The cosmological system of Ursus is similar to that of Tycho Brahe. Both place the Earth in the center, and set the other planets revolving around the Sun. For Ursus, in contrast to Tycho, the Earth rotates around its axis once a day, allowing the sphere of stars to stand still.
The sky appears the same whether the Earth rotates or the stars turn each day around a giant sphere. But if the region of the stars is not a rotating sphere, then one is freed to consider that stars might be positioned at immense and various distances from the Earth.
The system of Ursus, adopted by William Gilbert and others, remained a prominent rival to the Tychonic into the 17th century.
Ursus visited Tycho at Hven in 1584, and Tycho sent him away, fearing Ursus would steal his own system. As it turned out, they published their systems the same year, although Tycho published first: The preface to Tycho’s De Mundi is dated May, 1588 while that of Ursus is dated August. It was a period of experimentation with diverse cosmic systems, yet a bitter priority dispute broke out between Tycho and Ursus, which enveloped Kepler and other contemporary astronomers.
Ursus differed from Tycho by proposing that the Earth rotates daily around its axis and that the orbs of Mars and the Sun do not intersect. Both of these points were sometimes held by astronomers for the following century. In retrospect, it is easy to confuse these astronomers with those holding Tychonic views.
The History of Science Collections possesses another work by Ursus:
De astronomicis hypothesibvs (1597).