Universe Today
Ready for some astro-pareidolia? This week, we look no further than Earth's Moon, which reaches 1
st
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 12
th
, 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.
[caption id="attachment_129335" align="aligncenter" width="580"]
The annotated features on the lunar nearside. Image credit: Wikimedia Commons/ Peter Freiman(Cmglee). Background photograph by Gregory H. Revera.[/caption]
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 7
th
, 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.
[caption id="attachment_129338" align="aligncenter" width="580"]
All hail the 'Lunar X'... image credit: Dave Dickinson.[/caption]
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.
[caption id="attachment_129336" align="aligncenter" width="547"]
The Lunar X and the Lunar V features. Image credit and copyright:
Mary Spicer
[/caption]
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.
Doubloons! Look for the lunar 'S' on the 19th September 2015 http://t.co/qhW4krMIbapic.twitter.com/rTnApfKQXg — blobrana (@blobrana)
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
.
[caption id="attachment_129337" align="aligncenter" width="398"]
The 'Lunar Q' feature... Image credit: NASA/LROC.[/caption]
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.