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Kepler, Johannes

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Summary Article: Kepler, Johannes (1571–1630) from The Hutchinson Dictionary of Scientific Biography
Johannes Kepler
Image from: Johannes Kepler in The Cambridge Dictionary of Scientists
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Subject: biography, astronomy

Place: Germany

German astronomer who combined great mathematical skills with patience and an almost mystical sense of universal harmony. He is particularly remembered for what are now known as Kepler's laws of motion. These had a profound influence on Isaac Newton and hence on all modern science. Kepler was also absorbed with the forces that govern the whole universe and he was one of the first and most powerful advocates of Copernican heliocentric (Sun-centred) cosmology.

Kepler was born on 27 December 1571 in Weil der Statt near Stuttgart, Germany. He was not a healthy child and since it was apparently thought that he was capable only of a career in the ministry, he was sent for religious training in Leonberg, Adelberg, and Maulbronn. One event that impressed him deeply during his early years was the viewing of the ‘great’ comet of 1577 and his interest in astronomy probably dates from that time. He passed his baccalaureate at the University of Tübingen in 1588 and then returned to Maulbronn for a year. From 1589 to 1591 he studied philosophy, mathematics, and astronomy under Michael Mästlin at the University of Tübingen; yet although he showed great aptitude and promise and obtained his MA in 1591, he then embarked on a three-year programme of theological training. This was interrupted in the last year when he was nominated for a teaching post in mathematics and astronomy in Graz. It was during this teaching period that he abandoned his plans for a career in the ministry and concentrated on astronomy. He wrote his first major paper while at Graz and attracted the attention of other notable astronomers of the time, particularly Tycho Brahe and Galileo.

Kepler was a Lutheran and so was frequently caught up in the religious troubles of his age. In 1598 a purge forced him to leave Graz. He travelled to Prague and spent a year there before he returned to Graz; he was expelled again and arrived back in Prague, where he became Tycho Brahe's assistant in 1600. Brahe died a few months later, in 1601, and Kepler succeeded him as Imperial Mathematician to Emperor Rudolph II. On his deathbed Brahe requested that Kepler complete the Rudolphine Astronomical Tables, a task that Kepler finished in 1627.

Kepler lived in Prague until 1612 and produced what was perhaps his best work during those years. He was given a telescope in 1610 by Elector Ernest of Cologne and studied optics, telescope design, and astronomy. In 1611 Rudolph II was deposed, but Kepler was retained as Imperial Mathematician. In 1612, upon Rudolph II's death, Kepler became district mathematician for the states of Upper Austria and moved to Linz. But personal problems plagued him for the next ten years – the arrest and trial of his mother, who was accused of witchcraft in 1615 but exonerated in 1621, being particularly distressing.

It was in Linz that Kepler published three of his major works. In 1628 he became the private mathematician to Wallenstein, Duke of Friedland, partly because of the duke's promise to pay the debt owed Kepler by the deposed Rudolph II. In 1630 religious persecution forced Kepler to move once again. He fell sick with an acute fever, and died on the way to Regensburg, Bavaria, on 15 November 1630.

Kepler's work in astronomy falls into three main periods of activity, at Graz, Prague, and Linz. In Graz Kepler did some work with Mästlin on optics and planetary orbits, but he devoted most of his energy to teaching. He also produced a calendar of predictions for the year 1595, which proved so uncanny in its accuracy that he gained a degree of local fame. Kepler found the production of astrological calendars a useful way of supplementing his income in later years, but he had little respect for the art. More importantly, in 1596 he published his Mysterium cosmographicum, in which he demonstrated that the five Platonic solids, the only five regular polyhedrons, could be fitted alternately inside a series of spheres to form a ‘nest’ that described quite accurately (within 5%) the distances of the planets from the Sun. Kepler regarded this discovery as a divine inspiration that revealed the secret of the universe. It was written in accordance with Copernican theories and it bought Kepler to the attention of all European astronomers.

Before Kepler arrived in Prague and was bequeathed all Brahe's data on planetary motion, he had already made a bet that, given Brahe's unfinished tables, he could find an accurate planetary orbit within a week. It took rather longer, however. It was five years before Kepler obtained his first planetary orbit, that of Mars. In 1604 his attention was diverted from the planets by his observation of the appearance of a new star, ‘Kepler's nova’, to which he attached great astrological significance.

In 1609 Kepler's first two laws of planetary motion were published in Astronomia nova, which is a long text and as unreadable as it is important. The first law states that planets travel in elliptical rather than circular or epicyclic orbits and that the Sun occupies one of the two foci of the ellipses. What is now known as the second law, but was in fact discovered first, states that the line joining the Sun and a planet traverses equal areas of space in equal periods of time, so that the planets move more quickly when they are nearer the Sun. This established the Sun as the main force governing the orbits of the planets. Kepler also showed that the orbital velocity of a planet is inversely proportional to the distance between the planet and the Sun. He suggested that the Sun itself rotates, a theory that was confirmed by using Galileo's observations of sunspots, and he postulated that this established some sort of ‘magnetic’ interaction between the planets and the Sun, driving them in orbit. This idea, although incorrect, was an important precursor of Newton's gravitational theory. The Astronomia nova had virtually no impact at all at the time, and so Kepler turned his attention to optics and telescope design. He published his second book on optics, the Dioptrice, in 1611. That year was a difficult one, because Kepler's wife and sons died. Then, in 1612, the Lutherans were thrown out of Prague so Kepler had to move on again to Linz.

In Linz Kepler produced two more major works. The first of these was De harmonices mundi, which was almost a mystical text. The book was divided into five chapters and buried in the last was Kepler's third law. In this law he describes in precise mathematical language the link between the distances of the planets from the Sun and their velocities – a feat that afforded him extraordinary pleasure and confirmed his belief in the harmony of the universe. The second major work to be published during his stay in Linz, the Epitome, intended as an introduction to Copernican astronomy, was in fact a very effective summary of Kepler's life's work in theoretical astronomy. It was a long treatise of seven books, published over a period of four years, and it had more impact than any other astronomical text of the mid-17th century.

Soon after its publication, Kepler's Lutheran background caused yet another expulsion and this time he went to Ulm, where he finally completed the Rudolphine Tables. They appeared in 1627 and brought Kepler much popular acclaim. These were the first modern astronomical tables, a vast improvement on previous attempts of this kind, and they enabled astronomers to calculate the positions of the planets at any time in the past, present, or future. The publication also included other vital information, such as a map of the world, a catalogue of stars, and the latest aid to computation, logarithms.

Kepler wrote the first science fiction story, Solemnium, which described a man who travelled to the Moon. It was published in 1631, a year after his death, although it had been written 20 years earlier. Kepler was a remarkable man and a brilliant scientist. He kept a steady eye on what he saw as his true vocation as a ‘speculative physicist and cosmologist’ and, despite living in times of political unrest and religious turmoil, was never swayed by religious bigotry or political pressures. His new astronomy provided the basis on which Newton and others were to build, and to this day his three laws of motion are considered to be the basis of our understanding of the Solar System. His work and his strong support of Copernican cosmology mark a fundamental divide between two eras – that of the Ptolemaic Earth-centred view of the universe that had been accepted for the previous 15 centuries, and the new age given birth by the Copernican heliocentric, or Sun-centred, view of the Solar System.

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