Nicolaus Copernicus

Nicolaus Copernicus was an astronomer and church administrator from Royal Prussia, then part of the Kingdom of Poland, who proposed that the Earth and the other planets move around the Sun rather than the Sun around the Earth. His book On the Revolutions of the Heavenly Spheres, published as he was dying in 1543, began the long change in thinking known as the Copernican revolution and is often taken as the starting point of the Scientific Revolution.

Copernicus was born on 19 February 1473 in Toruń, a trading city on the Vistula that had passed from the State of the Teutonic Order to the Polish crown a few years earlier. His father, a merchant from Kraków, died when he was about ten, and he was brought up by his maternal uncle, Lucas Watzenrode, who became prince-bishop of Warmia in 1489. The family spoke German, and the question of whether to call him Polish or German was argued over fiercely in the nineteenth and twentieth centuries; he was a loyal subject of the Polish king, and the national labels of later times fit him poorly.

From 1491 he studied at the university of Kraków, where he first met serious astronomy. His uncle then sent him to Italy, the heart of the Renaissance. At Bologna from 1496 he studied canon law and lodged with the astronomer Domenico Maria Novara, with whom he made observations; in 1500 he spent time in Rome, and he then studied medicine at Padua, in the Republic of Venice. He took a doctorate in canon law at Ferrara in 1503 and returned home.

His uncle had secured him a post as a canon of the cathedral chapter at Frombork, a church office that gave him a lifelong income; he was never ordained a priest. For several years he served as his uncle's secretary and physician, and after the bishop's death in 1512 he lived mostly at Frombork. The chapter gave him many practical duties. He managed its estates, settled peasants on vacant farms, acted as a doctor, and during the war of 1519 to 1521 between Poland and the Teutonic Knights he organised the defence of Olsztyn castle against attack. He also wrote a treatise on the reform of the coinage, in which he observed that bad, debased money tends to drive good money out of circulation, an idea later named after the English financier Thomas Gresham.

The standard astronomy of the time went back to Ptolemy of Alexandria in the second century, building on Aristotle. It placed a motionless Earth at the centre of the universe, with the Sun, Moon and planets carried around it. To match what astronomers actually saw, such as the way planets seem to stop and move backwards for a time, Ptolemy had each planet move on a small circle, an epicycle, whose centre itself moved on a larger one, and he used other devices that Copernicus found unsatisfying because they broke the rule of perfectly uniform circular motion.

Some time before 1514 Copernicus set out a different scheme in a short manuscript, the Commentariolus, which he passed to a few friends. In it the Sun stands near the centre, and the Earth is a planet that turns daily on its axis and travels around the Sun once a year. This explained at once why the planets seem to go backwards: it is the effect of the moving Earth overtaking, or being overtaken by, the other planets. It also fixed the order of the planets by their periods, from Mercury, nearest the Sun, to Saturn, and allowed their distances to be worked out relative to the Earth's, something Ptolemy's system could not do. Ancient writers, notably Aristarchus of Samos, had suggested a moving Earth, but Copernicus was the first to work the idea out mathematically in full.

Copernicus spent decades developing the theory but hesitated to publish, perhaps fearing ridicule more than church censure. In 1539 a young Lutheran mathematician from Wittenberg, Georg Joachim Rheticus, came to Frombork to study with him. Rheticus published a first summary, the Narratio prima, in 1540, and persuaded Copernicus to release the full work. De revolutionibus orbium coelestium, in six books, was printed in Nuremberg in 1543 and dedicated to Pope Paul III. Andreas Osiander, a Lutheran theologian who oversaw the printing, added an unsigned preface saying that the theory was only a convenient way of calculating and need not be true, which many readers took to be Copernicus's own view. According to a later account, Copernicus received a copy of the book on the day he died, 24 May 1543, after a stroke.

The system was not simple. Copernicus kept circular orbits and still needed small epicycles, so his model was hardly more accurate than Ptolemy's and not much less complicated. Its strength lay in its overall harmony and in explaining features of the sky that the old system had to take as coincidences.

For decades few astronomers accepted that the Earth really moved, though many used Copernicus's mathematics and tables. Physical objections were serious, since no one could feel the motion and no shift in the positions of the stars could be detected, and the idea seemed to conflict with passages in the Bible. Some Protestant reformers, in the period of the Protestant Reformation, dismissed it. Johannes Kepler and Galileo Galilei became its leading defenders around 1600. In 1616, during the controversy over Galileo, the Catholic Church placed De revolutionibus on the Index of Forbidden Books until it was corrected, and in 1620 a small number of passages presenting the Earth's motion as fact were altered. Kepler's elliptical orbits and Isaac Newton's physics eventually supplied what Copernicus's system lacked.

Copernicus's grave in Frombork Cathedral was unmarked. In 2005 archaeologists found remains there that were identified in 2008, by comparing DNA with hairs found in a book he had owned, as very probably his, and he was reburied with honours in 2010. Today he is a national hero in Poland, and the phrase Copernican revolution is used for any change that overturns a basic assumption. His work removed the Earth from the centre of the universe and opened the way to modern astronomy.