June 17, 2021
First Quarter Phase
The left side of this view of the Moon is dominated by a vertically arranged trio of large craters. These are, from top to bottom, Ptolemaeus, Alphonsus, and Arzachel. Here they are caught within a few hours of daybreak. This timing makes certain features visible due to low-angle lighting, which emphasizes the vertical relief of even small hills and depressions.
For example: Look at Ptolemaeus crater (154 km diameter, 2.4 km depth). Notice on the floor of this vast walled plain that there are numerous shallow circular dimples, distributed across the floor. These are ghost craters, craters dug into the original floor of Ptolemaeus by a rain of later meteor, comet or small asteroid impacts. Sometime after their formation, other material, mostly rubble from other cratering impacts, filled the Ptolemaeus basin, including these craters, and left only the subtle ghostlike remnant circles where once ramparts stood tall. These features are most visible under lighting conditions such as I captured on June 17th.
Ptolemaeus is a basin so vast, that were you placed at its center, you would see a flat plain extending in all directions around you, the horizon interrupted by maybe one peak far to the East. That would be the highest point of the distant outer walls of Ptolemaeus.
Aside from the circular dimples, there is one crater prominent enough that it has its own name – Ammonius (9 km diameter). Other, lesser craters can be seen, as well as a multitude of tiny craterlets, barely within the resolution of my telescope, scattered across the plain of Ptolemaeus.
On the Northeastern wall of Ptolemaeus, starting at the 1:00 – 2:00 position, there is a linear arrangement of small craters that runs tangent to the rim of Ptolemaeus and ends near the southern end of another nearby crater, Müller (24 x 20 km oblong, 2 km depth). This linear feature has no official name. It is an example of a crater chain, called a “catena”, caused by a series of impacts from objects having a common source, such as pieces of a broken-up meteor or comet nucleus that hit the Moon, one after the other. Ratatat! This feature has been unofficially named “Catena Müller” by the lunar scientist, Danny Caes.
Now for Alphonsus (119 km diameter, 2.7 km depth). Alphonsus abuts the southern rim of Ptolemaeus, and, like its neighbor, is an ancient crater, a relic of the earliest eras of the Moon’s history. As such, it has survived all the insults the Solar System has dished out for 4 billion years or more. Its interior shows abundant craterlets. Its walls are slumped, battered and gouged by other smaller impacts. A lone pyramidal central peak rises toward the sky, its tip catching sunlight and its bulk casting a long shadow to the west. A ridge runs down the center of Alphonsus; it is described as consisting of ejected material from other impacts. To my eye it has a braided form, especially south of the central peak. Low chevron ridges of accumulated ejecta are seen in the southwestern quarter of the crater. Much of the crater interior remains in shadow; note the shape of the shadows cast by the eastern rim. Alphonsus was the impact site of the Ranger 9 mission in 1965. It was an alternate landing site for both the Apollo 16 and 17 missions.
The third and youngest of this trio is Arzachel (96km diameter, 3.6 km depth). This crater has ramparts that rise high above the surrounding terrain. The interior walls are terraced, and there is a prominent central peak, sited somewhat west of center. Only about a third of the crater floor is illuminated by sunlight in this early morning shot. Notice here again the prominent shadow cast by the central peak.
I will note one satellite crater of Arzachel crater in this photo: Arzachel B. On the rim of Arzachel, at roughly the 10:30 position, sits the half-lit Arzachel B. Not remarkable, true. But the creation of its near twin in the shallow seas of Cretaceous Alabama made for a very rough day in the Southeastern region of the North American continent of the time. Arzachel B is nearly identical in size to the Wetumpka Crater remnant located just north of Montgomery.
To the right (East) of the juncture of Ptolemaeus and Alphonsus is the hexagonal crater Albategnius (129 km diameter, 4.4km depth). Its walls are considerably degraded by slumping, cratering, and gouging. The smaller crater Klein sits on its western wall. Albategnius has a prominent central rebound peak, slightly off center to the west. Just visible in this photo is a tiny craterlet on the summit of the central peak.
As was the case with Ptolemaeus, Albategnius has small circular dimples in its floor, and numerous tiny craterlets as well. On the northwestern rim, what appears to be another unnamed catena reaches diagonally toward the northeast.
Stepping back from examination of individual craters, the whole western portion of the photo shows numerous radial valleys and gouges scarring the terrain. They point back toward some location out of the image to the Northwest. These are examples of “Imbrium Sculpture”. The Imbrium Event was a collision between the Moon and a large asteroid or protoplanet, which created the enormous Imbrium Basin and which sent great blocks of the Moon’s crust and pieces of the impactor bounding over the Moon’s surface at high speed. These massive chunks dug deep valleys into the Moon’s terrain, sculpting it as we see it now.
The right portion of the image has three features I’d like to highlight. In the lower left you can see another linear arrangement of craters, or catena. This one has a name, Catena Albufeda. Its name derives from a crater nearby. This is one of the longer catenae, 219 km from end to end. Crater Albufeda is the largest in this part of the photo. It’s namesake catena extends toward the lower right corner of the photo from the crater’s southern rim.
Above and right of Albufeda crater there is an area that is considerably lighter in color than is usual on the Moon. This area appears to be triangular in shape in the photo. The brightness is odd, for solar radiation darkens surface materials on the Moon. There are several such light areas on the Moon, with wildly differing shapes. Collectively they are called “Swirls” because that is what many of them look like. What they all have in common is a locally strong magnetic field. The “Why?” of the localized magnetic field is a topic of ongoing research. What seems evident, though, is that the magnetism shields surface materials from high-energy solar radiation in the same way that the Earth’s global magnetic field shields the planet from the same radiation. Areas like this might prove handy for human exploration of the Moon.
Speaking of human exploration of the Moon, the site of the Apollo 16 lunar landing is only some 60 km from this magnetic anomaly. Look above the area of the Swirl. There is a bright white dot. The dot marks a relatively fresh craterlet and its surrounding ejecta blanket; it is of no importance except as a guide. A little above and right of this dot is a cove or embayment into the rough hills to the dot’s right. That cove is the Apollo 16 landing site. There you will find the Apollo 16 Lunar Module, the Lunar Roving Vehicle, and a Passive Seismic Experiment Package. The Apollo 16 command module “Casper” is on display at the U.S. Space & Rocket Center, here in my hometown: Huntsville, Alabama.
Best 15% of 8175 frames.
Celestron Edge HD8
ZWO ASI 290MM
No filter