The Dialogue Concerning the Two Chief World Systems is revolutionary book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. In the Copernican system, the Earth and other planets orbit the Sun, while in the Ptolemaic system, everything in the Universe circles around the Earth. In 1633, Galileo was found to be "vehemently suspect of heresy" based on the book, which was then placed on the Index of Forbidden Books, from which it was not removed until 1835. Please register in advance for this one-time online seminar. TIMES IN PST.