**Johannes Kepler**, (born December 27, 1571, Weil der Stadt, Württemberg [Germany]—died November 15, 1630, Regensburg), German astronomer who discovered three major laws of planetary motion, conventionally designated as follows: (1) the planets move in elliptical orbits with the Sun at one focus; (2) the time necessary to traverse any arc of a planetary orbit is proportional to the area of the sector between the central body and that arc (the “area law”); and (3) there is an exact relationship between the squares of the planets’ periodic times and the cubes of their mean distances from the Sun (the “harmonic law”). Kepler himself did not call these discoveries “laws,” as would become customary after Isaac Newton derived them from a new and quite different set of general physical principles. He regarded them as celestial harmonies that reflected God’s design for the universe. Kepler’s discoveries turned Nicolaus Copernicus’s Sun-centred system into a dynamic universe, with the Sun actively pushing the planets around in noncircular orbits. And it was Kepler’s notion of a physical astronomy that fixed a new problematic for other important 17th-century world-system builders, the most famous of whom was Newton.

Among Kepler’s many other achievements, he provided a new and correct account of how vision occurs; he developed a novel explanation for the behaviour of light in the newly invented telescope; he discovered several new, semiregular polyhedrons; and he offered a new theoretical foundation for astrology while at the same time restricting the domain in which its predictions could be considered reliable. A list of his discoveries, however, fails to convey the fact that they constituted for Kepler part of a common edifice of knowledge. The matrix of theological, astrological, and physical ideas from which Kepler’s scientific achievements emerged is unusual and fascinating in its own right. Yet, because of the highly original nature of Kepler’s discoveries, it requires an act of intellectual empathy for moderns to understand how such lasting results could have evolved from such an apparently unlikely complex of ideas. Although Kepler’s scientific work was centred first and foremost on astronomy, that subject as then understood—the study of the motions of the heavenly bodies—was classified as part of a wider subject of investigation called “the science of the stars.” The science of the stars was regarded as a mixed science consisting of a mathematical and a physical component and bearing a kinship to other like disciplines, such as music (the study of ratios of tones) and optics (the study of light). It also was subdivided into theoretical and practical categories. Besides the theory of heavenly motions, one had the practical construction of planetary tables and instruments; similarly, the theoretical principles of astrology had a corresponding practical part that dealt with the making of annual astrological forecasts about individuals, cities, the human body, and the weather. Within this framework, Kepler made astronomy an integral part of natural philosophy, but he did so in an unprecedented way—in the process, making unique contributions to astronomy as well as to all its auxiliary disciplines.

# Astronomical work of Johannes Kepler

The ideas that Kepler would pursue for the rest of his life were already present in his first work, *Mysterium cosmographicum* (1596; “Cosmographic Mystery”). Kepler had become a professor of mathematics at the Protestant seminary in Graz, Austria, in 1594, while also serving as the district mathematician and calendar maker. In 1595, while teaching a class, Kepler experienced a moment of illumination. It struck him suddenly that the spacing among the six Copernican planets might be explained by circumscribing and inscribing each orbit with one of the five regular polyhedrons. Since Kepler knew Euclid’s proof that there can be five and only five such mathematical objects made up of congruent faces, he decided that such self-sufficiency must betoken a perfect idea. If now the ratios of the mean orbital distances agreed with the ratios obtained from circumscribing and inscribing the polyhedrons, then, Kepler felt confidently, he would have discovered the architecture of the universe. Remarkably, Kepler did find agreement within 5 percent, with the exception of Jupiter, at which, he said, “no one will wonder, considering such a great distance.” He wrote to Maestlin at once: “I wanted to become a theologian; for a long time I was restless. Now, however, behold how through my effort God is being celebrated in astronomy.”

Had Kepler’s investigation ended with the establishment of this architectonic principle, he might have continued to search for other sorts of harmonies; but his work would not have broken with the ancient Greek notion of uniform circular planetary motion. Kepler’s God, however, was not only orderly but also active. In place of the tradition that individual incorporeal souls push the planets and instead of Copernicus’s passive, resting Sun, Kepler posited the hypothesis that a single force from the Sun accounts for the increasingly long periods of motion as the planetary distances increase. Kepler did not yet have an exact mathematical description for this relation, but he intuited a connection. A few years later he acquired William Gilbert’s groundbreaking book *De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure* (1600; “On the Magnet, Magnetic Bodies, and the Great Magnet, the Earth”), and he immediately adopted Gilbert’s theory that Earth is a magnet. From this Kepler generalized to the view that the universe is a system of magnetic bodies in which, with corresponding like poles repelling and unlike poles attracting, the rotating Sun sweeps the planets around. The solar force, attenuating inversely with distance in the planes of the orbits, was the major physical principle that guided Kepler’s struggle to construct a better orbital theory for Mars.

But there was something more: the standard of empirical precision that Kepler held for himself was unprecedented for his time. The great Danish astronomer Tycho Brahe (1546–1601) had set himself the task of amassing a completely new set of planetary observations—a reform of the foundations of practical astronomy. In 1600 Tycho invited Kepler to join his court at Castle Benátky near Prague. When Tycho died suddenly in 1601, Kepler quickly succeeded him as imperial mathematician to Holy Roman Emperor Rudolf II. Kepler’s first publication as imperial mathematician was a work that broke with the theoretical principles of Ptolemaic astrology. Called *De Fundamentis Astrologiae Certioribus* (1601; *Concerning the More Certain Fundamentals of Astrology*), this work proposed to make astrology “more certain” by basing it on new physical and harmonic principles. It showed both the importance of astrological practice at the imperial court and Kepler’s intellectual independence in rejecting much of what was claimed to be known about stellar influence. The relatively great intellectual freedom possible at Rudolf’s court was now augmented by Kepler’s unexpected inheritance of a critical resource: Tycho’s observations. In his lifetime Tycho had been stingy in sharing his observations. After his death, although there was a political struggle with Tycho’s heirs, Kepler was ultimately able to work with data accurate to within 2′ of arc. Without data of such precision to back up his solar hypothesis, Kepler would have been unable to discover his “first law” (1605), that Mars moves in an elliptical orbit. At one point, for example, as he tried to balance the demand for the correct heliocentric distances predicted by his physical model with a circular orbit, an error of 6′ or 8′ appeared in the octants (assuming a circle divided into eight equal parts). Kepler exclaimed, “Because these 8′ could not be ignored, they alone have led to a total reformation of astronomy.” Kepler’s reformation of astronomy was of a piece with his reform of astrology’s principles and Tycho’s radical improvement of the celestial observations. Just as the spacing of the planets bore a close relation to the polyhedral forms, so, too, Kepler regarded only those rays hitting Earth at the right harmonic angles to be efficacious.

Kepler’s interest in light was directly related to his astronomical concerns: how a ray of light, coming from a distant heavenly body located in the outer regions of space, deflects when entering the denser atmosphere surrounding Earth; and then, in turn, what happens to light as it enters the relatively denser medium of the human eye. These problems had some medieval precedent, but, as usual, Kepler treated them in his own individual way. Although a court astronomer, Kepler chose a traditional academic form in which to compose his ideas on light. He called it *Ad Vitellionem Paralipomena, Quibus Astronomiae Pars Optica Traditur* (1604; “Supplement to Witelo, in Which Is Expounded the Optical Part of Astronomy”). Witelo (Latin: Vitellio) had written the most important medieval treatise on optics. But Kepler’s analysis of vision changed the framework for understanding the behaviour of light. Kepler wrote that every point on a luminous body in the field of vision emits rays of light in all directions but that the only rays that can enter the eye are those that impinge on the pupil, which functions as a diaphragm. He also reversed the traditional visual cone. Kepler offered a punctiform analysis, stating that the rays emanating from a single luminous point form a cone the circular base of which is in the pupil. All the rays are then refracted within the normal eye to meet again at a single point on the retina. For the first time the retina, or the sensitive receptor of the eye, was regarded as the place where “pencils of light” compose upside-down images. If the eye is not normal, the second short interior cone comes to a point not on the retina but in front of it or behind it, causing blurred vision. For more than three centuries eyeglasses had helped people see better. But nobody before Kepler was able to offer a good theory for why these little pieces of curved glass had worked.

After Galileo built a telescope in 1609 and announced hitherto-unknown objects in the heavens (e.g., moons revolving around Jupiter) and imperfections of the lunar surface, he sent Kepler his account in *Siderius Nuncius* (1610; *The Sidereal Messenger*). Kepler responded with three important treatises. The first was his *Dissertatio cum Nuncio Sidereo* (1610; “Conversation with the Sidereal Messenger”), in which, among other things, he speculated that the distances of the newly discovered Jovian moons might agree with the ratios of the rhombic dodecahedron, triacontahedron, and cube. The second was a theoretical work on the optics of the telescope, *Dioptrice* (1611; “Dioptrics”), including a description of a new type of telescope using two convex lenses. The third was based upon his own observations of Jupiter, made between August 30 and September 9, 1610, and published as *Narratio de Jovis Satellitibus* (1611; “Narration Concerning the Jovian Satellites”). These works provided strong support for Galileo’s discoveries, and Galileo, who had never been especially generous to Kepler, wrote to him, “I thank you because you were the first one, and practically the only one, to have complete faith in my assertions.”

In 1611 Kepler’s life took a turn for the worse. His wife, Barbara, became ill, and his three children contracted smallpox; one of his sons died. Emperor Rudolf soon abdicated his throne. Although Kepler hoped to return to an academic post at Tübingen, there was resistance from the theology faculty; Kepler’s irenic theological views and his friendships with Calvinists and Catholics were characteristic of his independence in all matters, and in this case it did not help his cause. Meanwhile, Kepler was appointed to the position (created for him) of district mathematician in Linz. He continued to hold the position of imperial mathematician under the new emperor, Matthias, although he was physically removed from the court in Prague. Kepler stayed in Linz until 1626, during which time creative productions continued amid personal troubles—the death of his wife and his exclusion from the Lutheran communion. Although he was married again in 1613 (to Susanna Reuttinger), five of his seven children from that marriage died in childhood. After the Counter-Reformation came in 1625, Catholic authorities temporarily removed his library and ordered his children to attend mass.

In 1615 Kepler used the occasion of a practical problem to produce a theoretical treatise on the volumes of wine barrels. His *Stereometria Doliorum Vinariorum* (“The Stereometry of Wine Barrels”) was the first book published in Linz. Kepler objected to the rule-of-thumb methods of wine merchants to estimate the liquid contents of a barrel. He also refused to be bound strictly by Archimedean methods; eventually he extended the range of cases in which a surface is generated by a conic section—a curve formed by the intersection of a plane and a cone rotating about its principal axis—by adding solids generated by rotation about lines in the plane of the conic section other than the principal axis.

The Linz authorities had anticipated that Kepler would use most of his time to work on and complete the astronomical tables begun by Tycho. But the work was tedious, and Kepler continued his search for the world harmonies that had inspired him since his youth. In 1619 his *Harmonice Mundi* (*Harmonies of the World*, which contained Kepler’s third law) brought together more than two decades of investigations into the archetypal principles of the world: geometrical, musical, metaphysical, astrological, astronomical, and those principles pertaining to the soul. All harmonies were geometrical, including musical ones that derived from divisions of polygons to create “just” ratios (1/2, 2/3, 3/4, 4/5, 5/6, 3/5, 5/8) rather than the irrational ratios of the Pythagorean scale. When the planets figured themselves into angles demarcated by regular polygons, a harmonic influence was impressed on the soul. And the planets themselves fell into an arrangement whereby their extreme velocity ratios conformed with the harmonies of the just tuning system, a celestial music without sound.

Finally, Kepler published the first textbook of Copernican astronomy, *Epitome Astronomiae Copernicanae* (1618–21; *Epitome of Copernican Astronomy*). The title mimicked Maestlin’s traditional-style textbook, but the content could not have been more different. The *Epitome* began with the elements of astronomy but then gathered together all the arguments for Copernicus’s theory and added to them Kepler’s harmonics and new rules of planetary motion. This work would prove to be the most important theoretical resource for the Copernicans in the 17th century. Galileo and Descartes were probably influenced by it. It was capped by the appearance of *Tabulae Rudolphinae* (1627; “Rudolphine Tables”). The *Epitome* and the Rudolphine Tables cast heliocentric astronomy and astrology into a form where detailed and extensive counterargument would force opponents to engage with its claims or silently ignore them to their disadvantage. Eventually Newton would simply take over Kepler’s laws while ignoring all reference to their original theological and philosophical framework.

The last decade of Kepler’s life was filled with personal anguish. His mother fell victim to a charge of witchcraft that resulted in a protracted battle with her accusers, lasting from 1615 until her exoneration in 1621; she died a few months later. Kepler used all means at his disposal to save his mother’s life and honour, but the travels, legal briefs, and maneuvers that this support required seriously disrupted his work. In 1627 Kepler found a new patron in the imperial general Albrecht von Wallenstein. Wallenstein sent Kepler to Sagan in Silesia and supported the construction of a printing press for him. In return Wallenstein expected horoscopes from Kepler—and he accurately predicted “horrible disorders” for March 1634, close to the actual date of Wallenstein’s murder on February 25, 1634. Kepler was less successful in his ever-continuing struggle to collect monies owed him. In August 1630 Wallenstein lost his position as commander in chief; in October Kepler left for Regensburg in hopes of collecting interest on some Austrian bonds. But soon after arriving he became seriously ill with fever, and on November 15 he died. His grave was swept away in the Thirty Years’ War, but the epitaph that he composed for himself survived:

I used to measure the heavens,

now I shall measure the shadows of the earth.

Although my soul was from heaven,

the shadow of my body lies here.