The Lunar X (sometimes called Werner X) is an optical illumination effect created when sunlight strikes the rims of three craters: Blanchinus, Purbach, and La Caille. As the Sun rises over this region of the Moon, the higher crater rims catch the light first, while the surrounding lower terrain stays in shadow. This contrast produces the temporary “X” shape.

The Lunar V forms in a similar way, near the crater Ukert. Like the Lunar X, it’s a transient lighting phenomenon, not a permanent marking.

The Lunar X and Lunar V are both brilliant examples of relief illumination, where topography becomes visible due to low-angle lighting near the Moon’s terminator (the boundary between lunar day and night).

Technically they can be seen for a few hours around first quarter once every lunar month (i.e. 29.5 days) although from a specific location this number will be lower, because sometimes the Moon will be below the horizon, or perhaps it will occur during daytime when it’s harder to see. And, as always, clouds can stop the fun! So, realistically, you might just have a few chances to see them every year. It’s entirely predictable though, so with a bit of research (or in my case, blind good luck!) you can see and photograph the Lunar X and Lunar V for yourself.