Last Sunday nights sojourn has us descending (telescopically-speaking) to the southern shores of Mare Nubium where there are some interesting features. The dominant crater Pitatus (106 km wide) has a central 500m high peak and a light dusting of ray material (lighter patches) on the western hemisphere of the crater floor (lunar north is to the right). Pitatus has spectacular concentric rilles that follow the inner walls of the crater. These crater floor rilles – which can be radial, transverse or concentric) are common in many larger craters that are 20-30 or more km in diameter, where the impact morphology has been modified by subsequent volcanic-related activity. It is a floor-flooded crater that was flooded by magma that emanated from cracks and fissures in the crater floor.

Just to the right of Pitatus, within Mare Nubium itself, lies the small half-buried crater rim of Pitatus S. Adjoining Pitatus is 42km wide Hesiodus with its central crater Hesiodus D. A distinct cleft in the wall of Hesiodus joins it to the larger Pitatus crater.

Hesiodis A is the ‘eye’-catching crater with a tight double-walled structure that looks somewhat artificial but is entirely natural, there are a number of similar small features on the lunar surface. Exactly why the lavas in concentric craters bubbled up to form donut-like rings remains unclear. It may have to do with cracks preferentially forming along the edges of crater floors, or the rate of lava flow as well as its texture. A plug of molten rock beneath the crater may have lifted up its floor, which later deflated in the centre like a collapsed soufflé, leaving a ring. It's even possible that dense impact melt on the crater floor resisted uplift from lavas pushing up from below. Whatever the specific reason, most lunar experts agree that concentric craters owe their formation to volcanic processes (Sky and Telescope article).

Heading WSW from Hesiodus (almost directly up the image) is the remarkably linear Rima Hesiodus, a linear depression running along the floor of Mare Nubium that is around 1.0 to 1.5 km wide and 256 km long. Light ‘washes’ of ray material that orthogonally intersect Rima Hesiodus trend towards the off-image (but naked eye visible) Tycho impact crater.

Moving south (left) from Rima Hesiodus we enter the bay-like morphology of crater Weiss, beyond which lies Cichus whose two thirds illuminated crater floor which is nicely illuminated at this time of the lunar cycle.

Continuing our ant-clockwise journey on the southern shores of Mare Nubium we come to two old and deeply incised ancient crater basins named Wurzelbauer and Gauricus named after German and Italian astronomers respectively.

Skywatcher 8 inch/f5 Newtonian, ASI1600mm Pro, Televue 5x Powermate, 27-09-2020, Melbourne, Australia.