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Johannes Kepler's obsession with Mars

In the dying years of the 16th century, Johannes Kepler – mathematician, astrologer and astronomer – became obsessed by Mars.

In particular, he became fascinated by the “Martian problem” – the fact that at certain times the Red Planet appeared to go backwards across the sky, before correcting itself and heading off again in the right direction. Solving the problem, he boasted, would take him just eight days. It ended up taking eight years, but solve it he did, laying the foundations for modern astronomy in the process.

Kepler was born in 1571 in the city of Weil der Stadt, not far from modern-day Stuttgart. He was raised by his mother, Katharina, a herbalist and folk healer – skills that would later dangerously affect both.

It was his mum who ignited the young boy’s interest in the movement of celestial bodies. When he was just six she took him to some high ground to witness a comet. As the little boy stood on a hill staring in awe, hundreds of kilometres to the north the comet was also observed by Tycho Brahe, an astronomer 25 years older than him. Decades later, Brahe would be central to the Martian solution.

Kepler attended the University of Tubingen, where he fell under the sway of Michael Maestlin, an astronomer and mathematician. Maestlin was a fervent supporter of Nicolaus Copernicus – the man who proposed, 50 years earlier, that the Earth orbited the Sun, and not the other way around.

In 1597, Kepler published a vigorous defence of Copernican thought. It was a brave move – the theory had been condemned by the Lutheran Church, and elements within the all-powerful Catholic Church also viewed it dimly (as Galileo was to discover in 1615).

Anxious to escape religious persecution, Kepler fled to Prague, where he got a job assisting Tycho Brahe. It turned out to be a short association. Brahe died a year or so later, leaving Kepler with a lifetime’s worth of meticulously recorded data concerning the movements of the stars and planets – especially Mars. Using this treasure chest, Kepler went on to formulate three laws of planetary motion, which have deeply influenced every astronomer since.

First, he deduced the planets do not move in circular orbits, but in ellipses, and that the Sun does not sit in the centre of these but off to one side.

Second, he realised the planets do not move at a constant speed, but slow down further from the Sun.

Third, he worked out the relationship between the orbits of any two planets is determined by their distances from the Sun – making their positions rigidly predictable.

With these laws he finally solved the Martian problem. He worked out that Mars took 687 days to orbit, while the Earth took just 365. This meant that at certain times the angle of view from Earth to Mars was so extreme that it made the Red Planet appear to be going backward, even though it wasn’t. The Martian problem was simply an optical illusion.

When not studying the sky, Kepler pursued his other passion: optics. He designed lenses to tackle both long- and short-sightedness.

In 1615, he had to rush home because his mother had been arrested for witchcraft. This was partly because she practiced herbalism and partly, perhaps, because in a flight of fancy Kepler once wrote a novel (called The Dream) that that featured a woman in contact with demons from the Moon. He secured her release, but she was arrested again in 1620, served 14 months in prison, and died not long after.

Kepler himself died in 1630. In his final years he made a living from writing horoscopes.

This article appeared in Cosmos 74 – Autumn 2017 under the headline "Conquering the Martian problem"

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