Tiedemann Giese (1 June 1480 – 23 October 1550), was Bishop of Kulm (Chełmno) first canon, later Prince-Bishop of Warmia (Ermland). His interest in mathematics, astronomy, and theology led him to mentor a number of important young scholars, including Copernicus. He was a prolific writer and correspondent, publishing a number of works on the reformation of the church. Tiedemann was a member of the patrician Giese family of Danzig (Gdańsk) in Poland. The Giese family ancestors originated from Unna in Westphalia, near Dortmund. His father was Albrecht Giese and his younger brother, the Hanseatic League merchant Georg Giese. Life and career Giese was the fifth child of Albrecht Giese and his wife, Elisabeth Langenbeck, both members of wealthy merchant families. His paternal family had emigrated from Cologne to Danzig in the 1430s. His father was the Mayor of Danzig, and his mother's uncle, Johann Ferber, had been Mayor of Danzig. At the age of 12 years, Tiedemann, along with his cousin, Johann Ferber, entered the University of Leipzig, and subsequently studied at Basel and in Italy. He earned a Master of Theology degree. Giese was one of the best educated scholars in Prussia, well versed in both theology and the sciences. At age 24, he and Mauritius Ferber (possibly a cousin) became priests at the Catholic Church of St. Peter and St. Paul. He was secretary to the King of Poland, and later appointed canon of Frauenburg (Frombork), where he remained for 30 years. His residence was the Episcopal Castle at Frauenburg. The King appointed him Bishop of Kulm on 22 September 1537 (ratified by the Pope on 11 January 1538). Toward the end of his life, he became Bishop of Ermland. Giese was supported by Chancellor Lucas David. He was a humanist and a liberal in the Erasmian mould. Although a Catholic, he demonstrated relative tolerance towards Lutherans. He made himself the spokesperson for a group of liberal and tolerant men who wanted to mediate between the "old-believers" and "the new-believers". In his writings, he expressed the aim of reconciling the Catholic and Protestant branches of the church, but ultimately alienated both of them. Bishop Giese was a lifelong friend and frequent companion of the astronomer and proponent of heliocentrism Nicolaus Copernicus and shared his interest in astronomy. As a very wealthy man, Giese had the best instruments which, from time to time, he loaned to Copernicus. Giese, seven