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== Kepler's other contributions ==

== Kepler's other contributions ==

In 1613, while at Linz, Kepler published his own calculation of the exact year of the birth of [[Jesus Christ]]. He it was who first concluded that the Julian-Gregorian calendar systems were off by four years, and that Jesus was born in 4 BC (Julian calendar), not 1 AD.<ref>James Ussher might have used those calculations in his ''[[The Annals of the World|Annals]]'' to fix the date of Christ's birth and the Crucifixion.</ref> Other tomes to his credit include ''Harmonices Mundi'' (1619, a defense of his Third Law of Planetary Motion) and ''Epitome Astronomiae'' (1621, a systematic defense of heliocentrism in seven volumes).<ref name=nasa/> His last work was the ''Rudolphine Tables'' (named for Rudolf II), a completion of Tycho Brahe's work. In that work he used logarithms, which he developed, to calculate predicted positions of all the planets known to that time. Those tables correctly predicted two solar transits of [[Mercury]] and [[Venus]], events that occurred after his death but on the dates he predicted.<ref name=nasa/>

In 1613, while at Linz, Kepler published his own calculation of the exact year of the birth of [[Jesus Christ]]. He it was who first concluded that the Julian-Gregorian calendar systems were off by four years, and that Jesus was born in 4 BC (Julian calendar), not AD 1.<ref>James Ussher might have used those calculations in his ''[[The Annals of the World|Annals]]'' to fix the date of Christ's birth and the Crucifixion.</ref> Other tomes to his credit include ''Harmonices Mundi'' (1619, a defense of his Third Law of Planetary Motion) and ''Epitome Astronomiae'' (1621, a systematic defense of heliocentrism in seven volumes).<ref name=nasa/> His last work was the ''Rudolphine Tables'' (named for Rudolf II), a completion of Tycho Brahe's work. In that work he used logarithms, which he developed, to calculate predicted positions of all the planets known to that time. Those tables correctly predicted two solar transits of [[Mercury]] and [[Venus]], events that occurred after his death but on the dates he predicted.<ref name=nasa/>

== Kepler's insights ==

== Kepler's insights ==