The Moon offers sky-watchers a spectacular celestial trick of light known as the Lunar X and Lunar V a few hours before it reaches its First Quarter phase. This phenomenon is entirely an optical illusion created by the specific angle of sunlight catching the rims of certain craters along the Moon's terminator—the ever-moving line that separates the illuminated day side from the shadowed night side.
The Optical Illusion
The appearance of the massive "X" and "V" is not a permanent feature of the lunar landscape, nor is it a sign of any new geological activity. It is a temporary visual effect, often referred to as a clair-obscur effect (French for "light-dark"), where bright light and deep shadows create a strong, striking pattern.
As the Moon approaches its First Quarter, the terminator sweeps across its surface. When the Sun's light grazes the mountainous terrain and crater rims at a very specific angle, only the highest peaks are illuminated, casting long shadows into the valleys below. From Earth, this configuration of light and shadow momentarily forms the distinct shapes of the letters.
Locating the Lunar X and V
The Lunar X: The "X" is the more prominent of the two and is located approximately 25 degrees south of the lunar equator. It is formed by the interplay of sunlight and shadow on the broken ramparts of three specific craters: Bianchini, Purbach, and La Caille. The brightly lit rims create the cross shape, which appears to float in the surrounding darkness near the terminator line.
The Lunar V: The "V" is typically located about 10 degrees above the equator, to the left of the vast dark expanse known as Mare Tranquillitatis (Latin for the Sea of Tranquility). It is formed by the illumination of the Ukert Crater along with several other small surrounding craters.
When and How to View It
The event is relatively fleeting, usually only visible for about four hours right around the time the Moon is nearing its exact First Quarter phase, which occurs roughly once a month.
For tonight, October 28, the Lunar X and V are expected to be at their most visible around 7 p.m. EDT (23:00 GMT) as dusk gives way to night, though the exact timing varies by location.
Observation Tips:
You will need a pair of binoculars or, preferably, a small telescope to clearly see the distinct letter shapes.
Focus your attention on the terminator line, which is the most interesting part of the Moon for observing such topographic features. The low-angle sunlight here emphasizes the Moon's mountains, valleys, and craters, giving the surface an almost three-dimensional appearance.
The Lunar X and V will gradually appear over an interval as the Sun rises over that particular spot on the Moon, and then fade as the terminator moves past the craters and the light begins to flood the valleys. If you miss it, don't worry—the phenomenon occurs every lunar cycle, though the visibility time changes each month depending on the Moon's orbit and your local moonrise/set times.