Kepler, Johannes .............................. K
(1571–1630)
German astronomer who was the first to
develop accurate theories of planetary mo-
tion. Born in the town of Weil, the son of
a mercenary soldier, Kepler was educated
at seminaries and the University of Tubin-
gen. He was named as a royal mathemati-
cian in 1594; in this position, the accurate
predictions of political and weather events
published in his almanacs gained him re-
nown as a prognosticator. In 1595 he be-
gan a serious study of planetary motions
that he published in his first bookThe Se-
cret of the Universe. This book defended
Nicolaus Copernicus’s theory of a helio-
centric universe, introducing the idea that
the planetary orbits were based on differ-
ent geometric shapes. In 1600 Kepler be-
came an assistant to Tycho Brahe, court
astronomer to Emperor Rudolf II, and
who remained an adamant supporter of
the geocentric (earth-centered) Ptolemaic
system, Brahe’s sudden death the next year
allowed Kepler to take over as court as-
tronomer. He publishedOptics, a treatise
on the use of telescopes in observational
astronomy, in 1604; in the meantime,
while studying the exhaustive calculations
and observations of Brahe, Kepler con-
cluded that the planets moved in ellipses,
rather than circles, a theory now known
as Kepler’s First Law of Planetary Motion.
This law proved superior to the theories
of Copernicus and Brahe in explaining
the irregular motion of the planets.
Kepler’s First Law, and a second law
stating that the radius of a planet sweeps
across equal areas in equal spans of
time, were published inNew Astronomy,
which appeared in 1609. HisConversation
with the Starry Messenger, published in
1610, supported the observations of Gali-
leo Galilei, whose sighting of Jupiter’s
moons Kepler confirmed with his own in-
struments. In his writings Kepler also hit
on the idea of an elemental force emanat-
ing from the sun and controlling the mo-
tion of the planets—an early notion of the
phenomenon of gravity.
In 1611, as Rudolf II gave up his
throne, Kepler moved from the imperial
court at Prague to Linz, in Austria, where
he became official mathematician. In
his Harmony of the World, Kepler at-
tempted a grand, unified theory of geom-
etry, mathematics, and astronomy, and
published his third law of planetary
motion, in which the square of the
sidereal period of a planet is proportional
to the cube of its distance from the sun.
Kepler summarized his laws inEpitome of
Copernican Astronomy, written between
1618 and 1621, which built on the helio-
centric model of the universe proposed by
Copernicus but also introduced Kepler’s
own theory of elliptical orbits. His book
Rudolfine Tablesset out charts of planetary
positions. Its accurate prediction of a tran-
sit of the sun by the planet Mercury
proved Kepler to be the most capable as-
tronomer of his time, and the book re-
mained in use for several generations after
his death in 1630.