Cleomedes - 126 km wide
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It's hard to get a good picture of things on the edge of the moon. Here we see Cleomedes crater almost touching the shore of Mare Crisium. Cleomedes is almost 4 billion years old, but escaped being filled with lava. It has pretty steep walls and a strangely off-center mountain. Mare Crisium is the most complete and roundest mare we can see. It is 638 km wide with a very flat and dark floor.
Image: March 24, 2008