Catch the Lunar X and V
BEST TIME TO SEE: Early evening, 8 February
The Lunar X and V are popular light and shadow effects, visible close to when the Moon reaches first quarter. They belong to a growing list of clair-obscur effects; light and shadow arrangements that appear like familiar, everyday things.
Clair-obscur effects can last for several days or, as is the case here, just a few short hours. The Lunar X and V take on the appearance of the letters X and V, glowing within the lunar terminator shadow. Best seen through a small scope, they are formed when the lunar dawn’s light hits elevated features, while the lower-level terrain remains in darkness.
The Lunar V is formed when elevated ridges near the 23km crater Ukert become illuminated. The arms of the V are a bit ragged, but the effect is convincing at low magnification. The Lunar X is more complex, being formed when portions of the rims of three intersecting craters (68km La Caille, 118km Purbach and 68km
Blanchinus) are illuminated.
In order to see clair-obscur effects, it’s necessary for the Moon’s terminator (the line between lunar day and night) to be in a precise position on the lunar surface, the Moon to be above the horizon and for clouds to be absent. In practice, having all three synchronise together isn’t as straightforward as you might imagine!
The Lunar X and V can be seen forming just after sunset on 8 February; the effect is at its best around 17:52 UT. Use a low magnification to start and look for the X one-quarter up the terminator from the southern edge. The V appears just north of the Moon’s centre on the 8th.