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A lunar Stonehenge

Our columnist observes a pattern on the Moon that resembles a well-known landmark on Earth.
OMearaStephen
Let’s have some fun. But first, a disclaimer: I claim no archaeo-astronomical, archaeomythological, or archaeomystical “whatevers” in what you are about to read. It’s just plain old apophenia (seeing patterns in random data), like transforming the pattern of stars in open cluster NGC 457 into Astronomy Editor David Eicher’s friendly owl, creating the Owl Cluster. In this case, I’ve transformed a random pattern of hills on the Moon into something that bears resemblance to an ancient monument on Earth. I dubbed it my “lunar Stonehenge.”

Pattern recognition at work

“Pattern recognition is one of the most fundamental cognitive skills we possess,” says British psychologist Keith Hillman, adding that while we can actively look for patterns — as we do with the Owl Cluster — pattern recognition also occurs unconsciously and automatically. The latter happened to me February 25 while testing my new 8-inch reflector on the waxingd gibbous Moon, 10 days after New Moon.

ASYOM0818_01
This Lunar Orbiter image shows Mare Insularum west (left) of Copernicus Crater. The author overlaid the inner structures of Stonehenge III (traced from Bulfinch’s Mythology) and added the Avenue, which belongs to Stonehenge I.
NASA
That night, the terminator sliced through Mare Insularum (the Sea of Islands) several degrees west of Copernicus Crater, where I noticed a ring of rugged terrain casting slight shadows. My mind’s neural pattern-recognition software automatically kicked in, suggesting here was a lunar Stonehenge! (Hillman was right. I couldn’t help it.)

On this night, the shadow conditions were not optimum, and as time passed, the northeastern shadows gradually slipped away, greatly diminishing the relief. Yet, my first visual impression was immediate, probably because I had just recently begun reading Gerald Hawkins’ book Stonehenge Decoded.

I referred to sketches of Stonehenge in Hawkins’ book, as well as some 1916 drawings in Bulfinch’s Mythology. I found a striking, though not perfect, similarity between the ring pattern of hummocky terrain in Mare Insularum and parts of Stonehenge III, especially its “horseshoe” of standing stones and the long ditch-bank entrance known as the Avenue, which leads to the famous Heel Stone and originally dates to Stonehenge I.

In Stonehenge III, the horseshoe is composed of five trilithons, each consisting of two upright stones capped by a third crosspiece. In my lunar horseshoe, only the five upright pairs are visible. Also, the lunar Avenue is much closer to the horseshoe pattern than the one on Earth. Still, it’s fun to see the similarities. I did not notice a Heel Stone.

ASYOM0818_02
It’s easy to see the location of the M15 dome and elongated vent in this Lunar Orbiter image. At this high Sun angle, the dome does not stand out in relief.
NASA
The secret

As I’ve mentioned, the uprights of the lunar horseshoe lie in a region of rough and hilly terrain made up of superimposed ejecta from the Copernicus, Kepler, and Eratosthenes impacts, over which basaltic lavas have flowed. Indeed, inside the lunar Stonehenge, on its floor, is a low dome with an elongated vent at its summit. Known as M15, this dome resembles a classic shield volcano (one with a pancakelike cross section), like those found on the Big Island of Hawaii.

As reported in the August 2007 issue of Icarus, M15 measures 13 miles (21 kilometers) in diameter but rises to an altitude of only 360 feet (110 meters), yielding a 0.6° slope; its rift vent measures 5.0 by 0.5 miles (8 by 0.8 km). So you need a very low Sun angle (on the order of 2° above the lunar horizon) to see the feature well. M15 lies on a 130-mile-long (209 km) volcanic dike that includes several other domes (M3, M4, and M10) whose vents are aligned in the same direction.

You can look for other lunar Stonehenge formations that mimic the original’s, such as a Slaughter Stone near the inner base of the horseshoe, as well as some Blue Stones, which on Earth also trace out a horseshoe pattern. As always, send your thoughts, comments, and imaginings to sjomeara31@gmail.com.

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