Tycho Brahe ( TY-koh BRAH(-hee) -⁠ BRAH-(h)ə; born Tyge Ottesen Brahe; 14 December 1546 – 24 October 1601) was a Danish astronomer, known for his accurate and comprehensive astronomical observations. Born in Scania, which became part of Sweden in the next century, Tycho was well known in his lifetime as an astronomer, astrologer, and alchemist. He has been described as "the first competent mind in modern astronomy to feel ardently the passion for exact empirical facts". Most of his observations were more accurate than the best observations available at the time.

An heir to several of Denmark's principal noble families, Tycho received a comprehensive education. He took an interest in astronomy and in the creation of more accurate instruments of measurement. As an astronomer, Tycho worked to combine what he saw as the geometrical benefits of Copernican heliocentrism with the philosophical benefits of the Ptolemaic system into his own model of the universe, the Tychonic system. His system correctly saw the Moon as orbiting Earth, and the planets as orbiting the Sun, but improperly considered the Sun to be orbiting the Earth. Furthermore, he was the last of the major astronomers to work without telescopes. In his De nova stella (On the New Star) of 1573, he refuted the Aristotelian belief in an unchanging celestial realm. His precise measurements indicated that "new stars" (stellae novae, now called supernovae), in particular that of 1572 (SN 1572), lacked the parallax expected in sublunar phenomena and were therefore not tail-less comets in the atmosphere, as previously believed, but were above the atmosphere and beyond the Moon. Using similar measurements, he showed that comets were also not atmospheric phenomena, as previously thought, and must pass through the supposedly immutable celestial spheres.

King Frederick II granted Tycho an estate on the island of Hven and the money to build Uraniborg, an early research institute, where he built large astronomical instruments and took many careful measurements. He later worked underground at Stjerneborg, where he discovered that his instruments in Uraniborg were not sufficiently steady. On the island (whose other residents he treated as if he were an autocrat) he founded manufactories, such as a paper mill, to provide material for printing his results. After disagreements with the new Danish king, Christian IV, in 1597, Tycho went into exile. He was invited by the Bohemian king and Holy Roman Emperor Rudolph II to Prague, where he became the official imperial astronomer. He built an observatory at Benátky nad Jizerou. There, from 1600 until his death in 1601, he was assisted by Johannes Kepler, who later used Tycho's astronomical data to develop his three laws of planetary motion.

Tycho's body has been exhumed twice, in 1901 and 2010, to examine the circumstances of his death and to identify the material from which his artificial nose was made. The conclusion was that his death was probably caused by uremia—not by poisoning, as had been suggested—and that his artificial nose was more likely made of brass than silver or gold, as some had believed in his time.