The week of April 29 – May 5 takes us from Day 20 to Day 27. This week we will highlight Rupes Recta, the Straight Wall, viewable early Wednesday morning, Day 22, and located in the SW quadrant of the Field Map at M9.

Rupes Recta fault straight wallStraight Wall: (Rupes Recta): [SW/M9] Two of the best-known faults on the Moon are Rupes Recta and Rupes Cauchy [Day 4]. They are fascinating to explore and they share remarkably similar neighborhoods: Both features are paralleled by a nearby rille, and in each case, an intervening small crater separates the rille and the fault.

Mare Nubium and Straight Wall on the moonStraight Wall is the showpiece lunar fault line and can be seen clearly in the smallest telescopes. It is 70 miles long and 1,300 feet high. If you view it either in the early morning on Day 8, or at lunar sunset around Day 22, you will see clear evidence that Straight Wall is an enormous fault line where the terrain to its west has fallen away. In the morning, the rising sun casts a thick black shadow over the lava plain to the west. At sunset two weeks later, the exposed slope of the fault is brightly illuminated.

At its south end, Straight Wall terminates in what looks like a small arc of mountains. The effect of the ensemble makes Straight Wall look remarkably like a sword with handle and hilt. This arc of mountains is actually the remains of an ancient unnamed crater whose western side, like Straight Wall itself, dropped as a result of faulting. When lava-flows later flooded Mare Nubium, they also covered the slumped western portion of this crater.


On Friday, Saturn is 0.8° north of the Moon.
On Saturday, the Eta Aquarid meteor shower peaks. Look to the SW after midnight.

It is highly recommended that you get a copy of Sky and Telescope’s Field Map of the Moon, the very finest Moon map available for use at the telescope. It is available for $10.95 at and on Amazon. All features mentioned in this blog will be keyed to the grid on the Field Map and will look like this: Plato: [NW/D9]

Courtesy of Gray Photography of Corpus Christi, Texas
Lunar photos: NASA / USGS / BMDO / LROC / ASU / DLR / LOLA / Moon Globe. Used by permission

Andrew Planck
Rupes Recta – The Straight Wall on the Moon
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