Vallis Rheita

Type: Crater chain valley

Size: 580km Longitude/latitude: 51.7° E, 42.5° S Age: Older than 3.9 billion years

Best time to see: Four days after new Moon (11–13 April) or three days after full Moon (25–27 April)

Minimum equipment: 50mm refractor

Vallis Rheita is a linear valley to the south-southeast of Mare Nectaris, not far from the Moon’s southeast limb. Its valley form is the result of multiple impacts assumed to be associated with the formation of Mare Nectaris, with which it’s radially aligned. The less well-defined Vallis Snellius, a similarly sized crater chain valley to the northeast, shows a similar alignment. It seems the impact that formed Mare Nectaris threw out objects that landed in such a way that they created the two linear valleys.

Vallis Rheita is named after the 70km crater Rheita near its northern extremity. Rheita appears to

The scalloped impacts make superb targets for observation

interrupt Vallis Rheita, indicating it’s younger than the crater chain. The other side of the valley is marked by 88km Metius, both Rheita and Metius appearing like gateway guardians to the valley. Metius is interesting under low-light conditions. A series of hills appear aligned southeast–northwest on its floor. When the lighting is just right, the hill peaks create the impression of a set of stairs, similar to the Zeno Steps clair-obscur effect that occur near the 65km crater Zeno, close to the Moon’s northeast limb.

Vallis Rheita appears to continue north of the Rheita–Metius line, but its form becomes hard to discern as it heads towards 50km Neander. In the opposite direction, heading southeast, you can see impressions of the different impacts that form the chain. An impressive interaction can be seen with 73km Young, 160km southeast of Rheita. Young is an ancient crater that appears overlaid by Vallis Rheita. It’s almost as if something has applied pressure on the southwestern two-fifths of Young and it has simply dropped in altitude as a result.

Immediately south of Young lies the younger form of 45km Young D. Unlike Young, Young D stamps its mark on Vallis Rheita, completely overlapping the crater chain and appearing to terminate its progress. However, look carefully and you’ll see evidence for a narrower, more furrowed gouge in the lunar surface starting at the southeastern rim edge of 34km Mallet C. Its track takes it over the more ancient and heavily worn form of 58km Mallet, which has a large portion of its floor taken up with 28km Mallet A. The 41km crater Mallet D interrupts Vallis Rheita adjacent to Mallet, the valley reappearing to the south of this crater’s rim where it interrupts 42km Mallet K, continuing south to terminate at 29km Reimarus A.

The scalloped impacts that form Vallis Rheita make superb targets for observation when the terminator is nearby, the low Sun really exaggerating the individual impact sites. Being relatively close to the southeastern limb, lunar libration also has an effect on the crater chain. The early-phase view on the evenings of 11–13 April sees the terminator crossing the feature, with Vallis Rheita appearing fully lit on 13 April, under favourable libration. A view after full Moon on the mornings of 25–27 April will show the valley under unfavourable libration; it will be interesting to compare the two views.