Nikolaus Copernicus (Polish Kopernik), a Polish or German astronomer, discoverer of the system of planetary revolutions, born at Thorn, in Prussia, Feb. 19, 1473, a few years after the annexation of the town to Poland, died at Frauenburg, May 24 (according to others, June 11), 1543. After receiving the rudiments of a classical and scientific education in his father's house, he studied medicine and received his doctor's diploma at the university of Cracow, and also devoted himself to mathematics and astronomy, to which he was incited especially by the example of Regiomontanus. He subsequently spent several years in Italy, studying astronomy under Domenico Maria at Bologna, and afterward at Rome, where he gained a reputation rivalling that of Regiomontanus. He taught mathematics at Rome with eminent success, and about 1503 returned to Prussia, having previously been made canon of Frauenburg by his uncle the bishop of Erme-land. Having overcome the opposition which was made to his settlement by the conflicting claims of others, he not only attended faithfully to the duties of his clerical office, but also gave free medical advice to the poor, and at the same time prosecuted astronomical labors.

Struck with the complexity of the Ptolemaic system of the universe, he sought to explain the planetary motions upon a more rational principle. After extending his researches over all the then known systems, he came to the following conclusions: The sun and stars are stationary; the moon alone revolves about the earth; the earth is a planet whose orbit is between Venus and Mars; the planets revolve about the sun; and the apparent revolution of the heavens is caused by the rotation of the earth on its axis. His discoveries had attracted the attention of astronomers, who were eager to see the data and proofs, while Copernicus, though he had already finished the composition of a work relating to his theories, hesitated to publish it till the most thorough investigations had satisfied him as to the perfect accuracy of its principles and details. His six books Be Orbium Gcelestium Revolutionibus were printed at Nuremberg in 1543 (later editions, Basel, 1566, and Amsterdam, 1617 and 1640), under the care of Rheticus, one of his disciples; and the first copy of the work was placed in the hands of Copernicus on the very day of his death. Besides his principal work, he published minor treatises on morals, trigonometry, and coinage.

The tower from which he made his observations, and the ruins of a hydraulic machine constructed by him, still exist at Frauenburg. Monuments were raised to his memory at Cracow in 1822, with the appropriate inscription, Sta, Sol, ne rnoveare, and at Thorn in 1853; and a colossal statue by Thorwaldsen at Warsaw in 1829, Poland claiming Copernicus as one of her sons. His life has been written by Gassendi (Paris, 1654), Westphal (Constance, 1822), Czynski (Paris, 1846), and Prowe (1853). In 1872 a prize of 500 thalers was offered for the best biography of him. The 400th anniversary of his birth was celebrated at Posen, Feb. 19, 1873.