 Galileo defended heliocentrism based on his astronomical observations of 1609 ( Sidereus Nuncius 1610 ) . In December 1613 the Grand Duchess Christina of Florence confronted one of Galileo 's friends and followers , Benedetto Castelli , with biblical objections to the motion of the earth . According to Maurice Finocchiaro this was done in a friendly and gracious manner , out of curiosity . Prompted by this incident , Galileo wrote a letter to Castelli in which he argued that heliocentrism was actually not contrary to biblical texts , and that the bible was an authority on faith and morals , not on science . This letter was not published , but circulated widely . 
