BBC Sky at Night Magazine
September’s top lunar feature to observe
Purbach is a large and quite ancient crater. If you’re a fan of the shadow and light play which produces clair-obscur effects, you may be familiar with at least part of Purbach as its eastern rim forms part of the Lunar X. The other craters involved are 68km La Caille to the northeast and 68km Blanchinus to the east.
Purbach is a fairly battered crater, although its eroded rim is largely intact, appearing best defined to the east. The crater’s floor sits at a depth of 3km and is largely flat in appearance apart from a series of flooded ghost craters in the northwest quadrant. The best defined of these is 20km Purbach W, slightly west of the centre of Purbach. To the south of Purbach W is Purbach A, an 8km craterlet with another small craterlet embedded in its northern rim.
If you’re able to see Purbach A, try for the trickier Purbach T close to the northeast rim. At 5km across this is a good test for a 100mm scope. Mid-way between T and A sits tiny Purbach X. At just 4km across at least a 200mm scope is needed to see it. A much easier member of the Purbach family interrupts the main crater’s rim to the northwest, another flatfloored feature, identified as Purbach G. Purbach’s western rim is interesting because there appears to be another rim arc that extends north, passing to the west of Purbach G. Inside Purbach, this gives the impression of a double rim. As ever, the appearance of these features is enhanced when lighting is oblique, a situation which occurs when the Sun is low in Purbach’s sky.
South of Purbach lies ancient Regiomontanus. At 126km across this crater is similar in size to Purbach, but it’s far less distinct. Its rim is eroded and irregular with an oval shape, 126km at the widest point but only 110km at the narrowest. A ridge of material aligned north-northwest to southsoutheast runs from its northern edge, where it connects to Purbach, into the centre of the crater. A tiny 6km craterlet, Regiomontanus A sits close to the centre of the main crater, on the ridge.
To the northwest of Purbach G lies the well-defined form of 58km Thebit. Fairly unremarkable at first glance, the circular rim of Thebit appears to have two similar-sized scalloped out regions to the south. The northwest portion of the rim is interrupted by 20km Thebit A. Like Russian dolls, look at Thebit A and its northwest rim appears interrupted by 12km Thebit L.
To the southwest of Thebit and northwest of Purbach sits the completely flooded crater Thebit P. Despite having a diameter of 78km, this is the most eroded feature yet, its outline being very difficult to discern at all. The western edge is marked by an arc-like ridge unofficially called the Stag’s Horn Mountains. The arc is part of another clair-obscur effect called the Cutlass. The effect is completed by the presence of Rupes Recta, the Straight Wall, which appears to stretch from the top of the arc for a distance of 120km to the north-northwest.