August 2014 Archive


August 1, 2014
Click for full size Guerike area

Fifty years ago yesterday, the NASA lunar probe Ranger 7 reached the Moon. Ranger 7 was the first successful American lunar probe, coming half a decade after the Russian Luna 2 impacted the Moon and Luna 3 photographed the previously unseen far side of the Moon. At a time when Cold War tensions were high, America was hungry for a lunar success in the space race with the Russians. Ranger 7's mission was to take closeup photos of the lunar surface during a suicide plunge into the Moon's surface. Thus fifty years ago today, newspapers across America featured front page closeup photos of the Moon, many of them showing a view like this one. Ranger 7 struck the Moon in the southern part of Oceanus Procellarum. That region of Procellarum was subsequently renamed Mare Cognitum, or “the Known Sea” in honor or Ranger's success. In this view, the crater Guerike with its broken walls is lower center. Above Guerike lie the side by side lava flooded craters Bonpland (left) and Parry (right). Above the pair is the even more ruined large crater Fra Mauro. Look one crater width west of Bonpland and you see two lonely elongated mountain peaks protruding from the smooth mare. Ranger 7 impacted just south of those two peaks. You had to be alive back then to understand the pride America took in the success of Ranger 7. It was a turning point in our journey to the Moon. I remember it like it was yesterday!


August 2, 2014
Click for full size Petavius area

In the highlands along the Moon's southeastern limb, we find a number of interesting features tucked between Oceanus Procellarum and the limb of the Moon. What strikes me the most are the two “Plato wannabes”, dark floored Cruger (left) and Billy (right). The half-sized cousins of the famous black floored crater Plato were undoubtedly created the same way as Plato, by magma welling up through cracks in the lunar crust and filling the interior of two otherwise relatively normal craters. At at one time, both Cruger and Billy had a central peak, but these features were buried by the lava flood. To the south of Cruger is the relatively fresh rayed crater Byrgius A. I can find no reference to its geologic age, but since crater rays fade with time, the brilliance of the ray structure tells me Byrgius A must be one of the youngest large craters in the region, perhaps only 200 million years old. Mare Orientale can be seen peeking around the western horizon while the river-like streaks of basalt that make Lacas Veris and Lacus Autumni are seen close to the limb.


August 3, 2014
Click for full size Cruger area

In the highlands along the Moon's southeastern limb, we find a number of interesting features tucked between Oceanus Procellarum and the limb of the Moon. What strikes me the most are the two “Plato wannabes”, dark floored Cruger (left) and Billy (right). The half-sized cousins of the famous black floored crater Plato were undoubtedly created the same way as Plato, by magma welling up through cracks in the lunar crust and filling the interior of two otherwise relatively normal craters. At at one time, both Cruger and Billy had a central peak, but these features were buried by the lava flood. To the south of Cruger is the relatively fresh rayed crater Byrgius A. I can find no reference to its geologic age, but since crater rays fade with time, the brilliance of the ray structure tells me Byrgius A must be one of the youngest large craters in the region, perhaps only 200 million years old. Mare Orientale can be seen peeking around the western horizon while the river-like streaks of basalt that make Lacas Veris and Lacus Autumni are seen close to the limb.


August 4, 2014
Click for full size Shickard area

In a region filled with unusual lava-filled craters, massive Schickard stands out as stranger than its neighbors. Spanning 137 miles, Schickard is filled nearly to its rim with multi-hued basalts that beg for an explanation as to why portions are lighter than others. It is a stretch to imagine different ancient lava flows producing contrasting basalts, so thoughts turn to what modified the basalt fields after they solidified. One thought is lava flows occurring after a nearby event showered the region with ejecta. Subsequent lava flows buried the light-colored ejecta, leaving darker fresh basalt to create the contrasting surface.


August 5, 2014
Click for full size Torricelli area

This is a fun place on the Moon. To set the location, the Apollo 11 landing area on Mare Tranquilitatis is at the top just left of center and the massive crater Theophilus is just protruding into the bottom. In the center, we see the curious pear-shaped crater Torricelli laying within the larger ghost crater Torricelli R. Both of these craters lay within Sinus Asperitatis, or the "Bay of Roughness", which forms the straight between Mare Tranquilitatis and Mare Nectaris. Torricelli is actually an overlapping pair of craters that creates the pear-shape illusion. On the southern plains of Sinus Asperitatis we can see both the streamers of the ground surge that splashed outward from the Theophilus impact and a field of smaller secondary craters formed by blocks of material thrown out of Theophilus. Also at the top of the image is the lengthy Hypatia Rille, a feature called a graben, or slumping of land between two parallel faults. If a 45-year old memory serves, I think this is the "Diamondback Rille" used as a navigation landmark by Apollo 11.


August 6, 2014
Click for full size Mare Imbrium

Wicked shadows! Last Sunday evening I was able to shoot the Moon for the first time in several months. The seeing wasn't great and we are under a haze of African dust that seems to have a special affinity for my wife's black car, but the ...shots turned out reasonably well. The seeing limited me to 2800mm focal length, or a "wide view", but I like the sunrise sweeping across Mare Imbium. I turned the image on its side to better fit Facebook. North is to the left. I enhanced the contrast along the terminator to bring up the shallow wrinkle ridges. I especially enjoy the starkness of the dagger-like shadows cast by the mountainous rim of the Imbrium Basin and the rim of Achimedes, the ringed dark-floored crater near the center. Sunrise casts a long shadow from these peaks, making them look more steep and sharp than they really are. The low sun also shows the streaks from the ground surge that spread out from Aristillus, the crater to the upper left of Archimedes.


August 7, 2014
Click for full size Sinus Iridum sunrise

Tuesday evening I had another decent run on the Moon. The African dust was still here (my wife's black car looks gold!) and the seeing was worse than Sunday, but I could see four craterlets in Plato, my sign that conditions are good enough to image.
Today's view continues the theme of sunrise crossing Mare Imbrium. By Tuesday it had reached the western plains that merge into Oceanus Procellarum. The horseshoe bay of Sinus Iridum is just breaking into dawn's light. I like how I caught the western rim of Iridum just catching the first rays of sunrise. I am also pleased at how the wrinkle ridges on western Imbrium showed up. They look like frozen waves washing up on shore.


August 8, 2014
Click for full size Mare Nubium

Here is a two-shot mosaic of the Mare Nubium area that I took last Tuesday evening. I like the Nubium region because in one view it shows samples of most of the geological features on the Moon. To the west you can see wrinkle ridges piled up on the mare plain. At center left you can see the three Hippalus Rilles that opened up from stretching of the surface. To the right is Straight Wall, a graben where the land slumps between two faults. A number of ghost craters dot the mare plain as do good examples of complex craters with central peaks and terraced walls and simple bowl-shaped craters. At the southern shore of Nubium lies the lava-flooded floor-fractured crater Pitatus. Just west of the floor-fractured crater Pitatus is the small bulls eye-looking concentric crater Hesiodus A. The lengthy straight Hesiodus Rille extends to the southwest from this area. Highlands and mountains lie to the southeast. There is something for everyone in the Nubium area.


August 9, 2014
Click for full size Mare Imbrium

Mare Imbrium (Sea of Rains) is the classic prototype for maria that formed within an impact basin. It is obvious that circular Mare Imbrium is bordered by curved adjoining mountain chains. With hindsight, we now know these mountains are actually the rim of a impact basin. By definition, a lunar basin is any huge crater exceeding 300 kilometers across. Imbrium more than fits the bill. In the billion years following the creation of the Imbrium Basin some 3.8 billion years ago, the excavation was filled by hundreds of lava eruptions until it was smoothly paved over like we see it today.
But this seemingly obvious explanation for this feature's origin was long in coming. It was 280 years after the perfection of the astronomical telescope before a geologist, not an astronomer, deduced the truth about the origin of Mare Imbrium. In the 1890's, Karl Grove Gilbert, chief geologist for the US Geological Survey, concluded that Imbrium formed by of a colossal collision on the Moon. But Gilbert's discovery was ignored for another half century until Ralph Baldwin independently came to the same conclusion in the late 1940's. Indeed, the universal acceptance of the impact origin of lunar features was not complete until the Apollo expeditions to the Moon. Sometimes the obvious takes a while to sink in.


August 10, 2014
Click for full size Fracastorius area

Fracastorius is an unsung crater with an unnerving name on the southern shore of Mare Nectaris. Like many large craters along the edge of a circular mare, Fracastorius' depressed northern rim allowed the lava flows that created Mare to spill into the crater, forming a horseshoe bay. The crater thus must have predated the lava flows that filled the Nectaris Basin, indicating it is an ancient Nectarian Epoch crater. To my eye, there is the barest hint of a central peak near the middle of Fracastorius. This shows the depth of the basalt within the crater must be at least a kilometer. I can find no reference to the name f the curved rille within Fracastorius, but by convention rilles are usually named after the most prominent nearby crater, thus I am assuming the rille is named after its parent crater.


August 11, 2014
Click for full size Bailly area

Last Saturday night, most lunar observers were enjoying the perigee full moon, hyped in the press as the "Supermoon". While I also enjoyed this natural curiosity, I was more excited by the fact that the south Texas seeing was calm enough to allow boosting my telescope to 7000mm focal length for the first time in months. Thus while others were imaging the full moon, I was marching up and down the thin terminator of the 98% illuminated Moon.
In this view we are looking at Bailly, a relic from the Nectrian Epoch that is the largest crater on the near side of the Moon that hardly anyone knows about. Tucked into the south-southwest limb of the Moon, Bailly spans an amazing 183 miles across, but its location near the edge of the visible moon severely foreshortens this massive crater and most novice observers do not notice it. Since it exceeds 300 kilometers in diameter, I argue that Bailly should technically be considered a basin, not a crater.
Last Saturday's "Supermoon" coincided with a good libration that pushed Bailly further into our view and allowed us to see the entire span of the crater and slightly beyond. The rough mountainous bump extending into the darkness to the right of Bailly is part of the ruined crater Le Gentil.


August 12, 2014
Click for full size Eddington area

Here is another of the far western lunar limb views I took during Saturday's “Supermoon”. In the upper half of this view along the northwest limb of the Moon we see three large overlapping lava-flooded craters. The foreground crater with the most prominent rim is Eddington, 80 miles in diameter, with its interior flooded by lavas that flowed in from Oceanus Procellarum well over two billion years ago. Behind Eddington's western rim lies even larger Struve, spanning 103 miles and even more ruined by lava fill than Eddington. To the north of Stuve, 62-mile diameter Russel partially merges with Struve where lavas overran the depressed rims of the overlapped craters. No central peaks protrude from the basalts within each of these craters, showing that the craters must be lava flooded to a depth of at least a kilometer. I have always heard Procellarum basalts are relatively shallow. These flooded craters on the western shore of Precellarum tell me Procellarum may be much deeper than thought.


August 13, 2014
Click for full size Copernicus rays

Without crater rays,the Moon would be far less interesting. Here we see the magnificent splash extending in all directions from Copernicus, once called the "Monarch of the Moon" because its spectacular ray system is so enchanting. For centuries, astronomers did not understand the nature of crater rays. One prominent early 20th century German lunar expert called rays a "riddle wrapped in an enigma".
Today we understand rays are actually a combination of material splashed out of a crater by the impact that created it, and thousands of small unseen (from Earth) secondary craters that have churned the surface of the Moon. The heat of a large crater-forming impact melts thousands of tons of rock and creates millions of glassy particles that are very efficient at reflecting light. These particles are much like the material in highway signs that reflects your car headlights. These particles are splashed across the Moon by the explosion that excavates the ray's parent crater.
The layer of material creating the rays is very thin. In fact, the Apollo 12 crew landed on a Copernican ray and could not detect it. "Space weathering", or the infall of micrometeorites, erases crater rays after about a billion years. Rays are thus useful in dating how old some craters are. In the case of Copernicus, the consensus averages about 800 million years old.


August 14, 2014
Click for full size Tycho rays

Here we see Tycho's confusing ray system at full moon. Thought to be only 108 million years old, Tycho's rays splash across half the face of the Moon. One rogue ray stretches as far as Mare Serenitatis to the far northeast. In this view we see the ray structure is heaviest to the east, indicative that Tycho was created by an asteroid strike arriving low from the west.
The bright ray system is visible to the naked eye. A binocular view of the rays draw the eye straight to Tycho like a bulls eye. However, Tycho's rays have some mysteries. We see several bright streamers radiating away from the crater, the "railroad tracks" extending to the upper left and the single streamer to the lower left. Look closely...see the mystery? You would expect crater rays to radiate from a central explosion point that created the parent crater. But these bright rays do not centrally converge on Tycho. They are in fact tangential to Tycho's rim. What the heck?? I have no hard explanation for this.
The Tycho ray mystery led some respected mid-20th century lunar experts to think Tycho crater and its ray system were created by two different events. My pet theory, open to correction, is that Tycho was created by the impact of a "rubble pile" asteroid. This is a class of asteroid that is not one solid body, but many big rocks weakly bound together by their own gravity. As this rubble pile asteroid approached the Earth/Moon system 108 million years ago, our gravity broke it up into multiple fragments, The fragments that hit the Moon first pushed out the offset rays, followed seconds later by the impact that blasted out Tycho crater itself. The multiple impacts were spread over a 30 to 40 mile area, creating offset rays and a big crater.


August 15, 2014
Click for full size Kepler-Aristarchus rays

Here we will take another look at crater rays. Relatively young Kepler (middle right) is only 19 miles in diameter, but its ray system makes quite a splash. Aristarchus at the upper left has a prominent ray system of its own, but in this image it is close to the terminator. Since ray material is mostly small glassy material formed in the heat of crater formation, it is very efficient at reflecting light back toward its source, which of course is the sun. Low sun illumination therefore suppresses ray visibility, so Aristarchus' rays do not fully show their glory here. At full moon, the earth is aligned between the Moon and sun, and the ray's are brightest. Together with giant Copernicus, just out of view at the top, Kepler and Aristarchus form a ray triad on upper Oceanus Procellarum that is a delight to view at moderate telescopic power.
An added bonus in this image is a good look at the volcanic Aristarchus plateau and Schroter's Valley at upper left and the fascinating Marius Hills at the left. The plateau built up from extended volcanic eruptions while the Marius Hills are composed of dozens of small individual volcanic mounds.


August 16, 2014
Click for full size Theophilus area

Here we see one-kilometer resolution in the classic Theophilus-Cyrillus-Catharina area on the northwest rim of Mare Nectaris. Theophilus is the younger looking Erastothinian Epoch crater at the upper right. The Nectarian Epoch crater Cyrillus lies to the left of Theophilus. Since Theophilus overlays the rim of Cyrillus, it is clearly the younger crater. At middle left is Catharina, also a Nectarian Epoch crater. All three of these craters are about 60 miles wide. Look closely and notice the hundreds of small mile-wide Theophilus secondary craters dusted across the landscape. Also notice a number of rilles that fracture the floor of both Catharina and Cyrillus. Within Catharina you can spot the remains of the relatively large crater Catharina P. The wave of ejecta from the Theophilus impact nearly buried Catharina P and heavily modified the floor of adjoining Cyrillus.


August 17, 2014
Click for full size Moretus area

This wide view is the southern highlands is centered on Moretus, the wide terraced crater with the pronounced central peak, and equally wide but older Curtius to the right of Moretus, This region is in the far southern latitudes of the Moon and is often ignored by the casual viewer because the craters appear to jumble together. But if you take the time to navigate by "crater-hopping", some interesting places can be found. Directly below Moretus is the crater Short, and to the lower left of Short is Newton, as in Issac, not my Canadian friend Jack. Continue south to the horizon and there are several pronounced bumps. If my navigation is correct, these are the mountainous rims near Cebeus, the crater where several years ago NASA crashed the probe that looked for lunar polar ice. I can pick out about 50 small craterlets within the big crater, most probably secondaries created by ejecta from nearby Tycho.


August 18, 2014
Click for full size Fracastorius
and Piccolomini sunset

I don't often get lost on the Moon, but I did with this photo. As you can guess, I am a fan of the big dramatic gaping holes along the terminator. I tripped over this scene Saturday morning while cruising for targets of opportunity at 7000mm focal length with my Skyris 274M camera. I am not sure what visual magnification that equates to, but it must be nearly 1000. At the time I didn't recognize the crater configuration due to the deep shadows. After processing it, it took me a while to realize the two big craters are Fracastorius at the top and Piccolomini as at the bottom. We visited a more illuminated Fracastorius a few days ago. Piccolomini however is a newcomer to 365 Days of the Moon. I'll bet you can't say that crater's name real fast five times and not have you day brightened! Fracastorius as we remember is a lava flooded horseshoe bay-like crater on the southern shore of Mare Nectaris. Its central peak has been buried by mare lava flows. The crater is so wide that the natural curvature of the Moon is protruding the center of its floor up into the rays of the setting sun that spill over the crater's rim. At the left, the tip of Piccolomini's central peak also just catch the setting sun.


August 19, 2014
Click for full size Mare Vaporum

Is the Moon all it's cracked up to be? It is if you look in the right places. In the center of the Moon's disk we see the dark patches of Mare Vaporum (Sea of Vapors) and Sinus Medii (Middle Bay). These dark areas make the Man-in-the-Moon's nose. This region is rich in large rilles, or cracks in the surface of the Moon. At the right we see the lengthy straight Ariadeaus Rille. This crack is actually a graben, a feature caused by the slumping of land between two parallel faults. To the left of Ariadaeus is the gull wing-shaped volcanic Hyginus Rille. Both "wings" branch off small Hyginus crater in the middle. But this is not an impact crater. Hyginus is actually a volcanic collapse pit. Below Hyginus is the network of Triesnecker Rilles. The Triesneckers are known as "irregular branching" rilles because they fork all over the place, through you can't see some of them in this high-altitude shot.
Notice the rough dark patch of volcanic deposits just above the Hyginus Rille. To my imagination, this looks like a heart. Look closely....can you see the rough area as the shape of a Valentine heart? I can, and thus I call this feature the "Heart of the Moon".

Cordillera Mountains

August 20, 2014
Click for full size Cordillera Mountains

Observers tend to avoid the full Moon because illumination is so flat that small details are hard to see. But near full Moon is also the only time you can see features along the Moon's far western limb. Here is where I search for glimpses of elusive Mare Orientale (Eastern Sea). Never mind the Eastern Sea is on the western side of the Moon. How that happened is fodder for another story. But in this image, my view was about a day too soon to see the actual mare patch of Orientale. Instead, we can see evidence of the impact rings the surround the Orientale Basin which contains Mare Orientale. At the left we see a wall-like feature casting a shadow. This is the Cordillera Mountains which are actually the outer impact ring of the Orientale Basin. On the horizon beyond the Cordillera lies the ridge of the Rook Mountains, the next impact ring. Territory beyond the Rooks is still in shadow and not visible in this image.
Also visible is the southern half of Rima Sirsalis, seen slashing in from the upper right. The other rille crossing Sirsalis is Rima Darwin. The dark lake of the lava filled crater Cruger is at the top while the fresh bright ray system radiating from Byrgius A is at lower right.


August 21, 2014
Click for full size Atlas and Hercules sunset

Here is another of the series of deep shadow terminator photos I took this last lunation. Similar to how deeply shadowed Fracastorius had me guessing a few days ago at what it was, it took me a few seconds to be assured this really was Hercules (left) and Atlas (right in deep shadow). Craters on the Moon are like human faces in that no two are like. But when you blur out the eyes, nose, and mouth in a picture with many faces, they all begin to look similar. It's the same with lunar craters if their interior is hidden in shadow, they look strikingly similar and I have to navigate by surrounding terrain. Though you can't see it in the shadow, Atlas is an Imbrian Epoch floor-fractured crater. Hercules is a younger Erastothenian Epoch crater that is surrounded by hundreds of small secondary craters strewn across the surrounding plains.


August 22, 2014
Click for full size Janssen sunset

More terminator madness! I have been having fun looking at familiar craters under very low illumination angles. The low sun creates a whole new personality for an otherwise overly-familiar crater. Here we see Janssen on the southeastern quadrant of the Moon's face. Under higher sun, shallow Janssen virtually disappears. Though it spans 115 miles, the novice observer often overlooks Janssen because it lacks depth compared to its width and the crater's rim blends into the surrounding highlands. Most people spot Fabricius, itself almost 50 miles wide, laying in the upper right corner of Janssen, and think that is Janssen, while overlooking the even bigger crater surrounding Fabricius. The low sun also accentuates the forked rille, Rima Janssen, within the floor of Janssen.


August 23, 2014
Click for full size Posidonius,
Le Monnier, and Serpentine Ridge

I love floor-fractured craters! Although no two craters are alike, there is a certain monotony to terraced walls...central peak...repeat. Floor-fractured craters are a nice diversion from that pattern. Here we see 58-mile wide Posidonius on the eastern edge of Mare Serenitatis. Perhaps three billion years ago, lavas from the same magma chambers that flooded Mare Serenitatis welled up through fractures in the floor of Posidonius and also flooded the crater to the point where even the central peak was covered. As the lava solidified into a basalt field, volcanic uplift further pushed up the now solid floor of Posidonius and created the system of rilles, or cracks across the crater floor. The result of this upward pressure created the classic "oatmeal cookie" appearance of the floor-fractured crater.
Below Posidonius is the half-moon bay of the crater Le Monnier. This was the landing site of the Russian Lunokhod 2 rover that traveled nearly 30 miles on the Moon back in 1973.
The rille Rima Chacornac extends below Posidonius while Rima Bond meanders along the right of the image. At the left is the winding wrinkle ridge popularly called the Serpentine Ridge.


August 24, 2014
Click for full size Clavius

Today we will take a break from the terminator madness of the past few days and look at an old classic crater, Clavius. Back when I first observed the Moon a time long ago when we didn't even know what the far side looked like), Clavius was considered the largest crater on the visible side of the Moon. Spanning 136 miles and containing a number of sizable craters within it, Clavius is indeed huge. But today we are a little more sophisticated in our view of the Moon and know that Bailly, along the southwestern limb not too far from Clavius, is actually about 50 miles wider. The vast expanse of Clavius, formed in the Nectarian Epoch, is host to other younger craters. Porter, from the Imbrian Epoch, lies on Clavius' northeast rim, while even younger Copernican Epoch Rutherford lies on the southeast rim. Blancanas, another ancient Nectarian Epoch crater, lies to the lower left of Clavius. Both of these craters have been degraded and filled with ejecta thrown from the impacts that created the great basins now hosting the Moon's dark maria.


August 25, 2014
Click for full size Tycho secondaries

A while back we took a wide view of Tycho under high sun illumination and looked at the parallel "railroad track" rays that seem to extend tangentially from the crater's rim. In addition to hundreds of tiny secondary craters peppering the area, here we also see the beginnings of the railroad tracks as a dual compact stream of secondary craters extending to the upper right. I still have not heard a convincing explanation of how these offset ray streamers came into being. The Moon may be the closest celestial body and the only one humans have explored, but Luna still hides some of its secrets.


August 26, 2014
Click for full size Theophilus sunset

Today I will show as bad of a Moon photo as I will ever post here. This was taken one blustery night on December 29, 2007 after I got my DMK-41 camera. The seeing was terrible, but I wanted some "seat time" with the new camera to learn its operation. Though pixelated to the max by very necessary overprocessing, there are some features here I find interesting.
The dark shadowed crater at the lower right is Theophilus on the Moon's southeastern quadrant. The mountains casting the sunset shadow down the middle of the image are the northern beginnings of the Altai Scarp, a cliff-like structure that is actually the outer impact ring of the Nectaris Basin. But the most interesting thing in this photo is something you almost can't see. I love to find evidences of ancient ruined craters that have no name on modern lunar charts. Let me help you find one. The largest crater on the upper left is Delambre. Look just below Delambre...see the curved shadowy structure? Let your eye follow that curve down through the very squished-looking crater Taylor, the continue the curve toward the right. Do you see evidence of the rim of an ancient obliterated crater? I think I do. I suspect that almost four billion years ago this was a crater about 60 miles in diameter. Debris thrown from the Nectaris Basin impact buried this old crater leaving only a hint of its former existence.


August 27, 2014
Click for full size Grimaldi and Riccioli

Again we are chasing craters few people pay attention to. In this view we are looking beyond the western shore to Oceanus Procellarum near the western limb of the Moon. The two large craters partially filled with dark basalt are Grimaldi (right) and Riccioli (left). Located at 68 and 74 degrees west longitude, these two craters are foreshortened to our earthly view and do not look as impressive as they really are.
Both Grimaldi and Riccioli were named after the mid 17th century Italian monks who defined the face of the Moon that we see today. In fact, they are self-named as Riccioli named about 280 of the major features on the Moon we still recognize today. For his lunar work, Riccioli used lunar maps drawn by Grimaldi, who in turn borrowed heavily from the works of Hevelius and van Langren (who were also rewarded with craters named after them).
Grimaldi, at 134 miles wide, is easily seen through a small telescope, but can be mistaken as an extension of nearby Oceanus Procellarum. While Grimaldi's basalt fill shares the same subsurface magma source as Procellarum, it is an independent feature. The crater is only partially filled with basalt and the Grimaldi's actual rim lies some distance from the edge of the lava fill. Riccioli, a floor-fractured crater, is smaller at 88 miles wide and is also partially lava flooded. The rim of the Moon holds delights for those who take the time to search for them.


August 28, 2014
Click for full size Pitiscus and Hommel sunset

Here is another of my recent terminator images that sent me on a search mission to identify what the heck these craters are. Since I knew the area had to be close to where my mystery image was a few days ago, it didn't take long to zero in on the proper targets. The crater at upper right with the central peak just catching the setting sun is 50-mile wide Pitiscus on the southeastern quadrant of the Moon. While Pitiscus is an old Nectarian Epoch crater, the complex shadowed crater below it is even older. Hommel is pre-Nectarian, which means it is one of the oldest surviving craters on the Moon. Too bad it is all in shadow, as its ruination if magnificent! At the bottom left is Mutus, another pre-Nectarian relic. The low sun accentuates the hundreds of small secondary craters dusting the sunset landscape.

Lacus Mortis

August 29, 2014
Click for full size Lacus Mortis

The smooth circular lava-flooded feature is 100 mile wide Lacus Mortis. I usually image Lacus Mortis at lower sun elevations so the shadows give more dramatic effect to the feature's name, which translates as the Lake of Death. When the Italian monk Giovanni Riccioli named this feature in the mid 17th century, Europe had just emerged from centuries of scientific ignorance and superstition. Even though Riccioli was learned man, one can imagine the social traditions of the time still leaned toward darker interpretations for the unknown. Thus it is not surprising that this feature gained such a bizarre name.
Lacus Mortis is actually a large pre-Imbrian crater that has filled with lava flows from nearby Mare Frigoris and Mare Serenitatis. Once the crater had been paved over with solidified basalt, the much younger Copernican Epoch crater Burg blasted the bull eye-like 24 mile wide crater in the middle of Lacus Mortis. The fingers of Rimae Burg criss cross the western side of Lacus Mortis.


Click for full size Proclus "bowtie" rays

A bow tie on the Moon! The target of today's 365 Days of the Moon is not what you think. The broad unwinking black eye of Mare Crisium is simply a bystander to the real point of interest, the 17-mile wide crater Proclus. OK, there are a lot of small craters here. Which one is Proclus? Look for what looks unusual and out of the strange ray system on the western rim of Mare Crisum. Part of the ray system is missing. This is what is called a bow tie ray system.
Ordinarily, meteor strikes create round craters regardless of what angle they strike the ground, and ray systems are more or less symmetrical around the crater. This is because the crater is not punched into the ground, but is created by the explosion of the meteor vaporizing from the heat of a hypervelocity impact. This process generally still holds true then the impactor arrives at an extremely shallow angle. The twist is oblique impacts alter the resulting crater ray system. The lower the impact angle, the larger the “zone of avoidance” in the direction the meteor came from. Thus highly oblique impacts create the bow tie effect where no crater rays extend in the direction the impactor arrived from.


August 31, 2014
Click for full size Endymion

The broad dark-floored crater is Endymion. At 76 miles wide, Endymion is a near clone of ever popular Plato. However, Endymion's location on the northeastern limb of the Moon places it away from most observer's attention. Endymion is pre-Nectarian, which means it is one of the oldest of lunar craters. Back during the Moon's volcanic era, magma seeped up through cracks in Endymion's floor and flooded the crater from the inside. In the more modern era (last billion years), light dusty streaks of ray material have deposited across the dark floor. I do not know the exact source of these ray streaks, but my bet is they came from Thales, a 19-milke wide Copernican Epoch crater about 150 miles north of Endymion.

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