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In 1600, Kepler left his job at Graz on account of the [[Counter Reformation]]. Unwilling either to convert to Catholicism or to sign the Lutheran Formula of Concord, he decided to leave Graz and travel to Prague.<ref name=nasa/><ref name=rice/> There he became the assistant to [[Tycho Brahe]], court mathematician to Emperor Rudolf II.<ref name=dao/>. Tycho was also makng observations on the orbit of the planets. In 1601, Tycho died, and Kepler was named Imperial Mathematician in his place,<ref name=nasa/> the highest ranking in Europe at the time.<ref name=rice/> After reading and studying the data that Tycho had developed and researched, Kepler was able to resolve a nagging problem concerning the motion of [[Mars]], and in particular its apparent [[retrograde motion]] in relation to the [[Earth]]. Kepler went on to calculate Mars' orbit--as an [[ellipse]], not a [[circle]].

In 1600, Kepler left his job at Graz on account of the [[Counter Reformation]]. Unwilling either to convert to Catholicism or to sign the Lutheran Formula of Concord, he decided to leave Graz and travel to Prague.<ref name=nasa/><ref name=rice/> There he became the assistant to [[Tycho Brahe]], court mathematician to Emperor Rudolf II.<ref name=dao/>. Tycho was also makng observations on the orbit of the planets. In 1601, Tycho died, and Kepler was named Imperial Mathematician in his place,<ref name=nasa/> the highest ranking in Europe at the time.<ref name=rice/> After reading and studying the data that Tycho had developed and researched, Kepler was able to resolve a nagging problem concerning the motion of [[Mars]], and in particular its apparent [[retrograde motion]] in relation to the [[Earth]]. Kepler went on to calculate Mars' orbit--as an [[ellipse]], not a [[circle]].

Kepler remained as Imperial Mathematician until 1612, when Rudolf II was deposed.<ref name=rice/> In the next year (1613) he accepted a teaching position in Linz, the last semi-permanent position he ever held. He held this job until 1626, when the pressures of the Thirty Years' War forced him to leave. Now out of a job and with no salary, he tried in vain to seek employment and to sue for back payment of his salary as Imperial Mathematician.<ref name=rice/> He died in 1630, at Regensberg.<ref name=rice/><ref name=nasa/>

Kepler remained as Imperial Mathematician until 1612, when Rudolf II was deposed.<ref name=rice/> In the next year (1613) he accepted a teaching position in Linz, the last semi-permanent position he ever held. He held this job until 1626, when the pressures of the [[Thirty Years' War]] forced him to leave. Now out of a job and with no salary, he tried in vain to seek employment and to sue for back payment of his salary as Imperial Mathematician.<ref name=rice/> He died in 1630, at Regensberg.<ref name=rice/><ref name=nasa/>

== Kepler's contributions to astronomy ==

== Kepler's contributions to astronomy ==