 The debate between Galileo and Grassi started in 1618, when the latter published "Disputatio astronomica de tribus cometis anni MDCXVIII", in which he asserted that comets are celestial bodies.Grassi adopted Tycho Brahe's Tychonic system, in which the other planets of the Solar System orbit around the Sun, which, in turn, orbits around the Earth.In his "Disputatio" Grassi referenced many of Galileo's observations, such as the surface of the Moon and the phases of Venus, without mentioning him.