Jupiter-Saturn conjunctions. Diagram from De Stella Nova (Kepler, 1606), showing the dates of conjunctions (close appearances in the sky) of the planets Saturn and Jupiter. The diagram shows the 20-year cycle of conjunctions of Saturn and Jupiter, from 1603 to 1763. The conjunctions are shown moving across the zodiac (symbols around edge) and into the three 'fire signs'. This only happened once every 800 years, and was taken to be a portent of great events to come. The text is in Latin. The title of the book, which was by the German astronomer and mathematician Johannes Kepler (1571-1630), refers to the discovery of a new star that appeared on 30 September 1604, what is now known to have been a supernova.