Lunar X Marks Spot of Optical Effect of Sunlit Craters at First Quarter Moon

Summary: Lunar X marks the spot of the optical effect of sunlit craters that is visible via binoculars or telescopes during the first quarter moon.

Lunar X (image center), taken afocally April 10, 2011, from United Kingdom with Panasonic Lumix camera and reflecting telescope: George Kristiansen from UK (George7378), CC BY SA 3.0, via Wikimedia Commons

The Lunar X is an optical effect created by the chiaroscuro interplay of sunlight with three of the moon's southern craters at the beginning of the first quarter moon, the third phase of the eight phase monthly lunar cycle.

The chiaroscuro (Italian: chiaro, "clear, bright" + oscuro, "dark") interplay between the sun's rays and the lunar surface spotlights the craters' elevated rims while their floors remain blackened. The moon's south central highlands serve as the dramatic topographic setting for the illusory Lunar X, which centers upon three adjacent craters.

Blanchinus: Sited at selenographic (Ancient Greek: σελήνη, selḗnē, "moon") coordinates 25.4 degrees south 2.5 degrees east, the degraded, irregularly rimmed crater plumbs a depth of 4.2 kilometers. Blanchinus crater's diameter measures 58 x 68 kilometers.

Its northwestern rim is adjacent to La Caille crater's southeastern rim. Purbach crater lies in proximity to the west.

Blanchinus crater is named for University of Ferrara's professor of astronomy and mathematics, Giovanni Bianchini (ca. 1410-ca. 1469), Latinized as Johannes Blanchinus.

La Caille: Sited at selenographic coordinates 23.8 degrees south 1.1 degrees east, the relatively smooth-floored lunar crater dips to a depth of 2.8 kilometers. La Caille crater traces a diameter of 68 kilometers.

The southeastern segment of La Caille's battered, notched and worn rim presents adjacency to Blanchinus crater's northwestern segment. La Caille's southwestern segment poises in proximity to Purbach crater's northeastern segment.

The crater is named for French astronomer Abbé Nicolas Louis de La Caille (Dec. 28, 1713-March 21, 1762).

Purbach: Sited at selenographic coordinates 25.5 degrees south 1.9 degrees west, the degraded, heavily worn large lunar crater descends to a depth of 3 kilometers. Purbach crater's diameter measures 118 kilometers.

Purbach's eastern border points as a caret eastward toward Blanchinus. La Caille hovers between the two, along their northern perimeters.

Purbach is named for Austrian astronomer and mathematician Georg von Peuerbach (May 30, 1423-April 8, 1461).

The Lunar X happens over an interval of about four hours. The optical effect peaks for about an hour. The effect then dissipates after the next hour as the sun's rays chase darkness and shadows from the craterous trio's surroundings and move the lunar terminator, the line between illumination and darkness, across the moon's surface.

On the dark side of the terminator, the sun's rays spotlight a peak on Purbach's southeastern wall and expand north-northeasterly toward the crater's common ground with La Caille and Blanchinus. The illumination of Purbach's northeastern wall completes the left side of the Lunar X.

Blanchinus' southwestern wall is lit to form the lower segment of the X's right side. Then the border shared by La Caille's southeastern wall with Blanchinus' northwestern rim catches the light and completes the Lunar X.

Lunar X is also known as the Purbach Cross in recognition of the major contribution made by Purbach crater to the calligraphic phenomenon.

Another popular name is Werner X, coined by David M.F. Chapman of the Royal Astronomical Society of Canada, Halifax Centre. The name recognizes the critical role played by well-lit Werner crater in locating Lunar X.

Sited at selenographic coordinates 28 degrees south 3.3 degrees east, the prominent impact crater with terraced walls plunges to a depth of 4.2 kilometers. Werner crater has a diameter of 70 kilometers.

Lying near Blanchinus crater's southern border, Werner is located southeast of Purbach.

Werner is named for Nuremburg-based parish priest, astronomer and mathematician Johannes Werner (Feb. 14, 1468-May 1522).

The Lunar X makes its appearance in tandem with the fresh hours of the first quarter moon. The third lunar phase characteristically exhibits 50 percent illumination of the lunar disk, as viewed by Earthlings.

Observation of Lunar X does not require extravagant astronomical equipment. Lunar X may be viewed through such simple optical aids as binoculars or a small telescope.

The quirky phenomenon, however, is not always visible at a given location during each month's first quarter moon, throughout the year. The fluctuations in the duration, or length, of lunation, or lunar month, occasioned by the moon's elliptical orbit around the Earth and the Earth's ellipsis around the sun facilitate or prohibit the appearance of the Lunar X for Earthlings.

For the moon's first quarter phase, X marks the spot of the fleeting calligraphic phenomenon solved by the mathematical expression of lunar topography + solar illumination. The fun sighting of Lunar X emphasizes the easy accessibility of moon watching and the enjoyable play performed by nature within the purview of Earthlings.

The magic trick of the Lunar X affirms moon watching as an amusing form of entertainment.

Lunar X craters Blanchinus + La Caille + Purbach: image by NASA/LRO/LROC Team

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Lunar X craters: NASA/LRO/LROC Team, Public Domain, via Wikimedia Commons @ http://commons.wikimedia.org/wiki/File:Purbach_%2B_Regiomontanus_%2B_Werner_-_LROC_-_WAC.JPG

Lunar X, imaged on April 14, 2008, with a DMK 21F04 firewire camera + SV152 apochromat + Televue 4X PowerMate: Jeff Barton (chipdatajeff), CC BY 2.0, via Flickr @ https://www.flickr.com/photos/chipdatajeffb/2413920726/

Lunar X, taken afocally April 10, 2011, from United Kingdom with Panasonic Lumix camera and reflecting telescope: George Kristiansen from UK (George7378), CC BY SA 3.0, via Wikimedia Commons @ http://commons.wikimedia.org/wiki/File:The_%27Lunar_X%27_taken_afocally.png

For further information:

Fazekas, Andrew. "Night Sky News: Watch Lunar Wonders This Week." National Geographic > Voices. Nov. 2, 2011.
Available @ http://voices.nationalgeographic.com/2011/11/02/night-sky-news-watch-lunar-wonders-this-week/

Fuller, David. "Lunar X and Lunar V." Eyes on the Sky > Moon > Lunar X.
Available @ http://www.eyesonthesky.com/Moon/LunarX.aspx

"Lunar X." The-Moon Wiki.
Available @ http://the-moon.wikispaces.com/Lunar+X

"Lunar X." NASA > Astronomy Picture of the Day (APOD). March 11, 2009.
Available @ http://apod.nasa.gov/apod/ap090311.html

Wood, Chuck. "Moon-Crossed." LPOD Lunar Photo of the Day > Archives. Jan. 8, 2010.
Available @ http://lpod.wikispaces.com/January+8,+2010

"X marks the moon on Thursday night." WRAL.com > Weather > Weathercenter Blog. March 29, 2012.
Available @ http://www.wral.com/weather/blogpost/10923597/