Nicolaus Reimers Baer (2 February 1551 – 16 October 1600), also Reimarus Ursus, Nicolaus Reimers Bär or Nicolaus Reymers Baer, was an astronomer and imperial mathematician to Emperor Rudolf II. Due to his family's background, he was also known as Bär, Latinized to Ursus ("bear").
Reimers was born in Hennstedt and received hardly any education in his youth, herding pigs until the age of 18. Yet, Heinrich Rantzau discovered his talents and employed him from 1574 to 1584 as geometer. Accordingly, Reimers in 1580 published a Latin Grammar and in 1583 his Geodaesia Ranzoviana. Rantzau also arranged a meeting with Tycho Brahe.
From 1585 to 1586 he was employed as a private tutor in Pomerania and from 1586 to 1587, Reimers stayed at the court of William IV, Landgrave of Hesse-Kassel in Kassel, where he met Swiss instrument maker Jost Bürgi (1552–1632). Both were autodidacts and thus had a similar background. As Bürgi did not understand Latin, Reimers translated Copernicus' De Revolutionibus Orbium Coelestium into German for Bürgi. A copy of the translation survived in Graz, it is thus called "Grazer Handschrift".
Reimers was a bitter rival of Tycho Brahe (his successor as imperial mathematician) after he tried to claim the Tychonic system as his own. 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.
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.
Johannes Kepler committed a faux pas early in his career by sending a laudatory letter to Reimers while seeking the patronage of Tycho. Ursus published the letter in the preface to his work claiming priority for Tycho's cosmological ideas.
But unlike Tycho's geoheliocentric system in which the Earth does not rotate and the Martian and Solar orbits intersect, in that of Ursus and his follower Roslin the Earth had a daily rotation and also the Martian and Solar orbits do not intersect, thus avoiding the Tychonic conclusion in respect of the Martian orbit that there are no solid celestial spheres on the ground that they cannot possibly interpenetrate. But on the other hand the orbits of Mercury and Venus would obviously intersect the Martian orbit in Reimers' illustration of his model, and indeed also intersect Jupiter's orbit.
However Kepler discovered Tycho had posited intersecting Martian and Solar orbits because he had mistakenly concluded from his data that at opposition Mars was closer to the Earth than the Sun was. The source of the error was a research assistants' mistaken calculation of Mars's daily parallax from observations during its 1582-3 opposition as greater than that of the Sun's presumed 3' parallax. Kepler discovered Tycho’s observations revealed little or no Martian parallax, implying it was further than the Sun at opposition. This would have refuted Tycho's system in favour of Ursus's and Roslin's. It seems it has yet to be determined whether the dominant astronomical system of the 17th century was the geoheliocentric system of Tycho or that of Ursus and Roslin at least in respect of non-intersecting Solar and Martian orbits, and also in that of the Earth's rotation or not.
Reimers died in Prague.
- "Grammatica Ranzoviana", 1580.
- Geodaesia Ranzoviana, Leipzig 1583.
- Nicolai Raymari Ursi Dithmari Fundamentum astronomicum, Straßburg 1588.
- Metamorphosis Logicae, Straßburg 1589.
- "Nicolai Raymari Ursi Dithmarsi Croius Puer seu Carmen Gratulatorium", Straßburg 1589.
- "Alt und auch Röm. Schreibkalender auff das Jahr 1593", Erfurt 1592.
- "Prognosticon Astrologicum dieses 1593. Jahrs", Erfurt 1592.
- "Alt und New Schreibcalender auff das Jahr 1594", Erfurt 1593.
- "Allegory to Emperor Rudolph II.", 1594.
- "Nicolai Raymari Ursi Dithmarsi Parentatio Iacobi Curtii", Prag 1594.
- Nicolai Raimari Ursi Dithmarsi de Astronomicis Hypothesibus, Prag 1597.
- Chronotheatron, Prag 1597.
- "Demonstratio Hipotheses Motuum Coelestium", Prag.
- Nicolai Raimari Ursi Dithmarsi Arithmetica Analytica vulgo Cosa oder Algebra, Frankfurt/Oder 1601.
- "Nicolai Raimari Ursi Ditmarsi Chronologische Beweisung", posthum Nürnberg 1606, Schleswig 1606, Schleswig 1666.
- UB-Graz / Handschriftenkatalog / Katalogisat Nr.:560
- Nicolaus Copernicus Gesamtausgabe: De revolutionibus: die erste deutsche Übersetzung in der Grazer Handschrift 
- Jürgen Hamel: Die astronomischen Forschungen in Kassel unter Wilhelm IV. Mit einer wissenschaftlichen Teiledition der Übersetzung des Hauptwerkes von Copernicus 1586 (Acta Historica Astronomiae ; Vol. 2) Thun ; Frankfurt am Main : Deutsch, 1998; 2., korr. Aufl. 2002, 175 S., ISBN 3-8171-1569-5 (1. Aufl.), 3-8171-1690-X (2. Aufl.), Abb., 15 x 21 cm, kartoniert EUR 14,80 / sFr 23,10. Inhalt: HTML PDF
- Max Caspar, Kepler, Translated and edited by C. Doris Hellman, New York, Abelard-Schuman, 1959
- See pp34 & 36 of Christine Schofield's 'The Tychonic and semi-Tychonic world systems' in Wilson & Taton 1989 'Planetary astronomy from the Renaissance to the rise of astrophysics Part A: Tycho Brahe to Newton' CUP
- See p178-80 of Dreyer's 1890 'Tycho Brahe'
- Owen Gingerich, Robert S. Westman: The Wittich Connection: Conflict and Priority in Late Sixteenth-century Cosmology, American Philosophical Society, 1988, 
- Dieter Launert: Nicolaus Reimers (Raimarus Ursus). Günstling Rantzaus – Brahes Feind. Leben und Werk. München 1999. ISBN 3-89241-030-5
- Dieter Launert: "Nicolaus Reimers Ursus - Stellenwertsystem und Algebra in der Geodaesia und Arithmetica". München 2007. ISBN 9783769609691
- Moritz Cantor (1888), "Raimarus Ursus: Nicolaus R.", Allgemeine Deutsche Biographie (ADB) (in Deutsch), 27, Leipzig: Duncker & Humblot, pp. 179–180<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Tycho Brahe and Mathematical Techniques
- Otto Liebmann (1895), "Ursus: Nicolaus Reimarus U.", Allgemeine Deutsche Biographie (ADB) (in Deutsch), 39, Leipzig: Duncker & Humblot, pp. 374–<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>