The old crater Sylvester (58-kilometer [36-mile] diameter) is on the western limb of the moon, near the north pole. It shows many younger craters on top of its flat floor. Scientists use these relations to map the relative ages of surface features; craters on top of other craters must post-date them. At left is a very young, fresh impact crater (3-kilometer [nearly 2-mile] diameter). The unrelated impact crater Sylvester N, 20 kilometers (12.5 miles) in diameter, is visible at the bottom of the image.