X Marks the Spot

March 02, 2013


Photographer: Geir T. Oye; Geir's Web site
Summary Author: Geir T. Oye

On the night of December 20, 2012, the Moon showed off a famous visual feature looking strikingly like the letter X, hence the name The Lunar X. To see it, pay attention to the waxing Moon close to its first quarter phase, when only half of its hemisphere is illuminated. The X will be visible on the Moon's terminator, but only for a little over four hours. This optical illusion is sometimes referred to as Werner X or the Purbach Cross. It's a beautiful example of how topography and lighting can create illusions. In this case, the effect is created when sunlight lights up the crater rims and ridges of La Caille, Blanchinus, and Purbach. Photo taken from Orsta, Norway at about 19:50 Central European Time.

Photo details: Canon EOS 500D, Borg 60ED, Sigma 2X converter, ISO 400; Exposure 1/60 sec.