Rupes Recta 22.1°S, 7.8°W

Rupes Recta – also known as the Straight Wall, Straight Scarp or Wall Beta – is a typical lunar undulation, an embankment. With grazing illumination it is a spectacular surface feature, even for observations with small telescopes. Lengths that are given vary between 110 km and 130 km, and the difference in height between the eastern and western sides amount to between 250 and 300 m, with a width of about 2.5 to 3 km. This gives a gradient of less than 10°, so Rupes Recta is rather a gentle slope and not – as the spectacular shadow might imply – a steep escarpment. The slope was probably created by a subsidence of the western side – the crater Birt side – of Mare Nubium, initiated by shock waves from the Imbrium impact and later altered and deformed by the Mare Nubium lava flows.

During the time around First Quarter, Rupes Recta is illuminated by the Sun at a low angle and casts a shadow that is easily visible even in small telescopes. A few days later, when the day/night border (the terminator) is about 20 to 30° away, the feature disappears and may only be made out in large telescopes. At the time of Last Quarter, Rupes Recta appears as a bright line, and a narrow rille (at right angles to Rupes Recta) becomes visible. North of Rupes Recta lies Promontorium Taenarium (19.0°S, 7.0°W).