There is a 110-kilometre cliff on the Moon that looks drawn with a ruler. It's a black line after first quarter and a white one after last.

Rupes Recta, the Straight Wall, is a fault where the crust dropped a couple of hundred metres along an almost perfectly straight break. Just after first quarter the low Sun throws its shadow toward us and it reads as a dark stripe; two weeks later the same slope is catching light and reads as a bright one. Same feature, opposite colour, and it is not actually a wall, the slope is gentler than most ski runs.