This 3.90-zł stamp honors Nicolaus Copernicus (Mikołai Kopernik) (1473-1543), the famous astronomer and mathematician who postulated the heliocentric theory that the earth was a moving heavenly body, one of the planets, that all revolved around the sun. Before Copernicus, the prevailing view favored the geocentric theory of Ptolemy, with a stationary earth at the center of everything. Copernicus based his theory on his own and others’ precise observations of planetary motions, and used complex geometric calculations to substantiate his theory. All of his observations were with the naked eye, since the telescope hadn’t yet been invented. His theory was revolutionary in that it provided a more accurate explanation of observed planetary motions compared to the geocentric theory.