X marks the moon on Thursday night
Posted March 29, 2012
Thursday starting shortly after 11 p.m., an X will be visible on the moon. It's not marking the spot of anything in particular. It's created by craters that are sufficiently close together and sunlight hitting them at just the right angle. While it's not quite visible with the naked eye, even a small telescope or well-supported pair of binoculars should provide enough magnification to see it.
To find it, aim at the the terminus, the dark vs. light line formed by the sun shining on the surface of the moon. Then find the middle of that line and look slightly south. If you are looking through a reflecting telescope remember that your image is probably flipped so move up instead of down along that line.
The X should be visible for about 3 hours (weather permitting).
The Lunar X (also known as the Werner X or Purbach Cross) is formed by the rims of the Blanchinus, La Caille and Purbach craters. This X is created by the sun's light hitting the rims of these craters at a very shallow angle. The phenomenon only occurs a few hours before the moon reaches first quarter, but some months it's more prominent that others based on whether the sun has set or not and how long ago it set.
The next few times this will be visible are May 27 starting around 9:45 p.m., July 25, starting around 8 p.m., and Aug. 24, shortly after 5:30 a.m.