   Nevertheless, Copernicus began to work on astronomy on his own. Sometime between 1510 and 1514 he wrote an essay that has come to be known as the Commentariolus (MW 75–126) that introduced his new cosmological idea, the heliocentric universe, and he sent copies to various astronomers. He continued making astronomical observations whenever he could, hampered by the poor position for observations in Frombork and his many pressing responsibilities as canon. Nevertheless, he kept working on his manuscript of On the Revolutions. He also wrote what is known as Letter against Werner (MW 145–65) in 1524, a critique of Johann Werner’s “Letter concerning the Motion of the Eighth Sphere” (De motu octavae sphaerae tractatus primus). Copernicus claimed that Werner erred in his calculation of time and his belief that before Ptolemy the movement of the fixed stars was uniform, but Copernicus’s letter did not refer to his cosmological ideas.