Nicholas Copernicus, a Polish astronomer, openly questioned the Ptolemaic system and proposed a heliocentric system in which the planets"including Earth" orbited the sun(Helios". This more mathematically satisfying way of arranging the solar system did not attract many supporters at first, since the available data did not yet support a wholesale abandonment of Ptulemy's system. By the end of the 16th century, however, astronomers like Johannes Kepler(1571-1630) had also begun to embrace Copermitin's theory.