Nine days after new moon the great crater Copernicus (diameter 95km) and its little brother Eratosthenes (60km) are on the terminator giving one of the more spectacular views the lunar observer can see. Younger observers have related how they can see Copernicus when it is on the terminator, but I never have though I have looked many times over the years. Both craters are known as “terraced” craters for the obvious reason that the interior walls appear terraced as if it were a farming project. In the lower left corner can be seen two shadow filled craters. The largest is Reinhold (49km) and the smaller polygonal one is Reinhold B (26km).In the opposite corner are two good sized craters, Pallas (51km) and Murchison (60km) which are usually seen in images of Triesnecker (27km) that is peeking in the extreme lower right corner and its rimae system.
Heading to the northeast (upper right) from Eratosthenes is the southern end of the Montes Apenninus including Mons Wolf (3500m high). Between Copernicus and Eratosthenes you can see a clustering of craters all roughly the same size. Notice they are laid out rather concentric to the walls of Copernicus and not all are round. These are “secondary” craters formed in low velocity impacts of the ejecta thrown out during the Copernicus impact that occurred in the last billion years, rather recent on the moon. In fact, this is the basis for the Copernican Period in the lunar geologic timescale while Eratosthenes defines the Eratosthenean Period that goes from 1.1 to 3.2 billion years ago. This is why you don’t see any ejecta blanket features on the side of that crater facing Copernicus. Secondary craters can be seen all over Sinus Aestuum, the plain below Eratosthenes to the east of Copernicus. Also on this plain you can see the rays from Copernicus as delicate shadings on the surface. During high sun, like full moon, these become overwhelmingly bright and dominant.
Lost in the secondary craters between the two great craters you can see a outline of a circle. This is Stadius (71km) a once great crater every bit as glorious as Eratosthenes when it was formed over 3.85-3.8 billion years ago (the Imbrium Period) but now buried in the flooding that created Sinus Aestuum and then overlain by ejecta from both the more recent craters. Oddly enough, all these secondary crater created from Copernicus ejecta are named as satellite craters of Stadius!
