Johannes Kepler

Johannes Kepler was a German astronomer and mathematician who discovered that the planets move around the Sun in ellipses, not circles, and formulated the three laws of planetary motion that bear his name. Working from the observations of Tycho Brahe, he turned the Sun-centred system of Nicolaus Copernicus into an accurate physical description of the heavens, also made founding contributions to optics, and prepared the ground for Isaac Newton's theory of gravity.

Kepler was born on 27 December 1571 in Weil der Stadt, a small free imperial city in Swabia, in the south-west of the Holy Roman Empire. His father was a mercenary soldier who abandoned the family when Johannes was a boy, and his mother Katharina was an innkeeper's daughter with a knowledge of herbal remedies. He was a sickly child, and smallpox left him with weak eyesight, but as a boy he was shown the great comet of 1577 and a lunar eclipse, which he remembered all his life. Talented and poor, he was educated at Protestant schools in the duchy of Württemberg and in 1589 entered the university of Tübingen to train as a Lutheran minister. There his mathematics professor, Michael Maestlin, taught him both the old Earth-centred astronomy and the new system of Copernicus, which Kepler adopted with enthusiasm.

In 1594, before finishing his theology, he was sent to teach mathematics at the Protestant school in Graz, in Styria, a province of the Habsburg Monarchy. One of his duties was to produce an annual almanac with astrological forecasts, and he went on casting horoscopes throughout his life. In Graz he came up with the idea behind his first book, Mysterium Cosmographicum (1596). Copernicus had six planets, and Kepler believed God had spaced their orbits by fitting the five regular solids described by Plato, the cube, tetrahedron and others, one inside another between their spheres. The scheme is wrong, but it showed his conviction that the cosmos follows a mathematical plan, and his attempt to explain why the planets move as they do, through a force coming from the Sun, rather than simply describing their paths. In 1597 he married Barbara Müller, a young widow. In 1600 the Catholic archduke Ferdinand expelled Protestants who refused to convert, and Kepler had to leave.

Kepler had sent his book to Tycho Brahe, the Danish nobleman whose observations of the planets were far more precise than any made before. Tycho had moved to Bohemia as imperial mathematician to the emperor Rudolf II, and in 1600 Kepler joined him near Prague as an assistant. The two men did not get on well, and Tycho guarded his data closely, but he set Kepler the task of explaining the orbit of Mars. When Tycho died suddenly in October 1601, Kepler succeeded him as imperial mathematician and gained access to his records.

Mars has an unusually eccentric orbit, and Kepler struggled with it for years. A circular model he built differed from Tycho's observations by eight minutes of arc, about a quarter of the width of the full Moon. Rather than dismiss the gap as error, he trusted the data and abandoned circles altogether. In Astronomia nova (1609) he published his first two laws: each planet moves in an ellipse with the Sun at one focus, and a line from the Sun to the planet sweeps out equal areas in equal times, so that a planet moves faster when it is nearer the Sun. Ending the ancient assumption that heavenly motion must be circular was one of the decisive steps of the Scientific Revolution.

Kepler's years in Prague were also productive in optics. In a work of 1604 he explained how the eye forms an image on the retina and how lenses for short and long sight correct vision, and he studied the bright new star of 1604, a supernova now named after him. When Galileo Galilei announced his telescopic discoveries in 1610, Kepler publicly supported him, and in Dioptrice (1611) he explained how telescopes work and described an improved design with two convex lenses.

In 1611 his wife and a son died and his patron Rudolf II was forced to abdicate. Kepler moved to Linz, in Upper Austria, as district mathematician, and in 1613 married Susanna Reuttinger. From 1615 he spent years defending his mother, who was accused of witchcraft in her home town of Leonberg and held in prison in 1620 and 1621 before he secured her release. In Harmonices Mundi (1619), a work on the musical and geometrical harmonies he saw in the universe, he stated his third law: the square of the time a planet takes to orbit the Sun is proportional to the cube of its average distance from it. This tied all the planets into a single system. His Epitome of Copernican Astronomy (1617 to 1621) presented the whole of astronomy in terms of his laws, and was placed on the Catholic Index of Forbidden Books.

The Thirty Years' War, which began in 1618, made Kepler's life increasingly unsettled. As a Lutheran who did not accept all of his own church's doctrines, he was excluded from communion by the Lutherans while refusing to become a Catholic. In 1626 Linz was besieged during a peasant revolt, and he left. In 1627 he published the Rudolphine Tables, based on Tycho's observations and his own laws, which allowed planetary positions to be predicted far more accurately than ever before. In his last years he worked for the general Albrecht von Wallenstein at Sagan, in Silesia. He died on 15 November 1630 in Regensburg, where he had travelled to try to collect money owed to him. His grave was lost when the churchyard was destroyed during the war. His Somnium, a story of a journey to the Moon written to show how astronomy would look from another world, was published in 1634 and is sometimes counted as early science fiction.

Kepler's laws were not widely accepted at first, and Galileo never took up his ellipses. Their accuracy, shown above all by the success of the Rudolphine Tables, gradually won astronomers over, and in 1687 Newton showed that all three laws follow from a single law of universal gravitation. Kepler is remembered as the first to insist that astronomy must explain the physical causes of motion and must match observation exactly. He is a central figure in the scientific history of Germany, and NASA's Kepler space telescope, which found thousands of planets orbiting other stars, was named after him.