Looking for Lunar Letters

Lunar W

Ready for some astro-pareidolia? This week, we look no further than Earth’s Moon, which reaches 1st Quarter phase this coming Sunday.

The Moon reaches First Quarter phase for lunation 1156 (which dates synodic cycles of the Moon using what’s called the Brown Lunation Number all the way back to January 17, 1923) this weekend on Sunday, June 12th, at 9:10 EDT/13:10 UT.

Every culture sees something different in the face of the Moon. The Chinese saw a rabbit, and named the Yutu ‘Jade Rabbit’ rover in honor of the myth. In Longfellow’s The Song of Hiawatha, it’s the body of the Iroquois Indian chief’s grandmother we see, flung up against the Moon. The Greeks believed the Moon was a large polished mirror, reflecting back a view of the Earth below. Of course, if this were the case, it would be hard to explain just how the image doesn’t shift during the night, as the Moon moves across the sky.

Moon Map
The annotated features on the lunar nearside. Image credit: Wikimedia Commons/ Peter Freiman(Cmglee). Background photograph by Gregory H. Revera.

A cosmic Rorschach test, the Moon is tidally locked in the Earth’s embrace, keeping its far side forever hidden from our terrestrial vantage point. The subtle rocking motions known as libration and nutation allow us to peer over the edge just a bit, allowing us to see 59% of the Moon’s total surface. A glimpse of the far side had to wait until the Soviet Luna 3 spacecraft flew past the Moon on October 7th, 1959 and returned the first blurry images.

One of the most famous of the lunar letters is the Lunar X, also referred to as the Werner X or Purbach Cross. This is the confluence of the rims of the craters La Caille, Blanchinus and Purbach located in the lunar highlands. The Lunar X becomes visible as the waxing gibbous Moon reaches seven days illumination, about 6 to 10 hours (depending on the incident sun angle) after First Quarter phase, and 6 to 10 hours before Last Quarter. The Lunar X can stand out in dramatic contrast against the darkness just beyond the lunar terminator, if you can catch it just as the first rays of sunlight hits the top of the ridge. Remember, the span of sunrise to sunset lasts two weeks on the Moon, and looking Earthward, you’d see the Earth in an opposite phase.

All hail the 'Lunar X'... image credit: Dave Dickinson.
All hail the ‘Lunar X’… image credit: Dave Dickinson.

Sometimes, the Curtiss Cross feature is referred to as a lesser known Lunar X; the confluence of two or more crater rims on the battered surface of the Moon is far from uncommon.

The Lunar X and the Lunar V features. Image credit and copyright: Mary Spicer
The Lunar X and the Lunar V features. Image credit and copyright: Mary Spicer

Sweeping northward, the Lunar V feature in the Mare Vaporum is also sometimes prominent around the same time as the Lunar X, and it’s possible to nab both in the same image.

Other lunar letters of note include the Lunar S in Sinus Asperitatis (visible at 47% illumination just before First Quarter), the Lunar W located near Mons Rümker on the lunar limb in the Oceanus Procellarum, and our favorite of the lesser known lunar letters, the Lunar Q of crater Kies in the Mare Nubium reaching favorable illumination 10 days after New. You can see a partial listing of lunar letters in the WikiMoon article here.

The 'Lunar Q' feature... Image credit: NASA/LROC.
The ‘Lunar Q’ feature… Image credit: NASA/LROC.

Of course, circular craters provide a wealth of candidates for the ‘Lunar O,’ and straight line features such as the Rupes Recta lunar straight wall feature in the Mare Nubium could easily pass for the ‘Lunar I’. Veteran lunar observer Charles Wood made a call in Sky and Telescope magazine to fill out the visual lunar alphabet in a similar fashion akin to Galaxy Zoo… hey, who wouldn’t love to spell out their name in craters? Maybe some of the recently mapped worlds such as Mercury, Pluto or Ceres could come to the rescue, filling in the final letters?

Many of these are optical illusions, tricks of lighting as the angle of the rising Sun slowly changes, casting shadows across the lunar landscape. Two illumination effects that are at work here straight out of art class are what’s known as the Clair-obscur or chiaroscuro phenomenon of light and shadow, and the Trompe l’Oeil effect, a three-dimension illusion of forced perspective. Follow features such as the Lunar X night to night as the Moon heads towards Full, and you’ll notice they nearly vanish amid the glare, as the Sun shines down from high overhead. The vanishing ‘face on Mars‘ was the result of the same trick of light seen in the early Viking 1 orbiter images. The ‘face’ vanished once the Mars Global Surveyor re-imaged the region during a pass at near-full illumination in 2001. Hey, why don’t conspiracy theorists ever cite the ‘Man in the Moon‘ as an artificial construct?

Why lunar letters? Well, I think its neat, to see something as familiar yet improbable as a gleaming letter on the lunar surface staring back at you at the eyepiece. If you look long and hard enough, the universe will produce just about anything, including telescope-building primates with language, and an accidental alphabet written in the heavens.

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.

A daytime capture of the Lunar X. (Photo by Author).
A daytime capture of the Lunar X. (Photo by Author).

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 19th 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 1st 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 17th.

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 1st and Last Quarter Moon in the daytime.

A 1st Quarter Moon with the Lunar X (inset). (Photo by Author).
A 1st Quarter Moon with the Lunar X (inset). (Photo by Author).

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 1st 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!

A simultaneous capture of the Lunar X & the Lunar V features. (Photo by Author).
A simultaneous capture of the Lunar X & the Lunar V features. (Photo by Author).

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.