including in Copernicus. In his thinking the Sun played a physical role as the source of a force that moved the planets. That explanation proved incorrect, but the mathematical regularities he found remained valid.

The Rudolphine Tables, based on Tycho's observations and Kepler's laws, gave more accurate planetary positions than earlier tables. Not all contemporaries adopted his laws: Galileo, for instance, continued to assume circular orbits. His work on optics, including Dioptrice (1611), contributed to the understanding of the telescope.

Kepler died on 15 November 1630 in Regensburg, during a journey to collect unpaid salary. He was buried in the Protestant cemetery at St Peter's, which was destroyed a few years later in the war; his grave is lost. His Somnium, a story about a journey to the Moon, appeared posthumously in 1634.

Newton used Kepler's laws as one of the foundations of his theory of gravitation (1687), and a transit of Mercury predicted by Kepler was observed by Pierre Gassendi in 1631. Historians stress that Kepler's mathematical astronomy was bound up with theological, musical and astrological motives; they therefore do not see his work merely as a forerunner of modern natural science.