April 19, 2014

Astro-Challenge: Spotting the Lunar X.

The Moon a few days before 1st Quarter showing the approximate position of the Lunar X. (Credit: Photo by Author).

     This week, we bring you a unique and off-beat challenge that comes to us via reader and expert astro-calculator Ed Kotapish. Right around 1st Quarter phase, the lighting angle on the Moon gets very interesting. Craters, mountains and rills may standout in stark contrast, as the shadowed crater floors may still be submerged in darkness. Some of our favorite sights are seeing the shadows cast of long mountain ranges in the lunar highlands, or a lit peak beyond the terminator, just catching the morning Sun… what would it be like to stand there and watch the lunar sunrise for its two week long day, a half phase Earth hanging overhead?

One of the more curious and sometimes elusive features in the lunar highlands has come to be known as the Lunar X. Also sometimes referred to as the Werner X or the Purbach cross, this is visible during some lunations in the lunar highlands. The X configuration is actually the convergence of three tightly packed crater rims; Purbach, Blanchinus and La Caille. Who first sighted or coined the term Lunar X is a bit of a mystery, but descriptions of it date back to Bill Buslers’ observation of it in June 1974. Dates and times in UT for optimal sightings in 2010 are as follows:

19 Jun 07:21 to 10:23 (Far East Pacific)
18 Jul 17:29 to 20:31 (Europe)
17 Aug 03:56 to 06:58 (North America)
15 Sep 15:14 to 18:16 (Central Asia)
15 Oct 03:48 to 06:50 (North America)
13 Nov 17:42 to 20:44  (Europe)
13 Dec 08:39 to 11:41 (Pacific)

The Moon at 1st quarter generally sets around midnite local, and this week’s June 19th apparition favors the Pacific area. I would still encourage observers worldwide to check out the arrowed area the evenings of June 18th-19th… good luck, and it’ll be a first for us as well if we catch it!

The astroword for this week is Nutation. Ever wonder why the lunar landscape looks slightly different from lunation to lunation? Nutation, or the slight “nodding” of the Moon is a secondary cause of the changing face of the Moon, behind the oscillation known as libration caused by tidal locking. The largest amplitude of nutation occurs over an 18.6 year period with a motion in obliquity of 9” and a maximum in longitude of 17”. Smaller periods are known, and the cause is the tugging of the Earth and Sun. All objects can nutate, much like the secondary observed motion of a gyroscope… the motion of the Moon is a tricky and complex affair!


  1. [...] If the region looks familiar, that because it’s just to the northwest of the Purbach cross, aka the Lunar X, another elusive feature that presents itself around 1st Quarter [...]

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