Only in the past 400 years has it been possible to study the moon with more than just the naked eye. With the invention of the telescope in the seventeenth century, astronomers such as Galileo Galilei could, for the first time, inspect its surface in closer detail. The Moon emerged as a whole world unto itself with mountains, "seas", and scars that astronomers called craters because of their resemblance to volcanic craters. The Moon became a place with topographical landmarks that one could name: the lunar Apennines, the crater Tycho, or the Sea of Tranquility, and later, minor features, like Mt. Hadley ( above ) photographed by Apollo 15.