Kepler was forced to leave his teaching post at Graz due to
the counter Reformation because he was Lutheran and moved
to Prague to work with the renowned Danish astronomer,
Tycho Brahe. He inherited Tycho's post as Imperial
Mathematician when Tycho died in 1601. Using the precise
data that Tycho had collected, Kepler discovered that the
orbit of Mars was an ellipse. In 1609 he
published Astronomia Nova, delineating his discoveries, which are now
called Kepler's first two laws of planetary motion. And
what is just as important about this work, it is the first
published account wherein a scientist documents how he has
coped with the multitude of imperfect data to forge a
theory of surpassing accuracy(O. Gingerich in forward to
Johannes Kepler New Astronomy translated by W. Donahue,
Cambridge Univ Press, 1992), a fundamental law of nature.
Today we call this the scientific method.
In 1612
Lutherans were forced out of Prague, so Kepler moved on to
Linz. His wife and two sons had recently died. He remarried
happily, but had many personal and financial troubles. Two
infant daughters died and Kepler had to return to
Württemburg where he successfully defended his mother
against charges of witchcraft. In 1619 he published
Harmonices Mundi, in which he describes his "third
law."
In spite of more forced relocations, Kepler
published the seven-volume Epitome Astronomiae in 1621.
This was his most influential work and discussed all of
heliocentric astronomy in a systematic way. He then went on
to complete the Rudolphine
Tables that Tycho had started
long ago. These included calculations using logarithms,
which he developed, and provided perpetual tables for
calculating planetary positions for any past or future
date. Kepler used the tables to predict a pair of transits
by Mercury and Venus of the Sun, although he did not live
to witness the events.
Johannes Kepler died in
Regensburg in 1630, while on a journey from his home in
Sagan to collect a debt. His grave was demolished within
two years because of the Thirty Years War. Frail of body,
but robust in mind and spirit, Kepler was scrupulously
honest to the data.
Kepler was assigned the task by
Tycho Brahe to analyze the observations that Tycho had made
of Mars. Of all the planets, the predicted position of Mars
had the largest errors and therefore posed the greatest
problem. Tycho's data were the best available before the
invention of the telescope and the accuracy was good enough
for Kepler to show that Mars orbit would precisely fit an
ellipse. In 1605 he announced The First Law:
Planets
move in ellipses with the Sun at one focus.
The figure
below illustrates two orbits with the same semi-major axis,
focus and orbital period: one a circle with an eccentricity
of 0.0; the other an ellipse with an eccentricity of
0.8.
Kepler published these two laws in 1609 in his book
Astronomia Nova.
For a circle the motion is uniform as
shown above, but in order for an object along an elliptical
orbit to sweep out the area at a uniform rate, the object
moves quickly when the radius vector is short and the
object moves slowly when the radius vector is long.
On
May 15, 1618 he discovered The Third Law:
The squares of
the periodic times are to each other as the cubes of the
mean distances.
This law he published in 1619 in his
Harmonices Mundi . It was this law, not an apple, that lead
Newton to his law of gravitation. Kepler can truly be
called the founder of celestial mechanics