The Lunar X (also known as the Werner X) is a clair-obscur effect in which light and shadow creates the appearance of a letter 'X' on the rim of the Blanchinus, La Caille and Purbach craters.[1]

The X is visible only for a few hours before the first quarter,[2][3] slightly below the lunar terminator. Near to the X, the Lunar V is also visible, formed by Ukert crater and several other small craters.[2]


  1. Rice, Tony, "X marks the moon on Thursday night".
  2. 2.0 2.1 Harrington, Philip S. (2010). Cosmic challenge : the ultimate observing list for amateurs. Cambridge: Cambridge University Press, 1999.
  3. Chapman, David M.F.. The Lunar X Files: a fleeting vision near the crater Werner. Willingboro Astronomical Society.

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