LROC Wide Angle Camera (WAC) visible to ultraviolet portrait of Copernicus Crater, image 458 kilometers (284 miles) wide.
Understanding how scientists determine the relative age of geologic units on the Moon is straightforward, most of the time. One simply follows the law of superposition; what is on top is younger, what is below is older. In some cases, superposition relations are not clear, so scientists then compare crater densities. That is the number of impact craters on a common size of ground. Since impacts occur randomly both in time and on the Moon's surface, any piece of ground has an equal chance of being hit. Over time, the number craters in a given area increases. Simply stated, the older an area the more craters you will find.
Photo credit: NASA/GSFC/Arizona State University
Note: For more information, see Absolute Time.
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