Lunar X and Lunar V are two challenging clair-obscur phenomena at the lunar terminator when the moon is about first quarter. Both phenomena are caused by sunlight hitting elevated terrain while the surrounding lower terrain is still in the shadows. Both are not that difficult to see. Even the smallest telescopes will allow one to view these fun features. The challenging part is that these are very short lived. The lunar X for instance is visible only for about four hours. I was lucky enough to photograph both on February 22th 2018
Lunar X, also sometimes called ‘Werner X’ or ‘Purbach X’ lies somewhat to the North of the crater Werner. Werner however makes no part of the Lunar X.
The elevated walls of the craters ‘La Caille’ to the North, ‘Blancinus’ to the East and ‘Purbach’ to the West, form the X as the tops of the crater walls are lit while the valley is still in the shadow behind the terminator.
Lunar V is a similarly interesting feature. It is visible when the walls of the crater ‘Ukert M’ are lit while the crater floor is still unlit. ‘Ukert M’ is only a small crater of a mere 20km. The surrounding area however is very interesting with the craters ‘Hyginus’ to the east , ‘Triesnecker’ to the South and their associated rimae.
Although observing these two phenomena has no real scientific value it is fun, somewhat challenging and requires some careful planning of, or least paying attention to, the lunar phase. The surrounding areas offer many fascinating features to check out.
I Believe the Lunar X and V should be on the bucket list of any lunar observer and offer some worthwhile views.
Have fun, and Clear skies.
Wouter D’hoye