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
Nicolaus 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.
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 Johannes.
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 Mathemati
