March’s top lunar feature to observe

Rupes Recta

Type: Fault line

Size: Length 120km, width 2-3km

Longitude/Latitude: 7.7o W, 21.7o S

Age: Approximately 3.2–3.9 billion years

Best time to see: One day after first quarter (11 March) or last quarter (25 March)

Minimum equipment: 50mm refractor

Rupes Recta, also known as the ‘Straight Wall’, is a remarkable feature on the Moon’s surface. Illuminated by morning light, it looks like a dark scratch, low resolution photos making it appear like a hair has crept into the image. In the period up to last quarter, Rupes Recta takes on a different persona, appearing as a bright line.

The variation is due to the fact that the Straight Wall forms a sloped transition from an upper surface level in the east to a lower one in the west. Viewed through a telescope, this gives the impression of a steep cliff. Rupes Recta means ‘straight cliff’.

However, as is often the case with stark light and shadow effects on the Moon’s surface, the cliff-like appearance of Rupes Recta is misleading. Measurements of the difference in height between surfaces suggests the vertical height of the ‘cliff’ to be less than half-a-kilometre, probably in the range of 400–500m. Doing the maths, this gives a slope angle of just 21˚ – hardly a cliff!

The Straight Wall is a fault line across the lunar surface. Running for 120km from northnorthwest to southsoutheast, Rupes Recta appears straight and an alternative name sometimes used to describe it is ‘the railroad’. It appears uninterrupted by younger impacts, although highresolution views and images of the fault line show kinks along its length. These appear to be features associated with the fault line itself rather than secondary external events.

Measured from the northern end, the kinks occur at distances of 15km, 25km, 42km and 86km. Close up images taken with the LRO (Lunar Reconnaissance Orbiter) suggest the kinks represent parts of the fault where one section has petered out and another started, along the same trajectory.

Rupes Recta sits in a southeast region of Mare Nubium, inside a feature that resembles a large crater, 185km in diameter. This is an unnamed feature which may just be an illusion caused by rounded highland sections to the north and southeast, with the other ‘rim section’ hinted at by wrinkle ridges in the mare to the west. The Straight Wall runs between 58km Thebit to the east and 17km Birt to the west.

The northern end of the fault begins adjacent to 5km Thebit D, itself lying just south of one of the highland regions mentioned earlier, an outcrop named Promontorium Taenarium. The other end of Rupes Recta passes across a raised mountain range, which includes a small, 17km crescentshaped range. The peaks here are known as the Stag’s Horn Mountains.

With a large telescope setup (300mm or larger), you may like to try for a narrow rille which starts to the west of Birt and gently arcs northwest, roughly parallel to Rupes Recta. Known as Rima Birt, it is around 1.5km wide and runs for 50km.