[Scientist Biography Series] Kepler and His Dream of the Heavens

If Stefanie Sun’s song ‘Kepler’ had not brought the name back into popular culture, I suspect quite a few people would still have no idea who Kepler was XD.

But anyone who has studied Kepler’s laws in high-school physics will quickly notice that the song and the astronomer share little beyond stars and planets. There is no ellipse, no area law, no three laws of planetary motion—nothing of the sort XDD.

Johannes Kepler (December 27, 1571–November 15, 1630) was a German astronomer and mathematician and one of the founders of modern astronomy. He is best known for the three laws later distilled from works including Astronomia Nova, Harmonices Mundi, and Epitome Astronomiae Copernicanae. His work would also help inspire Newton’s theory of universal gravitation.

Early Life and Childhood

Kepler was born on December 27, 1571, in the Free Imperial City of Weil, now Weil der Stadt in the German state of Baden-Württemberg, about 30 kilometers west of central Stuttgart.

His grandfather, Sebald Kepler, had once served as the town’s mayor. By the time Johannes was born, however, the Kepler family’s fortunes were already in decline.

His father, Heinrich Kepler, made a dangerous living as a mercenary and left the family when Johannes was five. He is believed to have later died in the Eighty Years’ War in the Netherlands. Kepler’s mother, Katharina Guldenmann, was an innkeeper’s daughter as well as a healer and herbalist.

Kepler showed an early interest in astronomy and mathematics. As a child he saw the Great Comet of 1577 and later a lunar eclipse, and both events left a deep impression on him.

Unfortunately, childhood smallpox left Kepler with poor vision and impaired hands, limiting his ability to make astronomical observations himself. His family was not well-off, and his physical condition also narrowed his prospects, so his parents sent him to school in the hope that he could eventually become a minister and at least secure a basic livelihood.

Education

With the ministry in mind, Kepler entered the Lutheran theological education system at Tübingen and studied theology. Once he entered the seminary, his exceptional abilities became increasingly obvious.

Under Michael Mästlin, professor of mathematics at the University of Tübingen from 1583 to 1631, Kepler studied both the Ptolemaic model of planetary motion and the Copernican system. In one student debate, he defended heliocentrism on both theoretical and theological grounds, arguing that the Sun was the principal source of cosmic motive power.

Apparently, Kepler also earned something of a reputation among his classmates as a skilled astrologer.

Teaching and Research

At the age of twenty-three, Kepler was recommended for a position teaching mathematics and astronomy at the Protestant school in Graz, an institution that would later become the University of Graz.

While teaching, he continued to study astrology and astronomy in his spare time. He believed he had found a relationship between the regular polyhedra and the distances of the planets, turned the idea into the wonderfully mystical Mysterium Cosmographicum, and sent a copy to the famous Danish astronomer Tycho Brahe.

Meeting Tycho Brahe

By 1600, Counter-Reformation policies and the expulsion of Protestants made it impossible for Kepler, a Protestant, to remain in Graz. Tycho had taken an interest in Mysterium Cosmographicum, so Kepler went to Prague and became his assistant and collaborator.

Since Tycho has entered the story, we might as well pause and talk about him too.

Tycho Brahe (December 14, 1546–October 24, 1601) was a Danish astronomer, astrologer, and alchemist.

Tycho’s interest in astronomical observation began when, at thirteen, he saw astronomers accurately predict a solar eclipse in 1560. He became fascinated by how such a prediction was possible and decided to devote himself to watching the sky—basically staying up night after night to stare at the stars.

By seventeen, Tycho had found serious positional errors in the astronomical tables of his day and decided to rebuild them. King Frederick II later funded one of the world’s most advanced observatories for him. Yet about twenty years later, a new king came to power and forced him to leave anyway. Orz.

No one could have guessed that the tables Tycho cared about so deeply would become one of the great unfinished tasks hanging over Kepler for the rest of his life.

Fortunately, Holy Roman Emperor Rudolf II later took Tycho in, allowing him to continue his observations in Prague.

Without a telescope, Tycho still pushed positional astronomy to an accuracy of roughly one to two arcminutes. That achievement was not merely a matter of good eyesight: it depended on enormous instruments, repeated calibration, and decades of records. It was genuinely astonishing.

Late in life, Tycho knew that his health was failing and that someone would have to continue his work. He had seen Kepler’s book, so he invited Kepler to join him and carry that work forward.

One astronomer had pushed naked-eye observation to the limit; the other wanted to explain celestial motion through mathematics and physical causes. Their meeting opened the door to the next stage of astronomy.

After Tycho died unexpectedly in 1601, Kepler succeeded him as imperial mathematician and took responsibility for completing the Rudolphine Tables. But Tycho’s data did not simply pass into his hands without dispute: Kepler and Tycho’s family continued to argue over rights to use the observations.

Tycho did not support the traditional Ptolemaic system. His hybrid geoheliocentric model kept Earth stationary, placed the Sun in orbit around Earth, and placed the other planets in orbit around the Sun. Kepler, by contrast, supported Copernican heliocentrism. Even with that theoretical divide, Kepler spent much of his life organizing Tycho’s observations, completing the Rudolphine Tables, and at times defending Tycho’s research in print. This scientific relay was not a perfectly harmonious master-and-student story—but the collision between observation and theory is exactly what made it so important.

Later Research and Kepler’s Three Laws of Planetary Motion

After taking over Tycho’s position, Kepler began an extraordinarily productive period of research.

In 1604, a brilliant new star appeared in the direction of Ophiuchus—the object now called Kepler’s Supernova. Kepler tracked changes in its position, brightness, and color over an extended period and published De Stella Nova in 1606. The supernova later took his name in recognition of that work.

During this period, he also completed Astronomiae Pars Optica, published in 1604. It examined optical problems in astronomical observation and offered an important account of how light forms an inverted image on the retina. Saying that everything we know about the eye rests entirely on Kepler would obviously be a little much XD, but his contribution to early modern optics and vision was genuinely foundational.

In 1611 he published Dioptrice, extending his work on lenses and refraction and proposing a telescope made from two convex lenses—the design later known as the Keplerian telescope.

Although Kepler inherited Tycho’s position, Tycho’s family remained deeply concerned about ownership of the observations, and the two sides argued for years over data and publication rights. Fortunately, Tycho had already assigned Kepler the most troublesome task of all: the orbit of Mars. That long war with Mars became the key to breaking free from circular orbits.

From Ptolemy through Copernicus and Tycho, astronomical systems had assumed that the heavens moved according to the most perfect forms and rules: celestial bodies were spheres, their paths were circles, and their motion was circular.

Kepler believed the same thing at first. But when he pressed Tycho’s Mars observations into circular models, the numbers still missed by as much as about eight arcminutes. That is only around 0.13 degrees, but it was far too large to ignore beside the precision of Tycho’s observations. Kepler trusted both the data and his own calculations.

He eventually abandoned uniform circular motion and tried an elliptical orbit, which finally matched the observations. By 1605, Kepler had at last ‘conquered Mars,’ replacing layers of uniform circles, concentric spheres, and epicycles with a simpler account of the orbit.

Kepler’s first law states that a planet travels around the Sun in an elliptical orbit, with the Sun at one focus of the ellipse.

Many astronomical models of the time focused mainly on calculating where a planet would appear. Kepler also tried to bring physical causes into astronomy. He found that the line joining a planet to the Sun sweeps out equal areas in equal times: Kepler’s second law. And no, this does not mean uniform circular motion—otherwise all that work abandoning the perfect circle would have been for nothing XD.

Even while fighting with the orbit of Mars, Kepler never forgot the astronomical tables Tycho had left unfinished.

In 1611, Kepler’s beloved son died of illness. Emperor Rudolf died not long afterward, while political instability and religious conflict continued to intensify.

Kepler was forced to leave Prague and moved to Linz, in present-day Austria, where he worked as a mathematician and teacher. There he pursued the harmonies of the cosmos and discovered the relationship now called Kepler’s third law, or the harmonic law: the square of a planet’s orbital period is proportional to the cube of its orbit’s semi-major axis. He published it in Harmonices Mundi, and it would later help guide Newton toward universal gravitation.

None of this made Kepler forget Tycho’s tables. He remained in Linz until 1626, when the Counter-Reformation, the Thirty Years’ War, and the Upper Austrian Peasants’ War forced him to leave. He then went to Ulm to oversee the printing and finally published the Rudolphine Tables in 1627. The tables joined Tycho’s precise observations to Kepler’s planetary laws, completing a project that had stretched across both men’s lives.

Later Life

By his later years, Kepler had still not received much of the salary owed to him by the imperial court, and he lived under severe financial pressure.

At one point he developed a high fever and, believing that he might not have long to live, wrote a few lines of verse and asked for them to be sent home.

In 1630, Kepler traveled to Regensburg to collect years of unpaid salary and other debts. He became ill shortly after arriving and died there on November 15. He was buried in a Protestant cemetery, but the grave was later destroyed during the Thirty Years’ War. The epitaph he wrote for himself, however, survived:

“Mensus eram coelos, nunc terrae metior umbras Mens coelestis erat, corporis umbra iacet.”

“I measured the heavens; now I measure Earth’s shadows. My mind belonged to the heavens; here lies the body’s shadow.”

Conclusion

When I first saw the song title, I was ridiculously excited. ‘Has a singer finally decided to promote science education through pop lyrics?’ I wondered.

Also me, a moment later: ‘…Where is the science…?’ And that is how this biography of Kepler came into existence XD.

Kepler strikes me as someone with an extraordinary sense of loyalty. He carried Tycho’s tables with him for much of his life, always wanting to finish the work his predecessor had left behind.

Reading this far, you may also feel that scholars in earlier centuries rarely had an easy life. Yet anyone who truly wanted to study needed powerful motivation and deep interest to stay on that road, which is one reason their achievements can feel so remarkable and moving.

Really, the same principle applies to almost any path. Enough motivation and interest can carry a person an astonishing distance. I hope every reader eventually finds a path that feels like their own :D.

References

Entering the Hall of Fame: Kepler, the Legislator of the Heavens, URL: http://technews.tw/2014/08/10/kepler-biography/

A Dialogue Between Galileo and Kepler, URL: http://memo.cgu.edu.tw/yun-ju/CGUWeb/SciKnow/PhyStory/Galileo/GalileoKeplerLetter.htm

Johannes Kepler (1571–1630) – Biography, URL: https://mathshistory.st-andrews.ac.uk/Biographies/Kepler/

Orbits and Kepler’s Laws, URL: https://science.nasa.gov/solar-system/orbits-and-keplers-laws/

Planetary Motion: The History of an Idea That Launched the Scientific Revolution, URL: https://science.nasa.gov/solar-system/planetary-motion/

Chapter 3: Gravity & Mechanics, URL: https://science.nasa.gov/learn/basics-of-space-flight/chapter3-3/

Johannes Kepler, URL: https://plato.stanford.edu/entries/kepler/

Johannes Kepler, URL: https://nmspacemuseum.org/inductee/johannes-kepler/