The Lunar “X” is a well-known “optical feature” on the Moon, which resembles the letter “X” when the lunar terminator is along just the right lunar longitude. It is an excellent example of how the combination of lighting and topography can combine to produce apattern that repeats on each lunation, but only for a short time.
The “X” is observable for about 4 hours around the lunar First Quarter. If one knows when and where to look, the “X” can be observed with a modest telescope or even well-supported binoculars.
The illusion of the “X” is created by sunlight falling on the rims/ridges between the craters La Caille, Blanchinus, and Purbach. It appears when there is a 1.2 degree sun elevation over crater Werner. It is located at lunar coordinates 25.3° S latitude and 0.9° E longitude, and is about 70 kilometers across.
The next opportunity to view the Lunar “X” is on Sunday, September 4th around 7:12 p.m. CDT.