Photograph of the central region of the Moon, to the east of the Mare Nubium. The day-night boundary bisects the dark, lava-floored Mare Nubium in this image. The chain of three impact craters to the right of the centre are, from top to bottom, Ptolemaeus, Alphonsus and Arzachel. Just below and to the left of Arzachel is a dark linear feature known as Rupes Recta or the Straight Wall. It is a 120 km long 'cliff' which is 360 metres high at its highest point and rises towards the east (right) at an angle of about 40 degrees - very steep by lunar standards.