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| Space on Jan 19, 2013

What is Lunar X?

Alien visitation? No. Lunar X is an example of how lighting and topography can combine to produce a pattern that seems familiar to the human eye.

Ron Bee photo of Lunar X on November 17, 2007.

Lunar X is a famous optical feature on the moon, visible through telescopes. When the terminator – or line between light and dark on the moon – is located in just the right place, it appears as the letter X on the moon’s surface. A sign of an alien visitation? No. Lunar X is a great example of how lighting and topography can combine on a planet or moon to produce a pattern that seems familiar to the human eye. In reality, the illusion of Lunar X is created by sunlight falling on the rims/ridges between the craters La Caille, Blanchinus, and Purbach.

In the case of Lunar X, the pattern repeats at each cycle of the moon, but only for a short time. The X is observable for about 4 hours around the first quarter moon phase.

EarthSky Facebook friend Raven Yu in Quezon City, Philippines took this photo of Lunar X on January 19, 2013. Thank you, Raven! View larger.

Bottom line: Lunar X is an optical feature on the moon, an apparent X on the moon’s surface, visible through telescopes. It’s caused by sunlight falling on the rims/ridges between the craters La Caille, Blanchinus, and Purbach.