 Later, in 1623, Galileo argued for a quite mistaken material thesis. In The Assayer, he tried to show that comets are sublunary phenomena and that their properties could be explained by optical refraction. While this work stands as a masterpiece of scientific rhetoric, it is somewhat strange that Galileo should have argued against the superlunary nature of comets, which the great Danish astronomer Tycho Brahe had demonstrated earlier.