Johannes Kepler (1571-1630)
Johannes Kepler was born in Weil der Stadt in Swabia, in southwest Germany. His
paternal grandfather, Sebald Kepler, was a respected craftsman who served as mayor of the
city; his maternal grandfather, Melchior Guldenmann, was an innkeeper and mayor of the
nearby village of Eltingen. His father, Heinrich Kepler, was "an immoral, rough and
quarrelsome soldier," according to Kepler, and he described his mother in similar
unflattering terms. From 1574 to 1576 Johannes lived with his grandparents; in 1576 his
parents moved to nearby Leonberg, where Johannes entered the Latin school. In 1584 he
entered the Protestant seminary at Adelberg, and in 1589 he began his university education
at the Protestant university of Tübingen. Here he studied theology and read widely. He
passed the M.A. examination in 1591 and continued his studies as a graduate student.
Kepler's teacher in the mathematical subjects was Michael Maestlin (1580-1635).
Maestlin was one of the earliest astronomers to subscribe to Copernicus's heliocentric
theory, although in his university lectures he taught only the Ptolemaic system. Only in
what we might call graduate seminars did he acquaint his students, among whom was Kepler,
with the technical details of the Copernican
system. Kepler stated later that at this time he became a Copernican for
"physical or, if you prefer, metaphysical reasons."
In 1594 Kepler accepted an appointment as professor of mathematics at the Protestant
seminary in Graz (in the Austrian province of Styria). He was also appointed district
mathematician and calendar maker. Kepler remained in Graz until 1600, when all Protestants
were forced to convert to Catholicism or leave the province, as part of Counter
Reformation * measures. For six years, Kepler taught arithmetic,
geometry (when there were interested students), Virgil, and rhetoric. In his spare time he
pursued his private studies in astronomy and astrology. In 1597 Kepler married Barbara
Müller. In that same year he published his first important work, The Cosmographic
Mystery, in which he argued that the distances of the planets from the Sun in the
Copernican system were determined by the five regular solids, if one supposed that a
planet's orbit was circumscribed about one solid and inscribed in another.
Kepler's model to explain the relative distances of the planets from the Sun in the
Except for Mercury, Kepler's construction produced remarkably accurate results. Because
of his talent as a mathematician, displayed in this volume, Kepler was invited by Tycho Brahe to
Prague to become his assistant and calculate new orbits for the planets from Tycho's
observations. Kepler moved to Prague in 1600.
Kepler served as Tycho Brahe's assistant until the latter's death in 1601 and was then
appointed Tycho's successor as Imperial Mathematician, the most prestigious appointment in
mathematics in Europe. He occupied this post until, in 1612, Emperor Rudolph II was
deposed. In Prague Kepler published a number of important books. In 1604 Astronomia
pars Optica ("The Optical Part of Astronomy") appeared, in which he treated atmospheric refraction * but also treated lenses and
gave the modern explanation of the workings of the eye; in 1606 he published De Stella
Nova ("Concerning the New Star") on the new star that had appeared in 1604;
and in 1609 his Astronomia Nova ("New Astronomy") appeared, which
contained his first two laws (planets move in elliptical orbits with the sun as one of the
foci, and a planet sweeps out equal areas in equal times). Whereas other astronomers still
followed the ancient precept that the study of the planets is a problem only in
kinematics, Kepler took an openly dynamic approach, introducing physics into the heavens.
In 1610 Kepler heard and read about Galileo's discoveries with the spyglass. He quickly
composed a long letter of support which he published as Dissertatio cum Nuncio Sidereo
("Conversation with the Sidereal Messenger"), and when, later that year, he
obtained the use of a suitable telescope, he published his observations of Jupiter's
satellites under the title Narratio de Observatis Quatuor Jovis Satellitibus
("Narration about Four Satellites of Jupiter observed"). These tracts were an
enormous support to Galileo, whose discoveries were doubted or denied by many. Both of
Kepler's tracts were quickly reprinted in Florence. Kepler went on to provide the
beginning of a theory of the telescope in his Dioptrice, published in 1611.
During this period the Keplers had three children (two had been born in Graz but died
within months), Susanna (1602), who married Kepler's assistant Jakob Bartsch in 1630,
Friedrich (1604-1611), and Ludwig (1607-1663). Kepler's wife, Barbara, died in 1612. In
that year Kepler accepted the position of district mathematician in the city of Linz, a
position he occupied until 1626. In Linz Kepler married Susanna Reuttinger. The couple had
six children, of whom three died very early.
In Linz Kepler published first a work on chronology and the year of Jesus's birth, In
German in 1613 and more amply in Latin in 1614: De Vero Anno quo Aeternus Dei Filius
Humanam Naturam in Utero Benedictae Virginis Mariae Assumpsit (Concerning the True
Year in which the Son of God assumed a Human Nature in the Uterus of the Blessed Virgin
Mary"). In this work Kepler demonstrated that the Christian calendar was in error by
five years, and that Jesus had been born in 4 BC, a conclusion that is now universally
accepted. Between 1617 and 1621 Kepler published Epitome Astronomiae Copernicanae
("Epitome of Copernican Astronomy"), which became the most influential
introduction to heliocentric astronomy; in 1619 he published Harmonice Mundi
("Harmony of the World"), in which he derived the heliocentric distances of the
planets and their periods from considerations of musical harmony. In this work we find his
third law, relating the periods of the planets to their mean orbital radii.
In 1615-16 there was a witch hunt in Kepler's native region, and his own mother was
accused of being a witch. It was not until late in 1620 that the proceedings against her
ended with her being set free. At her trial, her defense was conducted by her son
1618 marked the beginning of the Thirty Years War, a war that devastated the German and
Austrian region. Kepler's position in Linz now became progressively worse, as Counter Reformation * Counter Reformation measures put
pressure on Protestants in the Upper Austria province of which Linz was the capital.
Because he was a court official, Kepler was exempted from a decree that banished all
Protestants from the province, but he nevertheless suffered persecution. During this time
Kepler was having his Tabulae Rudolphinae ("Rudolphine Tables") printed,
the new tables, based on Tycho Brahe's accurate observations, calculated according to
Kepler's elliptical astronomy. When a peasant rebellion broke out and Linz was besieged, a
fire destroyed the printer's house and shop, and with it much of the printed edition.
Soldiers were garrisoned in Kepler's house. He and his family left Linz in 1626. The Tabulae
Rudolphinae were published in Ulm in 1627.
Kepler now had no position and no salary. He tried to obtain appointments from various
courts and returned to Prague in an effort to pry salary that was owed him from his years
as Imperial Mathematician from the imperial treasury. He died in Regensburg in 1630.
Besides the works mentioned here, Kepler published numerous smaller works on a variety of
The standard biography of Kepler is Max Caspar, Kepler, tr. C. Doris Hellman (New
York: Abelard Schuman, 1959 reprinted with a new instroduction and references by Owen
Gingerich, bibliographical citations by Owen Gingerich and Alain Segonds, New York: Dover,
1993). Arthur Koestler, The Watershed: a Biography of Johannes Kepler (Garden City:
Doubleday, 1960) is also useful. A complete list of his works can be found in Bibliographia
Kepleriana, 2d ed., ed. Martha List (Munich: Beck, 1968). There are two editions of
his works: Joannis Kepleri Astronomi Opera Omnia, ed. C. Frisch (Frankfurt and
Erlangen, 1858-1871), and Johannes Kepler Gesammelte Werke (Munich: Beck, 1937--).
Translations of single works in English are: Mysterium Cosmographicum--The Secret of
the Universe, tr. A. M. Duncan (New York: Abaris Books, 1981); New Astronomy,
tr. William H. Donahue (Cambridge: Cambridge University Press, 1992); Kepler's
Conversation with Galileo's Sidereal Messenger, tr. Edward Rosen (New York: Johnson
Reprint, 1965); The Six-Cornered Snowflake, tr. Colin Hardie (Oxford: Clarendon
Press, 1966); Somnium: the Dream, or Posthumous Work on Lunar Astronomy, tr. Edward
Rosen (Madison: University of Wisconsin Press, 1967). The introduction of Kepler's Dioptrice
can be found in The Sidereal Messenger of Galileo Galilei: and a Part of the Preface to
Kepler's Dioptrics, tr. Edward Stafford Carlos (London: Rivingtons, 1880; reprinted,
London: Dawsons of Pall Mall, 1960). Parts of the Epitome and Harmonice Mundi can
be found in vol. 16 of the "Great Books of the Western World" series (Chicago:
Encyclopaedia Britannica, 1952, 1955). A translation of Kepler's defense of Tycho Brahe
against the astronomer Ursus can be found in Nicholas Jardine, The Birth of History and
Philosophy of Science: Kepler's A Defence of Tycho against Ursus (Cambridge: Cambridge
University Press, 1984). Other works of interest are David C. Knight, Johannes Kepler
and Planetary Motion (London: Chatto & Windus, 1965); Angus Armitage, John
Kepler (London: Faber, 1966); J. V. Field, Kepler's Geometrical Cosmology
(London: Athlone Press; Chicago: University of Chicago Press, 1988); Bruce Stephenson, Kepler's
Physical Astronomy (New York: Springer Verlag, 1987); Fernand Hallyn, The Poetic
Structure of the World: Copernicus and Kepler, tr. Donald M. Leslie (New York: Zone
Books, 1990); Edward Rosen, Three Imperial Mathematicians: Kepler Trapped Between Tycho
Brahe and Ursus (New York: Abaris Books, 1986).
The Counter Reformation -- As dissenting groups split
off from the Catholic Church in what came to be known as the Protestant Reformation, the
Church began a series of reform measures of their own. These reform measures aimed to keep
Church members from becoming Protestants, and were known as the Counter Reformation.
atmospheric refraction -- The change in direction of
a ray of light as it passes from space into the atmosphere. This causes celestial objects
to appear to be in a location different from their actual ones.
Click here to go to the full Glossary.
Portrait: Johnnes Kepler Gesammelte Werke (Munich: C. H. Beck, 1937), Vol. I,
Nesting Spheres: ibid.
'De Stella Nova' (Prague, 1606).- Johannes Kepler