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Johannes Kepler

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Johannes Kepler (Weil der Stadt 1571 - Regensburg 1630) was an astronomer whose name lives on in his three laws on the motion of the planets orbiting the sun. Kepler was a genuine Copernican, in contrast to most of his contemporaries, who still adhered to the geocentric system of Ptolemy.


After the death of Tycho Brahe in 1601 Kepler became the Mathematicus Imperialis at the court of emperor Rudolph II in Prague. He stayed at the court until the abdication of Rudolph in 1612. During this period he wrote Astronomia Nova after an analysis of Tycho's detailed observations of the Martian orbit. It took Kepler eight years to discover that an elliptic orbit was needed to fit these data. In 1612 Kepler accepted a position in Linz (Austria).

Kepler lived and worked amid the great religious wars raging all over Europe. Kepler was Lutheran and had to flee several times during his lifetime from the Catholics. Belief in witchcraft was still widespread among both Catholics and Protestants. Between 1615 and 1621 it took Kepler much time, money, and use of his influence as Imperial Mathematician to get his mother absolved of the accusation of being a witch.


Johannes Kepler is sometimes described as a somewhat muddleheaded mystic. Usually, one then refers to the planetary theory that Kepler proposed at the age of twenty-four. He then put forward that the six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn (the only ones then known) move on surfaces of spheres that envelop the five regular polyhedrons. By imposing the order, octahedron, icosahedron, dodecahedron, tetrahedron, cube, Kepler found that the planets moving on the respective spheres, had the correct relative distances to the Sun. He was able to develop this theory as these Platonic solids can be inscribed and circumscribed by spheres. For a while Kepler was very enthusiastic about his idea, because he believed that he had found the symmetries that had guided the creation. However, when he discovered that his theory was only qualitatively correct, but not quantitatively, he dropped it.

The standard story has it that Danish astronomer Tycho Brahe (1546-1601), not knowing what to do with his observations, gave them to Kepler, who then solved the outstanding problem of the universe; the truth is more subtle. "Tycho has the world's finest observations," Kepler declared, "but he only lacks an architect to construct an edifice out of them." While Brahe had failed repeatedly to fit his brilliantly targeted observations into the outdated Ptolemaic model, Kepler, using Brahe's data, abandoned his own faulty theoretical framework and, using the Copernican model of the universe, creatively intuited his laws of planetary motion.[1]

Kepler transformed a theoretical astronomy that was understood in terms of orbs (that is, spherical shells to which the planets were attached) by introducing a single term, "orbit" (that is, the path of a planet in space resulting from the action of physical causes expressed in laws of nature). "Orbit" was introduced into astronomy by Kepler in his Astronomia Nova (1609).[2]

Three laws

Three astronomical laws pertaining to our solar system bear Kepler's name. The first and second law were published in his book Astronomia Nova (1609) and the third law was published in the book Harmonice Mundi (1619).

Kepler's first law states that the planetary orbits are ellipses not circles with the Sun in one of the foci of the ellipse. He surmounted here a great psychological barrier that even Copernicus had not been able to overcome. Since the days of Aristotle it was thought that the planetary motion was perfect, that is on circles or on spheres. Kepler's abandoning this perfectness was a great stride in the development of astronomy.

Kepler's second law is known as the law of equal areas: A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. We now recognize the law as conservation of angular momentum.

Kepler's third law states that the squares of the orbital periods T of all planets are directly proportional to the cubes of the semi-major axis a of the orbits, T2 = a3 (when the right units for a and T are chosen). Kepler himself pointed out the example of Saturn which is nine times further removed from the Sun than the Earth, and hence a Saturn year takes 27 Earth years. (Kepler's third law is a first approximation, in reality the Saturn year takes 29.46 Earth year).

Comets, astrology and the esoteric

Kepler (like Tycho Brahe[3]) devoted much of his time and energy to astrology as well as astronomy. Following the ideas developed by both Plato and Socrates, he believed that the cosmos was governed by the "anima," or soul, of all things that responded to external geometric configurations. However, Kepler could not identify the connection between the forces of the cosmos and the behavior of humans. In De Cometis Libelli Tres (1619), Kepler defined comets as ephemeral celestial phenomena originating from an ethereal aura whose essence was in many ways the same as that surrounding the earth in the form of air. Kepler linked the celestial and terrestrial realms through common physical characteristics, comparing the origins, activities, and eventual endpoints of comets with the corresponding attributes of earthly entities such as igneous outbursts and airborne projectiles. More fundamentally, Kepler relied in his "earthly account" of comets on a common metaphysical foundation, in which phenomena in the celestial and sublunary spheres exemplified the same underlying mathematical principles. Accordingly, Kepler claimed that the terrestrial realm realized the divine architectonic design originally implemented in the creation of the cosmos as a whole. His approach also explained how the sublunary world, in the form of the facultates animales of the earth and its inhabitants, discerned and responded to astrological influences from the heavens. Kepler did not, however, intend to make astrological predictions the focal point for De Cometis. Instead, he sought a more thorough natural knowledge of comets, in which mathematics, the metaphysical link between the celestial and terrestrial realms, was given a more prominent place in understanding the origins and interactions of earthly and heavenly phenomena.[4]

In his posthumous Somnium (1634), Kepler unveils a strange dream, an allegory, that portrays the heavens as if from a lunar perspective and via demons in caves where their minds reflect "in accordance with our inclination." Kepler insisted that this preoccupation with shadows, darkness, and mysteries was directly related to the observations and knowledge of his science. In part, his intention was to advance a Copernican position masked as a fable that might attract readers otherwise put off by the mathematics and empirical data which, in any case, he believed were inadequate to scientific breakthroughs. Moreover, two classical models provided examples for such allegorizing about the cosmos: Lucian's A True Story and Plutarch's Concerning the Face Which Appears in the Orb of the Moon. Somnium aimed to convert the reader's modes of vision and knowledge acquisition.[5]


  1. Owen Gingerich and James R. Voelkel, "Tycho and Kepler: Solid Myth Versus Subtle Truth." Social Research 2005 72(1): 77-106. Issn: 0037-783x Fulltext: Ebsco
  2. Bernard R. Goldstein, and Giora Hon, "Kepler's Move from Orbs to Orbits: Documenting a Revolutionary Scientific Concept. Perspectives on Science 2005 13(1): 74-111. Issn: 1063-6145 Fulltext: Project Muse
  3. Isaac Newton went in for alchemy
  4. Patrick J. Boner, "Kepler on the Origins of Comets: Applying Earthly Knowledge to Celestial Events." Nuncius [Italy] 2006 21(1): 31-47. Issn: 0394-7394
  5. Raz Chen-Morris, "Shadows of Instruction: Optics and Classical Authorities in Kepler's Somnium." Journal of the History of Ideas 2005 66(2): 223-243. Issn: 0022-5037 Fulltext: Project Muse