The first evidence of Galileo's espousal of heliocentrism came in a letter to Kepler of 1597, in which he described himself as a long-standing Copernican. His astronomical discoveries of 1610, achieved thanks to the telescope, thus came as a confirmation of earlier-held beliefs. With the Sidereus Nuncius, Galileo inaugurated modern cosmology. He showed that the Moon had Earth-like valleys and mountains; that the Milky Way is not a denser part of the heavens, but an impressive array of stars; and that Jupiter is surrounded by four satellites. These discoveries destroyed the very foundations of Aristotelian-Ptolemaic cosmology. Later, Galileo made other extraordinary observations that bolstered his Copernican faith: the strange appearances of Saturn, the phases of Venus, and sunspots.