This was only a theoretical presentation of the concept — due to topographical restrictions, it is impossible to see Middelburg from either Brussels or Antwerp. Nevertheless, the figure soon became well known all across Europe. Twenty years later, in ~1553, he was the first to describe how an accurate clock could be used to determine longitude. Jean-Baptiste Morin (1583–1656) did not believe that Frisius' method for calculating longitude would work, remarking, "I do not know if the Devil will succeed in making a longitude timekeeper but it is folly for man to try." In the event it took two centuries before John Harrison produced a sufficiently accurate clock. Frisius created or improved many instruments, including the cross-staff, the astrolabe, and the astronomical rings (also known as "Gemma's rings"). His students included Gerardus Mercator (who became his collaborator), Johannes Stadius, John Dee, Andreas Vesalius and Rembert Dodoens. Frisius died in Leuven at the age of 46. According to an account by his son, Cornelius, Gemma died from kidney stones, which he had suffered from for a minimum of 7 years. A lunar crater has been named after him. Gualterus Arsenius, the 16th century scientific instrument maker, was his nephew.