If you have a small telescope, or a powerful pair of binoculars, and look to the lower left of the great triple-crater chain of Ptolemaeus, Alphonsus and Arzachel you will see - depending on the time of the month - either a short, dark line or a short, bright line. Moon atlases and phone apps identify it as ‘Rupes Recta’, and to be honest it doesn’t look like much at first glance, nothing more than a dark pencil line or a white chalk scratch drawn on the Moon’s round, grey face. But Rupes Recta has another name, and it is one of the most famous and beloved features on the whole of the Moon. It’s known as the ‘Straight Wall’. It’s not actually a wall. It is an enormous scarp, a region where part of the Moon’s surface dropped dramatically away, leaving a steep cliff behind.
That cliff is very narrow, nowhere near as wide as the terraced rims of those three giant craters blasted out of the Moon to its north. It’s not that tall either: with a maximum height of around 450 metres (1,476 feet), it’s roughly as tall as the London Eye, or two nuclear submarines balanced end to end. And although the Straight Wall gives the impression of being a towering cliff face, it’s not. Pre-Apollo space artists depicted the Straight Wall as vertical - a frozen tsunami wave of grey lunar stone - but if you stood at its base its slope would rise at an angle of only 30 degrees or so.