Nicolai Reymers Baer

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Nicolai Reymers Baer, known as Ursus was a German mathematician. He lived during the approximate time period between 1550 and 1600.

He served as the official mathematician to the Holy Roman Emperor Rudolf II. In 1588 he claimed to have devised a model of the solar system where the planets revolved around the Sun, while the Earth only spun around on its axis. In this he differed from Copernicus, who had postulated also that the Earth orbited the Sun. Ursus objected to the Copernican model as it violated the Aristotelian principle of not allowing more than one natural movement by a body.

He was noted for a bitter dispute with Tycho Brahe regarding plagiarism of Tycho's work. Tycho complained that Ursus had plagiarized both his system of the world, as well as the publication of the mathematical model of prosthaphaeresis. History has sided with Ursus on the later issue, and he had stated that the technique was the invention of Paul Wittich and Jost Bürgi.