Johannes Kepler

German astronomer

• Born: December 27, 1571
• Birthplace: Weil, Swabia (now Weil der Stadt, Germany)
• Died: November 15, 1630
• Place of death: Regensburg, Bavaria (now in Germany)

Through the application of his exceptional intellect, faith, and tenacity, Kepler created the science of modern astronomy—the fusing together of physics and astronomy—and provided the solid foundation upon which Isaac Newton built his laws of universal gravitation. He also is considered the father of modern optics.

Early Life

Johannes Kepler (yoh-HAHN-ehs KEHP-lehr) was born prematurely, the first child of Heinrich and Katharine Kepler, and his childhood was exceptionally difficult. He was small and sickly, with thin limbs and a large, pasty face surrounded by dark hair. It is one of the ironies of history that the child who would one day revolutionize astronomy had poor eyesight. His mother was a small woman with a nasty disposition; his father appears to have had no established trade and, in 1574, simply left the family to fight for the Catholic duke of Alva in the Netherlands. His mother followed a year later, leaving Kepler in the care of his grandparents, who treated him badly. In 1576, his parents returned, but this provided only a dubious improvement in his family life.

Under such circumstances, it is not surprising that Kepler’s self-image as a child was terrible; he described himself once as a “mangy dog.” Possibly in response to the instability in his life, Kepler developed a pronounced religious disposition at a young age. Indeed, of all his childhood memories only two stood out as pleasant. When he was six, Kepler’s mother took him to a hill to see a comet, and at age nine his parents took him outside to observe an eclipse of the Moon. The seeds were planted that would influence the direction of his life.

Yet it was obvious even to his parents that Kepler was a bright child, and school provided a way for him to divert himself from the suffering of family life while building his self-assurance and developing his intellect. Kepler was fortunate that the Lutheran dukes of Württemberg provided generous educational scholarships for the intelligent sons of poor parents.

Kepler began his schooling at age seven at the Latin school in Leonberg. He completed the curriculum at age twelve, and his intellect, poor health, and pious nature preordained him to a clerical career. After passing a competitive exam on October 16, 1584, Kepler continued his studies at the higher seminary at Maulbronn. On September 17, 1589, he entered the University of Tübingen, where he was particularly influenced by his professor of astronomy, Michael Mästlin. Mästlin was unique in that he believed that Nicolaus Copernicus’s heliocentric theory was essentially true. Influenced by this exceptional teacher, Kepler accepted the Copernican view of the universe—an act that would have a profound impact on his life and the future of scientific thought.

At the age of twenty, Kepler was matriculated at the Tübingen Theological School. His career path seemed assured, but circumstances fatefully intervened. In 1593, the mathematician of the Lutheran high school of Graz died, and the school requested Tübingen to recommend a replacement. Kepler was nominated and agreed to accept the position.

His job was to teach mathematics and to publish the annual calendar of astrological forecasts. Kepler was lucky with his first calendar, correctly predicting a cold spell and a Turkish invasion. He would be involved with astrology all of his professional life, but he did it primarily to supplement his income. Still, while he considered popular astrological forecasts a “dreadful” superstition, Kepler believed that astrology could become an exact empirical science, and with that aim in mind, he would write several treatises on the subject.

Kepler was a terrible teacher. He was often unintelligible, launching into obscure digressions whenever a thought occurred to him. Yet it was during one of these lectures that the event happened that set Kepler on the path that would ultimately lead him to reinterpret the prevailing view of the universe.

Life’s Work

Kepler found his first year at Graz very trying. To escape its frustrations, he turned to the astronomical studies he had experimented with at Tübingen. The more he contemplated the Copernican system, the more he became convinced of its truth, but he was also aware that it was not the final, definitive explanation of the operation of the universe.

Already in 1595, Kepler was beginning to ask the questions that would determine that course of his scientific inquiry. Why are there only six planets? What determines their distances from the sun? Why do the planets move more slowly the farther away they are from the sun? These and other queries had been coursing through his mind when, on July 19, 1595, as he was teaching, an incredible thought struck him. On the blackboard, he had drawn a figure showing an outer circle circumscribing a triangle, which enclosed an inner circle. As Kepler looked at the two circles, he was suddenly dumbfounded by the realization that the ratios of the two circles were the same as those of the orbits of Saturn and Jupiter. Could it be that there were only six planets because planetary orbits were related to the five regular solids—the tetrahedron, cube, octahedron, dodecahedron, and icosahedron—of Euclid’s geometry?

This revelation was the basis of Kepler’s first major work, Mysterium cosmographicum (1596; The Secret of the Universe , 1981). His thesis was that one of the five regular solids fit between each of the invisible spheres that carried the six planets. The key insight of this book, however, revolved around his search for a mathematical relationship between a planet’s distance from the Sun and the time necessary for it to complete its orbit. Kepler concluded that there must be a force emanating from the Sun that swept the planets around their orbits. The outer planets moved more slowly because this force diminished in a ratio to distance, just as light did. Here is found the first hint of celestial mechanics, the joining of physics and astronomy, that would lead to the laws of planetary motion.

Kepler sent copies of the work to a number of scientists in Europe, including Tycho Brahe, who had spent several years making painstakingly accurate observations of planetary orbits and who would shortly become the imperial mathematician of the Holy Roman Empire. Although he did not agree with the Copernican underpinning of Kepler’s work, Brahe was impressed by Kepler’s knowledge of mathematics and astronomy, and he invited Kepler to join his staff in the observatory at Benatek, just outside Prague. Kepler was flattered by the invitation, but he had just been married and was too poor to afford the trip. The deteriorating religious situation in Graz, in which Protestants were being forced either to convert to Catholicism or to emigrate, compelled Kepler to make a decision. In 1600, Kepler joined Brahe in Prague, where he would remain until 1612. This period would be the most productive of his life.

Kepler’s first publication after arriving at Prague, De fundamentis astrologiae certioribus (1601; the more reliable bases of astrology), while rejecting the belief that celestial bodies direct people’s lives, supported the mystical view that there is a harmony between the universe and the individual. In 1606, after observing a supernova, Kepler published De stella nova in pede serpentarii (the new star in the foot of the serpent bearer), which argued that the universe of fixed stars was not pure and changeless, as had been believed. In 1604, Kepler authored a major work on optics, Ad Vitellionem paralipomena, quibus astronomiae pars optica traditur (Optics: Paralipomena to Witelo and Optical Part of Astronomy , 2000). The main subject was the atmospheric refraction, or bending, of light as it enters Earth’s atmosphere from space. An understanding of this phenomenon was vital if Kepler was to make optimum use of Brahe’s observational data. In this work, Kepler for the first time explained the fundamental structure and function of the human eye. Although he was unsuccessful in developing the law of refraction, he was able to create an improved table of refraction.

Kepler concluded his work in optics with Dioptrice (1611; partial translation of the preface, 1880), which not only restated his concept of refraction but also covered such subjects as reflections, images, magnification, and the optical principles of the astronomical telescope. These two works have justifiably established Kepler as the father of modern optics. That same year, Kepler wrote the Dissertatiocum nuncio sidereo (1610; Kepler’s Conversation with Galileo’s Sidereal Messenger , 1965), placing his then-considerable scientific prestige in support of Galileo’s astronomical discoveries. Kepler’s great magnum opus, published in 1609 after years of painstaking effort, was the Astronomia nova (New Astronomy , 1992).

When Kepler first arrived at Prague in 1600, Brahe immediately assigned him to investigate the orbit of Mars, which Brahe and his assistant, Danish astronomer Longomontanus (Christian Severin), had been unable to determine. The selection of Mars was particularly fortuitous because of all the planets, Mars has the most elliptical orbit and, therefore, provided the best opportunity for discovering the secrets of planetary motion. Once Kepler had full use of Brahe’s observational data following Brahe’s death in 1601—for now Kepler was the imperial mathematician—he became completely immersed in this project.

As he began the study of Mars, Kepler had a thoroughly Copernican concept of planetary motion. Each planet revolved around the Sun at a uniform speed in a perfectly circular orbit, considered the perfect geometrical shape. Accordingly, Kepler first tried to show that the Martian orbit was circular. After three years of intense, repetitive calculations, Kepler believed that he had proved Mars had a circular orbit, only to discover that two of Brahe’s innumerable observations of the planet differed from Kepler’s orbit by only 8 minutes of arc. (The width of a pinhead held at arm’s length approximately equals 8 minutes of arc.) Previous scientists would have made the evidence fit the theory, but Kepler would not. Believing that God would not create anything imperfect, Kepler knew that he could not ignore those 8 minutes of arc. The orbit of Mars could not be circular but had to be some other geometrical curve.

Kepler now had to redetermine the Martian orbit, but first he had to recalculate the orbit of Earth. Earth was his observatory, and if there were any misconceptions regarding its motion, then all conclusions regarding other planetary motion would be in error. He discovered that Earth did not revolve around the Sun at a uniform speed, but rather moved faster or slower depending upon its distance from the sun. Obviously, the Platonic and Copernican concept of uniform motion was incorrect. Kepler, however, discovered a new type of uniform motion: With the Sun as its focus, the planet, while revolving along the periphery of its orbit, will sweep out, in equal intervals of time, equal areas of the orbit, and unequal arcs along the periphery of the orbit.

The second law determined the variations of the planet’s speed along its orbit but not the shape of the orbit itself. For two more years, Kepler worked on that problem before coming upon the solution: an ellipse. This led to his first law (discovered after his second law): A planet moves in an ellipse with the Sun at one of the two foci.

Kepler’s third law was not published until 1619 in the Harmonices mundi (partial translation as Harmonies of the World , 1952). Here Kepler states that the squares of the planetary periods (the time it takes a planet to complete its orbit) are proportional to the cubes of their average distance from the Sun (the 3/2 ratio). The farther from the Sun an object is, the slower it moves.

These two works, the New Astronomy and Harmonies of the World, completed Kepler’s work on planetary motion and provided the basis for Isaac Newton’s explanation of universal gravitation, which affects every material object in the universe.

In the meantime, Kepler’s life underwent much upheaval. In 1610, his wife died, and the next year his patron, Holy Roman Emperor Rudolf II , slipped into insanity and was deposed by his brother, Matthias, who became the new king of Hungary. Although he was reappointed imperial mathematician, Kepler left Prague and moved to Linz, Austria. In 1623, he was married to Susanna Reuttinger, who although several years younger than Kepler and of lower social status, proved to be one of the few joys of his later life.

While in Linz, Kepler published his Epitome astronomiae Copernicanae (1618-1621; partial translation as Epitome of Copernican Astronomy, 1939). This relatively unknown and underrated work is highly significant, ranking next to Ptolemy’s Almagest (second century) and Copernicus’s De revolutionibus (1543) as the first systematic elaboration of the concept of celestial mechanics established by Kepler. In it, he not only conclusively proved the validity of the Copernican view of the universe but also revealed all the knowledge he had uncovered during his many years of research.

As he worked on Epitome of Copernican Astronomy, Kepler heard that his mother was going to be tried as a witch, a capital offense. Self-interest as well as familial devotion led him to conduct a successful defense. Had she been convicted, not only would there be severe consequences for her, including possible death, but Kepler’s status as imperial mathematician also could have been imperiled.

Kepler also planned to publish Tabulae Rudolphinae (English translation, 1675), named in honor of Rudolf II, in Linz, but a peasants’ rebellion and religious unrest forced Kepler to move to Ulm, where the work was first published in 1627. Based on Brahe’s observations, Tabulae Rudolphinae is a book on practical astronomy. It became an indispensable astronomical tool for more than a century, and Kepler considered it the crowning achievement of his life.

The last three years of Kepler’s life were a struggle. Albrecht Wenzel von Wallenstein, the famous mercenary general of the Thirty Years’ War (1618-1648), promised to meet Kepler’s financial needs, but the general was unreliable. In 1628, Kepler moved to Żagań, Silesia, and late in 1630 he left for Austria to collect some funds he was owed. He stopped at Regensburg, where the Imperial Diet was meeting, fell ill, and died on November 15, 1630. Scientists throughout Europe mourned his death.

Significance

Kepler’s impact on the development of astronomy and general science was enormous. By the sheer force of his intellect and the tenacity of his spirit, he forged ahead in the understanding of the cosmos, further than any of his contemporaries. Kepler not only provided the mathematical proof of the Copernican system but also went far beyond it, creating the science of modern astronomy, in which physics, the concept of physical force, and astronomy were fused together. He discovered the famous three laws, created the science of optics, and came very close to discovering gravity. His determination that the theory must fit the facts and not vice versa established the standard for future scientific inquiry. Without Kepler, there would not have been Newton’s laws of universal gravitation.

Kepler also had a more abstract impact on Western society. A society’s perception of the cosmos is reflected in the way it views itself. By demonstrating conclusively for the first time that the universe operates according to fixed, natural laws, Kepler assisted Western society in freeing itself from the shackles of superstition and ignorance.

Basic to Kepler’s success was his Christian faith. Although his belief in the harmony of the worlds led him into many mystical conjectures, Kepler’s firm belief that God would only have created a harmonious universe, where there had to be a predetermined reason for the occurrence of certain events, provided the proper attitude for discerning the existence of natural laws. Indeed, Kepler’s faith, rather than being a hindrance, was a creative force pushing him ever forward.

Bibliography

Armitage, Agnus. John Kepler. London: Faber & Faber, 1966. Although not as detailed as other biographies, this is a very good introduction to Kepler and his work. It is lucid, includes excellent illustrations, and provides clear explanations of Kepler’s calculations.

Bishop, Philip W., and George Schwartz, eds. Moments of Discovery. Vol. 1 in The Origins of Science. New York: Basic Books, 1958. Pages 265-277 contain the first partial translation of Astronomia nova, specifically Kepler’s explanation of his first law.

Caspar, Max. Kepler. New York: Abelard-Schuman, 1959. This translation of the 1947 German edition is the definitive work on Kepler, thoroughly investigating every aspect of his life and work. Well written and lucid, this work is a must for any study of Kepler.

Connor, James A. Kepler’s Witch: An Astronomer’s Discovery of Cosmic Order Amid Religious War, Political Intrigue, and the Heresy Trial of His Mother. San Francisco, Calif.: HarperSanFrancisco, 2004. A biography of Kepler, placing him within the context of seventeenth century life. Depicts him as one deeply driven by his Lutheran faith and one who was trying to survive the politics of the Counter-Reformation.

Gilder, Joshua, and Anne-Lee Gilder. Heavenly Intrigue: Johannes Kepler, Tycho Brahe, and the Murder Behind One of History’s Greatest Scientific Discoveries. New York: Doubleday, 2004. Depicts the troubled relationship of Kepler and his mentor, Brahe, contending Kepler murdered Brahe to obtain the scientific data Kepler needed to complete his laws of planetary motion. Portrays Kepler as a virtual sociopath, consumed with anger and beset with illness.

Kepler, Johannes. New Astronomy. Translated by William H. Donahue. New York: Cambridge University Press, 1992. The first full translation in English of Kepler’s major work. Includes a bibliography and an index.

‗‗‗‗‗‗‗. Optics: Paralipomena to Witelo and Optical Part of Astronomy. Translated by William H. Donahue. Santa Fe, N.Mex.: Green Lion Press, 2000. The first English translation of Kepler’s major work on optics. Includes a bibliography and an index.

Koestler, Arthur. The Watershed: A Biography of Johannes Kepler. Garden City, N.Y.: Anchor Books, 1960. This Science Study series volume is a fine introduction to Kepler and his work. The discussion on Kepler and Galileo is excellent, showing that Kepler had a much greater impact on the development of astronomy and physics than Galileo.

Small, Robert. An Account of the Astronomical Discoveries of Kepler. London: J. Mawman, 1804. Reprint. Madison: University of Wisconsin Press, 1963. Until the translation of Caspar, this was the best work on Kepler in English. It remains the best English work for readers interested in the mathematical details of Kepler’s theories. Contains reproductions of many of Kepler’s geometric figures.

Spiller, Elizabeth. Science, Reading, and Renaissance Literature: The Art of Making Knowledge. New York: Cambridge University Press, 2004. Studies literary works and the works of Kepler and other scientists to examine the changing disciplines of literature and science during the Renaissance.