Even beginners learn to recognize the great crater Theophilus. Just south of that crater is the the crater at the top right of this image, Cyrillus (100km dia.). South of Cyrillus is slightly larger Catharina (104km) both of Nectarian age about 3.9 billion years old. These give us the landmarks for an ignored region west (left) of the aforementioned craters. The terraced crater just left of center is Tacitus (41km) and a farther left is Almanon. Now before going any further I should explain the term catena. This is not a place where you go with a friend one evening, to have a couple Dos Equis and talk about old times observing. Rather this is a chain or line of small craters formed when a tidally disrupted object, like a comet, breaks up while the pieces maintain their orbit and collides with a slow rotating object like our moon. We see a good example of this between Almanon and Tacitus with the line of craters running from below Tacitus to above Almanon into the dark pool that is the edge of the crater Abulfeda (65km) giving this chain the name Catena Abulfeda with a length of 216km.
There’s a couple other interesting things going on in this region. Notice the 11km crater to the lower right of Tacitus (Tacitus A) with the sinusoidal scarp stretching to the north and south and thin rimae on either side of the crater. This scarp is the northern terminus of the great Rupes Altai that starts from Piccolomini well south of Fracastorius and Mare Nectaris.
