German astronomer whose observations of the nova of 1572 demonstrated it was a new star, indicating the heavens were changeable, not fixed as many had previously thought. Mästlin also failed to detect parallax for the comets of 1577 and 1580, suggesting they were supralunar bodies. These celestial events led him to reject Aristotelian cosmology. After attending Mästlin's lectures on the superiority of Nicolaus Copernicus's cosmology, Johannes Kepler embraced Copernicanism. Mästlin published the first correct explanation of earthshine—pale illumination next to the lunar disk crescent due to reflection from the sunlit Earth.