Danish astronomer and astrologer who is best known for his association with, and published support for, Tycho Brahe. Longomontanus used Tycho's data to compile the Astronomia danica (1622), an exposition of the Tychonic system, which holds that the Sun revolves around the Earth and the other planets revolve around the Sun. He began the construction of the Copenhagen Observatory in 1632 but died before its completion.