Johannes Kepler | 10 Facts About The German Astronomer


Johannes Kepler was a German astronomer who is most famous for his three laws of planetary motion. Born in a poor family, Kepler was a leading mathematician and astrologer of his time. He held the position of Imperial Mathematician, the most prestigious appointment in mathematics in Europe. Kepler had a tumultuous relationship with Danish astronomer Tycho Brahe; but it was the accurate planetary data of Brahe that enabled Kepler to make his great discoveries in astronomy. Johannes Kepler married twice and had more than 10 children, but few among them survived till adulthood. He was deeply religious, wanted to be a theologian initially and looked for God’s design in science and nature. Know more about the family, life, education, religion, scientific career and death of Johannes Kepler through these 10 interesting facts.


#1 He was born prematurely and was a feeble child with weak vision

Birthplace of Johannes Kepler
Birthplace of Johannes Kepler in Weil der Stadt, Germany

Johannes Kepler was born on 27th December, feast day of St John the Evangelist, 1571 in the town of Weil der Stadt, a free and imperial city subordinate only to the Holy Roman Emperor. Weil der Stadt is now a part of the Stuttgart Region in the German state of Baden-Württemberg. Johannes was the son of Heinrich Kepler, a mercenary who had fought in the Belgian war, and his wife Katharina Guldenmann, a healer and herbalist. He had three elder siblings; two brothers named Christoph and Heinrich; and a sister named Margaretha. Johannes was born prematurely and was a feeble child with weak vision. When he was five, his father Heinrich Kepler left to fight in the Eighty Years’ War, in the Netherlands. He never returned home and is believed to have died there.

#2 He was introduced to astronomy at an early age by his mother

Though his grandfather Sebald Kepler was a respected craftsman who served as mayor of the city, the Kepler family had became poor by the time Johannes was born. At the age of six, Johannes was sent to school, but was soon taken out to earn money for the family by working as a waiter in an inn. He was introduced to astronomy at an early age as his mother took him out at night to show him interesting things in the skies, including the Great Comet of 1577 and a lunar eclipse. Johannes was formally schooled in Latin and then attended the Protestant Seminary of Maulbronn. In 1589, at the age of 18, a scholarship enabled him to begin his university education at the Protestant University of Tübingen.

#3 He wrote the first defense of the heliocentric model of Copernicus

At the University of Tubingen, the professor of mathematics was Michael Maestlin, one of the most talented astronomers in Germany. Although at the university lectures Maestlin only taught the Ptolemaic system which placed earth at the center of the universe, he was one of the first to accept the heliocentric Copernican view. Maestlin corresponded with Kepler frequently and played a key role in Kepler believing in the Copernican system. He even lent Kepler his own heavily annotated copy of Copernicus’s masterpiece, De revolutionibus. In a student disputation, Kepler defended heliocentrism from both a theoretical and religious perspective. Though he had planned to become a theologian, in 1594 at the age of 23, Kepler became a lecturer in astronomy and mathematics at the Protestant school in the city of Graz, Austria. In 1596, while a teacher in Graz, he wrote the first published defense of the Copernican system, Mysterium Cosmographicum.

Johannes Kepler 1610 Portrait
1610 Portrait of Johannes Kepler


#4 He married twice and had a number of children

In December 1595, Kepler began courting Barbara Müller, a 23-year-old widow. Barbara was the daughter of a successful mill owner and her father opposed the match as Kepler was poor. However, he relented later, and Johannes Kepler was married to Barbara Muller on April 27, 1597. After their first two children died in infancy, they had a daughter Susanna (1602); and two sons Friedrich (1604) and Ludwig (1607). In 1611, Kepler’s wife Barbara and his son Friedrich died due to illness. Kepler is believed to have had an unpleasant marriage with Barbara. Following her death, he considered 11 different matches over two years. He returned to the fifth match as she won him over with “love, humble loyalty, economy of household, diligence, and the love she gave the stepchildren.” On October 30, 1613, Johannes Kepler married the 24-year-old Susanna Reuttinger. Their first three children died in childhood but the next three survived, Cordula (1621), Fridmar (1623) and Hildebert (1625). Kepler’s second marriage is considered successful.

Johannes Kepler and his wife Barbara Muller
Portraits of Johannes Kepler (right) and his first wife Barbara Muller (left)


#5 He served as Imperial Mathematician to the Holy Roman Emperor

Tycho Brahe was a renowned Danish astronomer known for his accurate planetary observations. He was Imperial Mathematician to the Holy Roman Emperor, the most prestigious appointment in mathematics in Europe. However, Tycho was known to be notoriously quarrelsome and Kepler had send a laudatory letter to Nicolaus Reimers, a bitter rival of Brahe. In February 1600, Tycho invited Kepler to be his assistant but the two didn’t get along. However, each knew the value of the other and ultimately they reached an agreement. Kepler moved to Prague the same year and through most of 1601 he worked as Tycho’s assistant. They continuously fought during this time as Tycho refused to share all his meticulous observations with Kepler. Tycho died mysteriously on October 24, 1601 and two days later Kepler was appointed his successor as Imperial Mathematician. He served in the post under Emperor Rudolf II and his two successors Matthias and Ferdinand II.

Monument to Tycho Brahe and Johannes Kepler in Prague
Monument to Tycho Brahe and Johannes Kepler in Prague, Czech Republic


#6 Kepler is most famous for his three laws of planetary motion

Using the accurate data of Tycho Brahe, Johannes Kepler discovered his three laws of planetary motion that describe the motion of planets around the Sun. Kepler’s 1609 work, Astronomia nova, records the discovery of the first two of the three planetary laws while the third law was first published in his 1619 work Harmonices Mundi. Among other things, his laws state that planets move in elliptical orbits; and at different speeds at different times, according to their distance from the sun. Before Kepler it was believed that planets moved in circular orbits and at constant speed; and it was only several decades later when Isaac Newton derived them from his set of general physical principles that Kepler’s findings were established as laws. Kepler’s laws of planetary motion were hugely influential and laid the foundation of modern astronomy and physics.

Kepler's three laws of planetary motion
Kepler’s three laws of planetary motion


#7 He considered his three laws as celestial harmonies that reflected God’s design

Statue of Katharina Kepler
Statue of Katharina Kepler in Eltingen, Germany

Johannes Kepler incorporated religious arguments and reasoning into his work. He believed that God had created the world according to an intelligible plan. He regarded his three laws of planetary motion as celestial harmonies that reflected God’s design for the universe. In his famous work Harmonices Mundi, he found harmonies in nature to claim that the Earth has a soul because it is subjected to astrological harmony. It was while finding these harmonies that Kepler discovered what came to be known as the third law of planetary motion: that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. Kepler seeking a hand of God in his discoveries was in keeping with his time. Isaac Newton, who was to build on his work, also dedicated more time to theology than mathematics or physics.

#8 His mother was accused and tried for witchcraft

Christoph, brother of Johannes Kepler, was involved in a financial dispute with a woman named Ursula Reingold. Ursula claimed that Kepler’s mother Katharina had made her sick with an evil brew. The dispute escalated and Katharina was accused of witchcraft. It was claimed that she had been instructed in magic by an aunt, who had in fact been burned for sorcery. There was a witch hunt going on in the region and Katharina was one of 15 women who were accused. Johannes Kepler, who prepared an extensive defense for his mother, played a part in securing her release. One of his student friends, Christopher Besoldus, assisted her juridically. Katharina was imprisoned in August 1620 but released 14 months later in October 1621. She died the following year.


#9 He had to undergo hardships due to Counter Reformation measures

Johannes Kepler 1971 German stamp
1971 German stamp featuring Johannes Kepler

Johannes Kepler was a Lutheran. Lutheranism is a major branch of Protestant Christianity. In 1600, when Kepler was seeking appointment as assistant to Tycho, all Protestants in Graz were forced to convert to Catholicism or leave the province, as part of Counter Reformation measures. As Kepler refused to convert, he and his family were banished from Graz. While he served as Imperial Mathematician under Rudolf II in Prague, he was allowed to practice his faith due to his position although the only acceptable religious doctrines were Catholic and Utraquist. Still he didn’t enjoy complete religious freedom. After the death of Rudolf II, Kepler moved to Linz in 1612. Though he enjoyed more religious freedom here, still he was excluded from the sacrament in the Lutheran church as he refused to sign a Lutheran statement of faith known as Formula of Concord. In 1618, Counter Reformation measures put pressure on Protestants in Linz. Though Kepler was exempted from banishment, he still suffered persecution.

#10 Kepler laid the foundation of modern optics

In his last years, Kepler traveled a lot, from Prague to Linz and Ulm to a temporary home in Sagan, and finally to Regensburg. Soon after arriving in Regensburg, he became seriously ill with fever. Johannes Kepler died on 15th November 1630 in Regensburg and was buried there. His grave was lost after the Swedish army destroyed the churchyard during the Thirty Years’ War (1618 – 1648). Apart from his laws of planetary motion, Kepler made important contributions to optics including formulation of the inverse-square law governing the intensity of light; inventing an improved refracting telescope; and correctly explaining the working of a human eye. He is considered to have laid the foundation of modern optics. As founder of celestial mechanics and optics, he is considered one of the most influential scientists in history.

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