Stalking the Lunar X

This week offers observers a shot at capturing a fascinating but elusive lunar feature.

But why study the Moon? It’s a question we occasionally receive as a backyard astronomer. There’s a sort of “been there, done that” mentality associated with our nearest natural neighbor in space. Keeping one face perpetually turned Earthward, the Moon goes through its 29.5 synodic period of phases looking roughly the same from one lunation to the next. Then there’s the issue of light pollution. Many deep sky imagers “pack it in” during the weeks surrounding the Full Moon, carefully stacking and processing images of wispy nebulae and dreaming of darker times ahead…

But fans of the Moon know better. Just think of life without the Moon. No eclipses. No nearby object in space to give greats such as Sir Isaac Newton insight into celestial mechanics 101. In fact, there’s a fair amount of evidence to suggest that life arose here in part because of our large Moon. The Moon stabilizes our rotational axis and produces a large tidal force on our planet. And as all students of lunar astronomy know, not all lunations are exactly equal.

This week, we get a unique look at a feature embedded in the lunar highlands which demonstrates this fact. The Lunar X, also sometimes known as the Purbach cross or the Werner X reaches a decent apparition on March 19^th at 11:40UT/7:40EDT favoring East Asia and Australia. This feature is actually the overlapping convergence of the rims of Blanchinus, La Caille and Purbach craters. The X-shaped feature reaches a favorable illumination about six hours before 1^st Quarter phase and six hours after Last Quarter phase. It is pure magic watching the X catch the first rays of sunlight while the floor of the craters are still immersed in darkness. For about the span of an hour, the silver-white X will appear to float just beyond the lunar terminator.

Visibility of the Lunar X for the Remainder of 2013.
Lunation	Date	Time	Phase	Favors
1116	March 19th	11:40UT/7:40EDT	Waxing	East Asia/Australia
1116	April 3rd	3:20UT/23:20EDT*	Waning	Africa/Europe
1117	April 17th	23:47UT/19:47EDT	Waxing	Eastern North America
1117	May 2nd	16:19UT/12:19EDT	Waning	Central Pacific
1118	May 17th	10:51UT/6:51EDT	Waxing	East Asia/Australia
1118	June 1st	4:31UT/0:31EDT	Waning	Western Africa
1119	June 15th	21:21UT/17:21EDT	Waxing	South America
1119	June 30th	16:04UT/12:04EDT	Waning	Western Pacific
1120	July 15th	7:49UT/3:49EDT	Waxing	Australia
1120	July 30th	3:16UT/23:16EDT*	Waning	Africa/Western Europe
1121	August 13th	18:50UT/14:50EDT	Waxing	South Atlantic
1121	August 28th	14:27UT/10:27EDT	Waning	Central Pacific
1122	September 12th	9:50UT/5:50EDT	Waxing	East Asia/Australia
1122	September 27th	2:00UT/22:00EDT*	Waning	Middle East/East Africa
1123	October 11th	19:52UT/15:52EDT	Waxing	Atlantic Ocean
1123	October 26th	14:12UT/10:12EDT	Waning	Central Pacific
1124	November 10th	10:03UT/5:03EST	Waxing	East Asia/Australia
1124	November 25th	3:14UT/22:14EST*	Waning	Africa/Europe
1125	December 10th	00:57UT/19:57EST	Waxing	Western North America
1126	December 24th	17:07UT/12:07EST	Waning	Western Pacific
*Times marked in bold denote visibility in EDT/EST the evening prior.

 

Fun Factoid: All lunar apogees and perigees are not created equal either. The Moon also reaches another notable point tonight at 11:13PM EDT/ 3:13 UT as it arrives at its closest apogee (think “nearest far point”) in its elliptical orbit for 2013 at 404,261 kilometres distant. Lunar apogee varies from 404,000 to 406,700 kilometres, and the angular diameter of the Moon appears 29.3’ near apogee versus 34.1’ near perigee. The farthest and visually smallest Full Moon of 2013 occurs on December 17^th.

The first sighting of the Lunar X feature remains a mystery, although modern descriptions of the curious feature date back to an observation made by Bill Busler in June 1974. As the Sun rises across the lunar highlands the feature loses contrast. By the time the Moon reaches Full, evidence of the Lunar X vanishes all together. With such a narrow window to catch the feature, many longitudes tend to miss out during successive lunations. Note that it is possible to catch the 1^st and Last Quarter Moon in the daytime.

Compounding the dilemma is the fact that the lighting angle for each lunation isn’t precisely the same. This is primarily because of two rocking motions of the Moon known as libration and nutation. Due to these effects, we actually see 59% of the lunar surface. We had to wait for the advent of the Space Age and the flight of the Soviet spacecraft Luna 3 in 1959 to pass the Moon and look back and image its far side for the first time.

We actually managed to grab the Lunar X during a recent Virtual Star Party this past February. Note that another fine example of lunar pareidolia lies along the terminator roughly at the same time as the Lunar X approaches favorable illumination. The Lunar V sits near the center of the lunar disk near 1^st and Last Quarter as well and is visible right around the same time. Formed by the confluence of two distinct ridges situated between the Mare Vaporum and Sinus Medii, it is possible to image both the Lunar X and the Lunar V simultaneously!

This also brings up the interesting possibility of more “Lunar letters” awaiting discovery by keen-eyed amateur observers… could a visual “Lunar alphabet be constructed similar to the one built by Galaxy Zoo using galactic structures? Obviously, the Moon has no shortage of “O’s,” but perhaps “R” and “Q” would be a bit more problematic. Let us know what you see!

-Thanks to Ed Kotapish for providing us with the calculations for the visibility of the Lunar X for 2013.