February’s top lunar feature to observe

Walther

Type: Crater

Size: 141km

Longitude/Latitude: 0.6o E, 33.2o S

Age: Approximately 3.9 billion years

Best time to see: First quarter (8-9 February) or six days after full Moon (22-23 February)

Minimum equipment: 10x binoculars

Walther is a large, rugged crater located 440km northeast of the prominent 86km ray crater Tycho. It sits within the complex, highly cratered southern region of the Moon and takes a bit of navigational finding despite its size.

Walther sits adjacent to the large walled plane of 235km Deslandres extending to the west. From the centre of Walther, 75km Miller lies 200km to the south and 70km Werner lies 174km to the northeast. Werner sits near the trio of craters, 118km Purbach, 68km Blanchinus and 68km La Caille. These are components of the popular Lunar X clair-obscur effect, which is visible around first-quarter phase.

Walther’s ancient rim shows complexity, which is most apparent under oblique lighting. Multiple terraces have become smoothed and rounded over time, but still retain their underlying structure.

Various peaks appear around the rim, one to the east rising to a height of 3km. This casts an impressive shadow when the morning terminator approaches in the run up to firstquarter. The rim has been subject to a large number of impacts and these look incredible under oblique lighting, giving the rim sections a complex texture in which it’s easy to get lost. This is especially true to the north where Walther borders 126km Regiomontanus.

Apart from the northeast quadrant, most of the inner floor appears smooth, save for an interruption from 13km Walther E to the west, and the trio of Walther N (6km), Walther M (5km) and Walther Q (4km) in the southwest. The central mountain is around 2km high.

It’s more-or-less central on the northsouth axis, but definitely offset east.

The asymmetry is accentuated by the presence of rough terrain to the northeast and includes the craterlets Walther A (12km) to the north of the central mountain, together with Walther T (8km) and Walther F (6km) to the east.

Walther’s outline stands up well, considering the erosive pounding that its rim sections have taken over time. A curious intrusion occurs in the northwest, where a progression of overlapping craters – arranged radially from Walther’s centre – appear to break through the rim.

This line includes 14km Walther C, 9km Walther B, 5km Regiomontanus Y and 5km Regiomontanus M. A more noticable breach occurs to the east. The northeast rim section is interrupted by 18km Walther D while the eastern section is more demonstrably breached by 31km Nonius L, itself overlaid southeast by 18km Nonius K. As a test of resolution for a 200mm telescope, see whether you can spot the tiny 4km craterlet Walther U, which lies on the floor of Walther immediately west of Nonius L.

Walther used to be called Walter until it was realised that there were two craters with the same name. The other Walter is a tiny 1.2km craterlet in the western part of Mare Imbrium. Apparently, the duplication had existed for 20 years before anyone noticed. The renaming of Walter to Walther was contentious and a number of amateurs and professionals still use the old name today.

Photo: PETE LAWRENCE