 Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without " a true physical demonstration that the sun does not circle the earth but the earth circles the sun " . Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth . This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea . The reference to tides was removed from the title by order of the Inquisition . 
