Fundamentum Astronomiae is a historic manuscript presented by Jost Bürgi to Emperor Rudolf II in 1592.
It describes Bürgi's trigonometry based algorithms called Kunstweg which can be used to calculate sines at arbitrary precision.
However, Henry Briggs (mathematician) (1561-1630) was acquainted with the method, likely via a link to John Dee who knew Christoph Rothmann, a colleague of Bürgi at the court.
[2] Ursus, his friend wrote in his 1588 Fundamentum astronomicum, "I do not have to explain to which level of comprehensibility this extremely deep and nebulous theory has been corrected and improved by the tireless study of my dear teacher, Justus Bürgi from Switzerland, by assiduous considerations and daily thought.
And we want to point out now how all sines can be found without the troublesome inscription [of polygons], namely by dividing a right angle into as many parts as one desires.