This is a famous "optical feature” on the Moon, which appears like the letter "X" when the terminator is at a suitable position. It is a fine example of how the combination of lighting and topography can combine to produce a pattern that repeats on each lunation, but only for a short time. The illusion of the "X" is created by sunlight falling on the rims/ridges between the craters La Caille, Blanchinus, and Purbach.
A telescope is required for viewing this feature. Here is a scheduele for dates in 2011.
The "X" is observable for about 4 hours around the lunar First Quarter.
The start time is for a fully formed “X”. The end time is subjective. To watch the formation from darkness subtract 2 hours from the start times below
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