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Lunar X (also known as Werner X, or the Purbach Cross) is a "clair-obscur" phenomenon, occurring for a few hours around the first quarter phase. Sunlight striking the high ridges between the craters Purbach, Blanchinus, and La Caille creates an isolated X-shaped figure on the dark side of the terminator. At the same time, just a little further north, a V-shaped figure is formed by crater Ukert. Imaged with Celestron NexImage5 camera on 114 x 500mm telescope + 2xBarlow:
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