27th December: Johannes Kepler

Born this date in 1571 was Johannes Kepler, German astronomer best known for his laws of planetary motion.

1. Kepler was born in the Free Imperial City of Weil der Stadt, which is today part of the Stuttgart Region of Germany.
2. His grandfather had been Lord Mayor of the city, but the family's fortunes had dwindled, so that his father had to work as a mercenary. He left the family when Johannes was five, and never came back - he was believed to have died in the Eighty Years' War in the Netherlands. Kepler's mother Katharina was a healer and herbalist. In 1615, Katharina was accused of using witchcraft to make a woman involved in a financial dispute with the family ill. She was sent to prison, but released after fourteen months after Kepler drew up a compelling legal defence for her.
3. His interest in astronomy started at a young age, encouraged by his mother, who took him to look at the Great Comet of 1577 at the age of six. However, he suffered from smallpox as a child which damaged his sight.
4. In Kepler's day, astronomy and Astrology were virtually the same thing, and Kepler was known at university for casting horoscopes for other students. In later life, when working as imperial mathematician his primary function was to provide astrological advice to the emperor, even though he didn't believe astrology could predict the future. When Emperor Rudolph and his brother Matthias both sides sought Kepler's astrological advice during a time of political tension, his advice made little reference to the stars, except in fairly general statements. Mainly he was trying to advise them not to do anything drastic.
5. Kepler's first major astronomical work was Mysterium Cosmographicum (The Cosmographic Mystery) published in 1596. Its full title (translated from the original Latin) was the catchy Forerunner of the Cosmological Essays, Which Contains the Secret of the Universe; on the Marvelous Proportion of the Celestial Spheres, and on the True and Particular Causes of the Number, Magnitude, and Periodic Motions of the Heavens; Established by Means of the Five Regular Geometric Solids. In it, Kepler proposed a model of the Solar System with each planet's movements represented by a different solid shape within a sphere representing Saturn, the most distant planet known at the time.
6. Kepler sent this work to numerous prominent astronomers of the day, hoping to get patronage. He was acknowledged as a skilled astronomer but the paper wasn't widely read (perhaps the long title put people off). One of these people was a man called Reimarus Ursus who was in a bitter dispute with the astronomer Tycho Brahe. He used Kepler's paper to get one over on Brahe. Despite this, Kepler and Brahe started corresponding with each other when Brahe critiqued Kepler's work. This led, eventually, to Kepler working as Brahe's assistant.
7. When Brahe died, Kepler was appointed his successor as imperial mathematician with the responsibility to complete his unfinished work.
8. As well as the movements of planets, Kepler also wrote papers on optical theory, in which he became the first to observe that images are projected onto the retina inverted, and a description of the hexagonal symmetry of snowflakes. The latter was written as a New Year's gift to one of his patrons.
9. He married twice - his first wife was Barbara Müller, who had already been widowed twice at the age of 23. Third time lucky for her, as he outlived her and later married Susanna Reuttinger, 24 after considering eleven different women over two years.
10. His epitaph reads: I measured the skies, now the shadows I measure; Skybound was the mind, earthbound the body rests.