In De mundi aetherei (1588), Tycho reported that the comet of 1577 displayed no detectable parallax and thus moved, contrary to Aristotle, in the regions of the heavens beyond the Moon, passing through multiple celestial spheres. The ancient solid spheres melted. No longer were planets carried by giant transparent spheres. Rather, planets must swim through fluid heavens as fish swim through the sea. In the Tychonic system, the Earth lies at the center of the cosmos. The planets revolve around the Sun, while at the same time the Sun revolves around the Earth. Because the sphere of Mars intersects the sphere of the Sun, the Tychonic system was incompatible with solid celestial spheres. So long as one abandoned solid spheres, the Tychonic system provided all of the mathematical elegance of the Copernican system, but without the arbitrary supposition of unreasonably large stars.