"Rupes Recta, Ancient Thebit, Huygens’ Sword and Rima Birt"
This photo is a northward extension of my previously posted shot of Deslandres Crater. The focus of this photo is the region that occupies the center of the image. First, let me outline the lay of the land. Deslandres Crater and the adjoining Regiomontanus Crater are in the bottom right corner of this image. Moving up the right side of the image, Purbach Crater sits about 1/3 of the above the bottom. Higher up, Arzachel Crater can be seen. Above and left of Arzachel, near the top, is Alpetragius, sporting an oversized central peak. The dark area on left top and down the left side is Mare Nubium. At bottom left is Pitatus Crater.
Moving toward the center from the right side, is Thebit crater, inside from both Arzachel and Purbach. It sits on an arc of hilly terrain that appears as a bay connecting to Mare Nubium. If you look closely, you can see that the arc of hills continues out into Mare Nubium, in the form of curving wrinkle ridges that circle all the way around to meet again the arc of hills upon which Thebit sits. This circle, formed from the arc of hills and the wrinkle ridges, bears memory of an old, unnamed, ruined crater that is unofficially called "Ancient Thebit". The 57-km-wide crater Thebit sits on its battered eastern rim.
The interior of Ancient Thebit holds several features of interest. Most notable is the long black linear feature that cuts across the floor of Ancient Thebit. This feature often evokes wonder from new observers of the Moon. “What is that straight black line?”, they ask, realizing this is something unusual. What it is, is the best example of a geological fault to be found on the surface of the Moon. It is called “Rupes Recta”, the Straight Wall. Close examination reveals its nature. First, it is almost, but not quite straight. It casts a shadow when illuminated from the East, as in this photo. That indicates it is higher on the east side than on the west. Its height is estimated between 250 and 450 meters. It is 120 kilometers long. To the east of the Straight Wall, the floor of Ancient Thebit is rough, hilly and peppered with craterlets. To the west of the Straight Wall, the terrain is smoother, and sparsely cratered. The western floor of the ancient crater was flooded with lava, as was Mare Nubium. It is likely that the weight of the flooding of the old crater floor caused the western side to subside, and then break downward along the line of the Straight Wall.
At the southern end of Rupes Recta, the fault terminates in an area of low mountains. These mountains are called by some “the Stag’s-Horn Mountains” for that is what they resemble. The entire feature comprised of Rupes Recta and the Stag’s Horn Mountains is known as “Huygens’ Sword”, with the mountains forming the hilt and guard of the sword, and the Wall the blade. Take a look for yourself. What do you think?
West of the wall, the 17 km wide crater Birt stands pretty much alone in this part of the ancient crater floor: the 7 km wide Birt A crater hugs its southeastern rim.
Originating immediately to the west of Birt crater and running northwestward, a second elongated feature can be seen running roughly parallel to the Straight Wall. The northernmost end of this feature arises on the top of a somewhat darker, possibly dome-shaped structure sited nearly on the edge of Ancient Thebit. The southernmost end seems to terminate abruptly in a circular pit just beyond the shadow of Birt’s western rim. This elongated fissure is Rima Birt. It is likely a relic of the volcanism than flooded this part of Ancient Thebit. Because the tiny pit at the Rima's northern end is located near the rim of Ancient Thebit, one can imagine speculate that the fractured rock of the old crater’s rim allowed lava fountains to erupt onto the lunar surface, producing a dome, collapse pits, a lava channel, and pyroclastic deposits. If you recall the lava fountains and rivers of lava you saw in videos of the recent Kilauea eruption, you might be able to imagine the scene at the head of Rima Birt, except the Rima Birt eruption poured out enough lava to fill the Ancient Thebit and break the crater’s floor. That would have been a spectacle!
Photographed in the early evening of February 2, 2020, during a period of outstanding seeing that coincided with the first half of the Super Bowl.
This photo has more noise than other shots from this day, so it looks granier by comparison. Sorry...
Celestron EdgeHD 8 telescope, ZWO ASI290MM monochrome camera, Celestron Advanced VX mount.
Best 10% of 2083 video frames, stacked with AutoStakkert 3, wavelets processing with Registax 6, and final processing in Photoshop CC 2019.