Galileo Galilei (1967). “Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, Second Revised Edition”, p.58, Univ of California Press
Essay published in 1615, in response to enquiries of Christina of Tuscany, as quoted in "Aspects of Western Civilization : Problems and Sources in History" by Perry McAdow Rogers, (p. 53), sourcebooks.fordham.edu. 1988.