8. Dialog on Two World Systems In dialog form, arguments are presented in the Italian idiom, comparing the Ptolemaic and Copernican systems. Two interlocutors argue for Copernicus, one for Ptolemy, thus leaving Galileo free to claim impartiality. Galileo meets the two chief onjections to heliocentricity--stellar parallax and vertical fall of terrestrial bodies. In this book Galileo introduced hisimportant theory of the tides. The book was placed in the Index where it remained until 1822. Galileo was tried and confined by the Inquisition.