Johannes Kepler

Johannes Kepler, a German scholar who lived from 1571 to 1630, worked across astronomy, mathematics, astrology, natural philosophy, and music theory. He played a central role in the 17th-century Scientific Revolution, most famous for formulating the laws that describe how planets move. His major written works include *Astronomia nova*, *Harmonice Mundi*, and *Epitome Astronomiae Copernicanae*. Because his contributions were so broad and influential, he is considered a founding figure in modern astronomy, the scientific method, natural science, and modern science itself. He also earned the title "father of science fiction" thanks to his novel *Somnium*.

Kepler began his career teaching mathematics at a seminary school in Graz, where he became connected to Prince Hans Ulrich von Eggenberg. He later worked as an assistant to the astronomer Tycho Brahe in Prague, then served as imperial mathematician under Emperor Rudolf II and his successors Matthias and Ferdinand II. He also taught mathematics in Linz and advised General Wallenstein.

During Kepler’s lifetime, astronomy and astrology were not clearly separated, but a sharp divide existed between astronomy (a mathematical branch of the liberal arts) and physics (a part of natural philosophy). Religious ideas shaped his work; he believed God had created the universe according to a rational plan that human reason could grasp. He called his new astronomy "celestial physics," describing it as an "excursion into Aristotle's Metaphysics" and a "supplement to Aristotle's On the Heavens." This approach transformed ancient cosmology by treating astronomy as part of a universal mathematical physics. He also made fundamental advances in optics, earning recognition as the father of modern optics, especially for his work *Astronomiae pars optica*. He invented an improved refracting telescope—the Keplerian telescope—which became the basis for modern refracting telescopes, building on Galileo’s earlier design. Galileo himself mentioned Kepler’s discoveries. Kepler also proposed what is now known as the Kepler conjecture. His work influenced Isaac Newton, providing a foundation for Newton’s theory of universal gravitation.

**Early life**

**Childhood (1571–1590)**

Kepler was born on 27 December 1571 in the Free Imperial City of Weil der Stadt, now part of the Stuttgart Region in Baden-Württemberg, Germany. His parents were Lutheran, but he was likely baptized a Catholic because Protestant baptisms were not allowed in Weil at that time. His grandfather, Sebald Kepler, had served as the city’s Lord Mayor. By Johannes’s birth, the family’s fortunes had declined. His father, Heinrich Kepler, worked as a mercenary, barely making a living, and left the family when Johannes was five. He is believed to have died in the Eighty Years’ War in the Netherlands, fighting in the Catholic Spanish army despite being a Protestant. His mother, Katharina Guldenmann, was the daughter of an innkeeper and worked as a healer and herbalist. Johannes had six siblings, but only two brothers and one sister lived to adulthood. Born prematurely, he later described himself as weak and sickly as a child. Still, he often amazed visitors at his grandfather’s inn with his extraordinary mathematical ability.

He encountered astronomy early in life and developed a lasting passion for it. At age six, he saw the Great Comet of 1577, writing that his mother took him to a high place to watch it. At nine, he observed a lunar eclipse, recalling that he was called outside and saw the Moon appear quite red. However, childhood smallpox left him with poor eyesight and crippled hands, limiting his ability to observe the skies directly.

Kepler attended the Grammar School in Weil until 1577, when his family moved to Leonberg in Protestant Württemberg. He spent a year at the elementary German school there, then moved to the Latin Grammar School, where all lessons and books were in Latin. He later attended two monastic schools: from 1584 at Adelberg, and from 1586 at the seminary in Maulbronn. In September 1589, he entered the Tübinger Stift at the University of Tübingen, a seminary that trained Lutheran pastors for Württemberg. There, he studied philosophy under Vitus Müller and theology under Jacob Heerbrand, a student of Philipp Melanchthon who had also taught Michael Maestlin. Heerbrand became Chancellor at Tübingen in 1590. Kepler proved to be an excellent mathematician and gained a reputation as a skilled astrologer, casting horoscopes for other students. Under Michael Maestlin, Tübingen’s mathematics professor from 1583 to 1631, he learned both the Ptolemaic and Copernican systems of planetary motion. He became a Copernican during this time. In a student debate, he defended heliocentrism on both theoretical and theological grounds, arguing that the Sun was the main source of motive power in the universe. Although he wanted to become a Lutheran minister, he was denied ordination because his beliefs conflicted with the Formula of Concord, the Lutheran statement of faith adopted in 1577. Near the end of his studies, he was recommended for a teaching position in mathematics and astronomy at the Protestant school in Graz, Styria, Inner Austria. He accepted the post in April 1594, at age 22.

**Graz (1594–1600)**

While in Graz from 1594 to 1600, Kepler issued many official calendars and prognostications, which boosted his reputation as an astrologer. He had mixed feelings about astrology and criticized many common astrological practices.