In 1604, he entered the service of emperor Rudolf II in Prague. Here, he befriended Johannes Kepler. Bürgi constructed a table of sines ("Canon Sinuum"), which was supposedly very accurate, but since the table itself is lost, it is difficult to be sure of its real accuracy (for instance, Valentinus Otho's "Opus Palatinum" had parts which were not as accurate as it was claimed). An introduction to some of Buergi's methods survives in a copy by Kepler; it discusses the basics of Algebra (or "Coss" as it was known at the time), and of decimal fractions. Some authors consider Bürgi as one of the inventors of logarithms. His legacy also includes the engineering achievement contained in his innovative mechanical astronomical models. During his years in Prague he worked closely with the astronomer Johannes Kepler at the court of Rudolf II.