Though Hooke had experimented with musket balls and mud in making impact craters, he believed that lunar craters probably had a volcanic origin. At that time, interplanetary space was thought to be empty, and Hooke couldn't imagine where the impact objects came from. Today, we know that most of the moon's surface formed more than three billion years ago when the solar system was heavily populated with asteroids. Earth was also hit many times, but water, life, and atmospheric and geologic processes have eroded many of Earth's craters until they are barely recognizable today. Because most of those processes do not affect the moon, its surface has remained relatively unchanged since the end of the period of heavy cratering.
Impact craters are a natural lure for children.
The spectacular way in which they form spurs a child's imagination, while the features that result are easily spotted in the most modest equipment. Anyone can look up at the full moon and identify its largest seas and craters. With a pair of binoculars, lunar mountain ridges, crater rims, and central peaks come into focus. He was accustomed to seeing a much larger phase. The moon holds craters, basins, mountains, rilles, and rays.