While you admire Queen Anne’s Summer Palace in the beautiful gardens of Prague Castle, remember that two of the greatest astronomers in history observed the stars here together: Tycho Brahe and his assistant Johannes Kepler. Yes, this is where Kepler invented the first two laws of planetary motion. The most beautiful Baroque library in the world, the Clementinum, preserves a memory of Kepler in several of his writings with a personal dedication to his friends from Charles University. Kepler’s legacy lives on at the Prague headquarters of the European Union’s Agency for the Space Programme, which implements the Copernicus Earth observation programme. It sounds like a fairy tale: The poor, short-sighted son of a mercenary and a herbalist accused of witchcraft, he became one of the greatest astronomers in history thanks to his talent and diligence. His three laws of physics about the movement of the planets around the Sun are now known to the entire scientific world. Two of them were formulated by this German mathematician and astronomer in Prague. In 1600, he joined the court of Emperor Rudolf II in Prague as an assistant to the famous astronomer Tycho Brahe. Both astronomers lived in Prague’s Hradčany neighbourhood, and the owner of the house, Jakub Kurz from Senftenava, added an observatory to the building. Neither the observatory nor the house can be seen at this site anymore, as they were demolished and buried long ago. However, Kepler and Brahe also observed the night sky from the nearby Queen Anne’s Summer Palace, where Emperor Rudolf II had an astronomical observatory built. Today, the Summer Palace houses exhibitions of fine art and craftsmanship. Emperor Rudolf II, benefactor to both Kepler and Brahe, demanded that the scientists constantly produce horoscopes, which he trusted implicitly. Kepler was rather sceptical about them, but as they represented a welcome source of income, he compiled them with a slight sense of embarrassment not only for the emperor, but for many other interested parties. Yet this care for the emperor gave him the freedom to research. Kepler was also commissioned to compile the horoscope of Duke Albrecht of Wallenstein, who submitted the details of his birth anonymously. Kepler was thus boldly able to add to the horoscope that its holder was an intelligent individual with pronounced antisocial tendencies, predatory of nature and morbidly ambitious. In 1625, Kepler corrected the horoscope, whose owner had been identified by then, based on new observations and warned Albrecht that the beginning of the year 1634 would be critical for him. Indeed, on Saturday, 25 February 1634, Albrecht von Wallenstein was murdered in a burgher houses in Cheb. This is vividly described by Friedrich Schiller in his Wallenstein trilogy, the source of the famous saying “I know my Pappenheims.” The mutually beneficial collaboration between the astronomers (Brahe was the most accurate observer of the starry sky of his time and the short-sighted Kepler was a brilliant mathematician) did not last long. Tycho Brahe died less than two years after Kepler’s arrival in Prague. He was buried in the Church of Our Lady before Týn on Old Town Square and his tombstone is located near the first south pillar of the main nave. Kepler was then appointed imperial court mathematician. In 1607, he moved with his family to the house U Francouzské koruny (At the French Crown) in Karlova Street. Its facade now bears a massive bronze plaque commemorating the astronomer’s residence. There is also a Kepler Museum in the courtyard of the house, and a small Kepler fountain adorns the inner yard. It was created by students of the Faculty of Mathematics and Physics of Charles University together with Czech sculptor Zdeněk Kolářský. At the time when Kepler moved into Karlova Street, it was the site of lively construction. The Jesuit Fathers were building their Clementinum compound nearby. The St. Salvator Church and the Vlašská Chapel were already standing, while the Baroque Library, where several of Kepler’s writings were later housed, was still waiting to be built. It later became one of the most beautiful libraries in the world. Among the most precious works now housed in the Clementinum is the Ad Vitellionem Parali-pomena. Omnibus Astronomie Pars optica traditur with Kepler’s handwritten dedication to his friends, the dean and other professors of the University of Prague. After the death of Emperor Rudolf II in 1612, Kepler moved from the country and visited Prague for the last time in 1627, three years before his death, to hand over to Emperor Ferdinand II the completed astronomical tables that Rudolf II had commissioned. He stayed at the U Velryby (At the Whale) inn in Mostecká Street, almost in the shadow of the majestic Lesser Town Bridge Tower. It wasn’t like visiting the U Mecenáše (At the Benefactor’s) pub in the Lesser Town with Tycho, but even so, Prague bid quite a nice farewell to the man who gazed into the heavens.