**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 Copernican System.*

**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**

**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 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
subjects.**