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We do not know precisely when or why Nicholas Copernicus adopted a heliocentric system. Before 1514, he wrote a brief prospectus for his radical rearrangement of the planets (the so-called Commentariolus), but he realized that to compete with Ptolemy's Almagest, he had to prepare a major treatise that included key observations distributed over each planet's orbit. Copernicus remained hard at work on this task in his late 60s. His still partly unfinished magnum opus would not have been printed in his lifetime except for the arrival of a young disciple from Wittenberg, Georg Joachim Rheticus, who eventually took a copy of the manuscript to Nuremberg for publication. The printing of approximately 400 copies of the book was completed in April of 1543, and the final sheets (actually the front matter, which was struck off last) reached Copernicus only on the day he died, 24 May 1543.
Copernicus had found a ``theory pleasing to the mind," but he had no observational evidence to prove the sun-centered layout. As for his contemporaries, the entire weight of tradition reinforced the notion that astronomers dealt with geometry and hypotheses, not physics or physical reality. Hence, in the sixteenth century, heliocentrism was viewed almost universally as a curious hypothesis, not as a viable cosmology. The annotations that early owners made in the margins of their copies of De revolutionibus substantiate this view. Yet, despite the fact that the earth's motion seemed contrary to the evidence of the senses, a brilliant cosmological vision had seized Copernicus' imagination, one that would eventually capture Kepler's as well. When Galileo saw that a moving Jupiter did not lose its satellites, he, too, became an enthusiastic heliocentrist. De revolutionibus rapidly became an icon, rather than a handbook for the new astronomy; even today, at a price of over \$100,000 for a first edition, it remains a symbol of the revolution in our world view.
