Johannes Kepler

NAME Johannes Kepler. He is occasionally rendered in older English texts as "Johann Kepler." 

WHAT FAMOUS FOR Johannes Kepler was a German mathematician and astronomer best known for his three laws of planetary motion, which established that planets orbit the Sun in ellipses rather than perfect circles and that their speeds vary depending on their distance from the Sun. His analysis of Tycho Brahe's observational data led him to discover these laws, published first in Astronomia Nova (1609) and completed in Harmonice Mundi (1619). His work provided crucial evidence for the heliocentric model of Copernicus and later underpinned Isaac Newton's theory of universal gravitation. (1) 

BIRTH Johannes Kepler was born on December 27, 1571, at the Free Imperial City of Weil der Stadt, roughly 50 miles west of Stuttgart in what is now Germany. He arrived as a premature "seven-month" child, a fact he later documented with characteristic precision, noting the pregnancy had lasted exactly 224 days, 9 hours, and 53 minutes. (2)

Kepler's birthplace, in Weil der Stadt, Germany By MarkusHagenlocher

FAMILY BACKGROUND Kepler's father, Heinrich Kepler, earned a precarious living as a mercenary soldier and abandoned the family when Johannes was five years old; Kepler himself later described him as "vicious, inflexible, quarrelsome, and doomed to a bad end." 

His mother, Katharina Guldenmann, was the daughter of an innkeeper and worked as a healer and herbalist, though Kepler described her too in unflattering terms as "small, thin, swarthy, gossiping and quarrelsome." 

His paternal grandfather, Sebald Kepler, had served as mayor of Weil der Stadt, while his maternal grandfather was an innkeeper and mayor of the nearby village of Eltingen. (2) (3)

CHILDHOOD Kepler was introduced to astronomy at an early age and, at age six, observed the Great Comet of 1577; two years later his mother took him to a hilltop to witness a lunar eclipse. 

Around the same period he survived a severe bout of smallpox that left his hands partially crippled and his eyesight permanently impaired with multiple vision, afflictions that made him unsuited for physical labor but pushed him toward intellectual pursuits. 

He spent time being raised largely by his grandparents while his parents were often absent, and guests at his grandfather's inn were reportedly astonished by the boy's ability to solve numerical problems. (4)

EDUCATION Kepler attended the Tübinger Stift at the University of Tübingen, where he studied philosophy and theology while proving himself a superb mathematician. 

He earned a reputation as a skillful astrologer even as a student, casting horoscopes for his fellow students, and his mathematics professor Michael Maestlin introduced him to the Copernican heliocentric theory. He passed his Master's examination in 1591 and continued as a graduate student before being recommended, near the end of his studies, for a teaching post rather than the ministry he had originally intended to pursue. (5) 

CAREER RECORD 1594 Kepler accepted a position as teacher of mathematics and astronomy at the Protestant school in Graz, Austria, at the age of 23.

1600 Kepler first met the Danish astronomer Tycho Brahe on February 4, at Benátky nad Jizerou near Prague, where Brahe's new observatory was being constructed; he stayed as a guest for two months analyzing Brahe's observations of Mars.

1601 Kepler was supported directly by Tycho Brahe, who assigned him to analyze planetary observations; after Brahe's unexpected death on October 24, Kepler was appointed his successor as Imperial Mathematician to Emperor Rudolf II.

1609 Kepler published his first two laws of planetary motion in Astronomia Nova (New Astronomy).

1611 Kepler published Dioptrice, a study of telescopic optics that helped legitimize Galileo's telescopic discoveries.

1619, Kepler outlined his third law of planetary motion, discovered on March 8, 1618, in Harmonice Mundi (Harmony of the Worlds).

1628, following the military successes of General Wallenstein's armies during the Thirty Years' War, Kepler was appointed an official adviser to Wallenstein, providing astronomical calculations and occasional horoscopes.

APPEARANCE Kepler was described as short, slight, and physically fragile. His bout with smallpox as a young child left him with permanently damaged eyesight—including severe myopia and multiple vision (polyopia)—as well as skin lesions and crippled, weak hands. Contemporaries described his appearance as modest, studious, and somewhat unkempt. (6)

Portrait by August Köhler 1910 after 1620 original

FASHION Kepler dressed in the sober, dark clothing typical of a Lutheran academic and court mathematician of his era, favoring plain scholarly attire over ostentation. There is little evidence he took any particular interest in fashion, his energies being devoted almost entirely to his studies. (6)

CHARACTER Kepler possessed an intensely curious, rigorous, and open-minded intellect combined with deep humility and spiritual piety. He was intensely self-analytical, frequently writing candid assessments of his own personal flaws and intellectual struggles. Known for his persistent work ethic, he spent years performing tedious hand calculations to prove planetary orbits.

Kepler was  described by contemporaries as amiable and affectionate in private, generous in recognizing the achievements of others, and largely free of envy despite the professional rivalries he faced. He was also solitary and prone to melancholy, with a strongly speculative, almost mystical bent of mind that coexisted with rigorous scientific precision — a combination that made his character, in the words of one early biographer, "especially difficult to estimate." (7)

SPEAKING VOICE The evidence suggests Kepler was not a naturally compelling lecturer: his mathematics classes at Graz reportedly drew very few students, and by his second year none at all, forcing him to teach other subjects like Virgil and rhetoric to justify his salary. 

His written "voice," however, is extensively documented and offers the best window into how his mind worked: contemporaries and later historians describe his prose as notoriously fast-moving, digressive, and intellectually intense, frequently interrupting scientific argument with personal anecdotes, numerological asides, or excited exclamations about a new geometrical relationship. Kepler himself acknowledged this restless quality, explaining that he wrote as he did in order to "lighten the hard work and difficulty of a subject by mental relaxation," and his surviving letters reveal the same quick, associative, sometimes tangential energy that likely characterized his conversation. (8) (9)

SENSE OF HUMOUR Kepler's writings show flashes of dry wit, particularly in his astrological polemics, where he compared popular astrology to a "foolish daughter" dependent on her "wise mother," astronomy. He also wrote a satirical piece imagining a lunar-based perspective on Earth's astronomy, a device that reveals a playful, imaginative side balanced against his more severe scientific temperament. (10)

RELATIONSHIPS Kepler married twice. On April 27, 1597, he married his first wife, Barbara Mühleck (Müller), then 23, in a match arranged by friends and matchmakers; the marriage was reportedly unhappy owing to her difficult temperament, and she bore him five children, only two of whom survived, before her death in 1611.

Oil on copper portraits of Kepler and his wife, Barbara, c. 1600

In 1613, at age 41, Kepler married Susanna Reuttinger, then 24, having methodically evaluated eleven proposed candidates before choosing her; this second marriage, which produced seven children of whom three died young, was said to be considerably happier. (11) (3)

MONEY AND FAME Kepler's income was frequently precarious, and he supplemented his salary throughout his career by casting horoscopes and almanac forecasts for patrons and clients, a practice he privately regarded with some disdain even as he relied on it financially. 

His scientific reputation grew steadily through his roles as Imperial Mathematician to Emperor Rudolf II and adviser to General Wallenstein, though fame did not translate into lasting financial security, and he died while traveling to collect debts owed to him. (12)

FOOD AND DRINK Kepler was born and raised in the wine-growing region of Swabia, and his father kept a tavern in which the young Johannes reportedly worked as a pot-boy for a period after being withdrawn from school. (13)

His adult habits leaned toward austerity, driven as much by chronic financial hardship as personal taste: in a startlingly candid self-portrait written at age 25, comparing himself to a dog, Kepler noted that his own appetites were dog-like — "he liked gnawing on bones and dry crusts of bread, and was so greedy that whatever he saw he grabbed; yet like a dog he drinks little and is content with the simplest food." Chronic stomach ailments and fevers plagued him throughout his life, though no surviving source directly links these complaints to specific foods or to alcohol. (14)

MUSIC AND ARTS Kepler held a lifelong fascination with music, which played a central role in his astronomical theory of celestial harmony. In Harmonice Mundi, he mapped the movements and speed ratios of planets to musical intervals, arguing that planetary motion follows music's template; for example, the ratio between Jupiter's maximum speed and Mars's minimum speed corresponds to a minor third, while that between Earth and Venus corresponds to a minor sixth. (1)

LITERATURE Kepler is sometimes described as a father of science fiction for his novel Somnium (The Dream), which imagined a journey to the Moon and used the device to explain astronomical principles from a lunar viewpoint. It was published posthumously in 1634

He also wrote extensively in Latin across astronomy, optics, and mathematics, and left behind a substantial body of personal correspondence that offers rare insight into his inner life. (10)

Reproduction of 1634 title page of Somnium (The Dream)

NATURE Kepler held a deep awe for the natural world, viewing the entire cosmos as a reflection of an underlying divine and mathematical harmony. 

He integrated his religious convictions directly into his understanding of nature, regarding the Sun as a physical image of God's power and light. This philosophical view drove his exploration of geometry, the five regular polyhedra, and the close packing of spheres, which he applied to explain natural structures like the hexagonal shape of honeycomb cells. 

Alongside his broader planetary work, Kepler investigated a wide array of natural phenomena, studying atmospheric optics, the mechanics of human eyesight, and snow crystal structures, the latter leading to his pioneering 1611 treatise On the Six-Cornered Snowflake. (5) (15)

HOBBIES AND SPORTS Kepler's chronic ill health, including weak eyesight and general physical frailty, ruled out strenuous physical activity, and his intellectual energies were absorbed almost entirely by mathematics, astronomy, and astrology. He did, however, pursue practical inventive projects as something of a side interest, including efforts to design and patent an improved water pump. (5)

ASTRONOMY Johannes Kepler spent roughly forty years making a career out of trying to persuade the heavens to behave mathematically. It was not a straightforward undertaking. He worked through religious persecution, political upheaval, financial uncertainty and, for several years, the deeply irritating planet Mars. Yet from this rather unpromising mixture emerged the three laws of planetary motion that helped transform astronomy into modern science.

Kepler's professional career began in a slightly unexpected place: not an observatory, but a classroom. In April 1594, aged 23, he took a post teaching mathematics at the Protestant school in Graz, Austria. His original ambition had been to become a Lutheran minister, but life, as it occasionally does, had other plans.

While in Graz he produced his first important work, Mysterium Cosmographicum (1596). Kepler proposed that the distances between the planets could be explained by nesting the five Platonic solids — a wonderfully elaborate solution to a problem that, unfortunately for him, nature had not actually set. The idea was wrong, but it demonstrated his fierce commitment to the Copernican model of a Sun-centred universe.

Kepler's Platonic solid model of the Solar System, from Mysterium Cosmographicum (1596)

Graz was becoming increasingly uncomfortable for Protestants, however, and by 1600 the pressure had become impossible to ignore. Kepler refused to convert to Catholicism, and he and his family were effectively expelled. He therefore packed up and moved to Prague, which was already home to one of Europe's most remarkable astronomers.

That astronomer was Tycho Brahe, the Danish nobleman with an enormous collection of astronomical observations and, by all accounts, an enormous personality to go with it.

Kepler joined Brahe near Prague in 1600. His principal assignment was to investigate the orbit of Mars, which was proving particularly troublesome. Mars had the unfortunate habit of refusing to fit neatly into the circular planetary orbits that astronomers had been using for centuries. Kepler was given Brahe's observations and set about making sense of them.

He did not have long with his new employer. Brahe died unexpectedly in October 1601, leaving Kepler with something far more valuable than a comfortable working relationship: access to the painstaking observations that would eventually unlock the true shape of the planetary orbits.

After Brahe's death, Kepler succeeded him as Imperial Mathematician to Emperor Rudolf II, an extraordinarily prestigious appointment. It also gave him access to the observations he needed for his increasingly obsessive investigation of Mars.

The next twelve years were astonishingly productive. Kepler published Astronomiae Pars Optica (1604), an important study of optics; De Stella Nova (1606), concerning the new star, or supernova, he had observed in 1604; and Astronomia Nova (1609), which contained his first two laws of planetary motion.

The great breakthrough was that planets did not travel in perfect circles after all. Their orbits were ellipses, with the Sun occupying one focus. This sounds obvious now, but Kepler reached it only after years of calculations, revisions, false starts and what has become known as his "war with Mars." It was, in effect, a six-year argument with a planet, and Mars was winning for much of it.

Kepler also made important advances in optics. His Dioptrice (1611) helped explain the discoveries being made with telescopes and described the design of what became known as the Keplerian telescope.

The death of Rudolf II in 1612 brought another change. Kepler moved to Linz in Upper Austria, where he became district and provincial mathematician. He remained there until 1626, although political and religious turmoil made the place progressively less congenial.

Despite the troubles, Kepler continued working. In 1618 he discovered his third law of planetary motion: the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. He published it in Harmonice Mundi in 1619.

He also produced the Epitome Astronomiae Copernicanae (1618–1621), a systematic presentation of Copernican astronomy incorporating all three laws. The work became influential throughout Europe and helped establish the mathematical framework that would later prove so useful to Isaac Newton.

Eventually, however, religious and political upheaval associated with the Counter-Reformation made life in Linz untenable. In 1626 Kepler moved on again, this time to Ulm.

In Ulm, Kepler finally completed one of the projects that had occupied him for years. In 1627 he published the Rudolphine Tables, combining Tycho Brahe's extraordinarily precise observations with Kepler's new mathematical model of elliptical planetary orbits.

The result was the most accurate set of astronomical tables yet produced. It was also a fitting culmination of the unlikely partnership between Brahe, the great observer, and Kepler, the man who spent years trying to make sense of what Brahe had observed. One supplied the numbers; the other supplied the explanation. Science has rarely looked so much like a very long group project.

From 1628 Kepler also served as an adviser to General Albrecht von Wallenstein during the Thirty Years' War. Among other things, he provided astrological calculations for his patron, a reminder that even one of the founders of modern astronomy lived in a world where astronomy and astrology had not yet completely gone their separate ways.

Kepler's career ended, rather unglamorously in 1630, when he died in Regensburg while travelling to collect money that was owed to him. After a lifetime spent calculating the movements of planets, he was still having difficulty getting people on Earth to pay their bills.

Across four turbulent decades, Kepler had transformed astronomy. The old system had insisted that the heavens must move in perfect circles because circles were considered philosophically superior. Kepler eventually abandoned the philosophy in favour of what the observations were actually telling him. The planets travelled in ellipses, at changing speeds, according to mathematical relationships that could be measured and tested.

It was a decisive change in the way humanity understood the universe. Kepler supplied the laws; Newton would later explain the force behind them. Together, they helped turn astronomy from a subject in which the heavens were expected to conform to elegant assumptions into a science in which the heavens were allowed, at last, to have the last word. 

SCIENCE AND MATHS Kepler made fundamental contributions to mathematics and optics. He developed early methods of integration (infinitesimal calculations) to compute the volumes of wine barrels (Nova Stereometria Doliorum), which contributed to the later development of calculus. 

In optics, he correctly explained how light enters the human eye, how the retina projects inverted images, and how lenses correct near-sightedness and far-sightedness. 

PHILOSOPHY & THEOLOGY Brought up in the Protestant faith, Kepler had originally intended to become a minister before circumstances led him toward mathematics and astronomy instead. He believed his scientific discoveries revealed the divine order underlying the universe, famously stating, "Science is thinking God's thoughts after him," and he wrote of his convictions, "I believe only and alone in the service of Jesus Christ. In him is all refuge and solace." He remarked that his discoveries "may well wait a century for a reader, as God has waited 6,000 years for an observer," reflecting his confidence that his work served a divine as well as scientific purpose. (16) (17)

POLITICS Kepler operated largely within the patronage system of the Holy Roman Empire, serving as Imperial Mathematician under Emperor Rudolf II and his successors, and later as an adviser to General Wallenstein during the Thirty Years' War, providing astronomical calculations for Wallenstein's astrologers. As a Protestant working within a predominantly Catholic imperial court, he navigated considerable religious and political tension throughout his career, particularly as the confessional conflicts of the era intensified. 

SCANDAL In 1615, at the height of his scientific career, Kepler's mother Katharina was accused of witchcraft in Leonberg. Her defense initially relied on local lawyers, and the trial dragged on for years through delays and procedural complications until, in 1620, a concerned Kepler took a more active role, using his scholarly connections to gather evidence refuting the charges and writing letters and petitions on her behalf. Katharina was ultimately acquitted in 1621, narrowly avoiding execution at the stake, though she died just six months later, reportedly worn down by the ordeal. (17)

MILITARY RECORD Kepler was closely connected to military affairs through his role as an adviser to General Wallenstein from 1628 onward during the Thirty Years' War, supplying astrological and astronomical calculations relevant to Wallenstein's campaigns. (7)

HEALTH AND PHYSICAL FITNESS Kepler was sickly from birth as a premature child and nearly died of smallpox around age four, an illness that left him with crippled hands and permanently impaired, multiple vision. 

Throughout his life he suffered from headaches, sensitivity to cold, hemorrhoids that made calculation while seated difficult, and other chronic ailments, some of which historians have suggested may have included an element of hypochondria. He was also reportedly reluctant to bathe, going as long as eight years at one point before his first wife persuaded him to do so. (3) (18)

HOMES Kepler's life was markedly itinerant, shaped by religious upheaval and shifting patronage: he lived successively in Weil der Stadt, Leonberg, Tübingen, Graz, Prague, and later Linz, before his final years took him to Ulm, Sagan, and finally Regensburg. This constant relocation reflected the instability of Protestant scholars working within the religiously and politically fractured Holy Roman Empire during the era of the Thirty Years' War. 

House of Kepler and Barbara Müller in Gössendorf, near Graz (1597–1599) by Mrbeachguide

TRAVEL Kepler's travels were largely dictated by his career and patronage, taking him from Graz to Prague to work with Tycho Brahe in 1600, and later to Linz and other imperial territories as his position and the ongoing war required. His final journey, in 1630, took him to Regensburg to collect interest owed to him on Austrian bonds, a trip from which he never returned. (12)

In 1608, he introduced the concept of solar sails, writing: "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void."

DEATH Johannes Kepler died in the southeastern German city of Regensburg on November 15, 1630, at the age of 58, apparently following a brief feverish illness. His burial site in Regensburg was lost after Swedish forces destroyed the churchyard during the ongoing Thirty Years' War, and only his own self-composed poetic epitaph survives: "I measured the skies, now the shadows I measure / Skybound was the mind, earthbound the body rests." (17)

APPEARANCES IN MEDIA  Kepler has been featured across modern media and scientific tributes:

Film: The biographical film Johannes Kepler (1974) produced in East Germany, and the 2002 film Eye of the Astronomer.

Literature: John Banville's 1981 historical novel Kepler, which dramatizes his life and scientific struggles.

Spaceflight: NASA named its planet-hunting space telescope the Kepler Space Telescope, launched in 2009 to search for exoplanets.

Astronomy: Features named in his honor include the lunar crater Kepler, the Martian crater Kepler, and the asteroid 1134 Kepler.

ACHIEVEMENTS Kepler's three laws of planetary motion remain foundational to modern astronomy and provided one of the essential building blocks for Isaac Newton's theory of universal gravitation. 

He also advanced the science of optics through his work on the Keplerian telescope and the study of light within the eye.

Kepler wrote what is considered an early work of science fiction in Somnium, 

He anticipated the modern concept of solar sails more than three centuries before their practical realization.