L44: Mersenius

Mersenius is located west of Mare Humorum not far from the moon’s western limb. It is of average size but still rich in detail. The crater walls are broad and terraced; the floor has been flooded with lava and bulges markedly upwards in the middle.

My observation was done under excellent conditions with steady, transparent skies and favorable illumination. The terraces were prominent, especially on the western side (right i sketch). The convex shape of the floor was indicated by a soft shadow tracing the walls in the northern half of the crater. Two lighter rays were also clearly visible on the floor. (After checking up on the observation it seems that the northern and thinner ray, is a rille rather than a ray.) Several secondary craters were also seen, most notably Mersenius H on the southern part of the rim, and Mersenius L adjacent to the crater on the northern side.

Sketch made on 25 September 2015, UT 21:00, using a SkyWatcher 10″ and a TeleVue Nagler zoom 3-6 mm. Seeing 4-5/5. South is up.

L9: Clavius

Clavius is the third largest crater on the moon’s near side, and it can be found close to the southern limb. The crater is also one of the oldest, which is evident from the many smaller craters that mark the Clavius plain. At local sunrise the two larger of these — Clavius C and D — gives rise to a clair-obscur phenomena, giving the impression of two eyes staring back at you.

My observation of Clavius was done under a 10 days old moon, so I didn’t get to see the Eyes of Clavius (which only shows at 9 days). But the illumination nicely captured the dramatic landscape of the crater. When I started out the seeing was terrible, and I hesitated whether to continue or to call it a night. Eventually I decided to give it a shot, hoping that it might improve after a while. The turbulence settled somewhat as the moon rose, but it wasn’t enough to capture any finer detail.

Clavius C and D are the two largest craters just right of the center. The rim is marked by Porter (top right) and Rutherford (bottom right). The large adjacent crater towards the bottom left is Blacanus.

Sketch made 23 September 2015, UT 17:30, using a Celestron XLT 120 and a TeleVue Nagler zoom 3-6 mm. Seeing 1/5, gradually improving to 3/5. South is up.

Clavius have also made an imprint on popular culture: In Arthur C. Clarke’s 2001: Space Odyssey American activities on the moon are centered around a base build underneath the surface of the crater plain. When Dr Floyd sets out to investigate the monolith excavated in Tycho, he takes the moon shuttle from the Clavius Base.

Clavius Base. Screen capture from 2001: Space Odyssey.

Below is a clip from 2001: A Space Odyssey showing the encounter with the monolith found in Tycho crater. The soundtrack still gives me creeps.

L42: Marius Hills

The largest gathering of volcanic domes on the moon can be found in the middle of Oceanus Procellarum and close to the ring-plain Marius. Squeezed into an area of approximately 200 km in diameter the Marius Hills contains about 300 domes, half of the moon’s known population. The domes are just a few hundred meters high and thus best observed under low sun.

The sketch was made under a waxing gibbous moon, just a few days from full, and the sun angle was perfect for highlighting the many domes. Sketching them was another matter, quite cumbersome at that. Marius is the crater to the middle left in the sketch. Top right is the end of the Reiner Gamma swirl.

Sketch made on 25 September 2015, UT 20:30, using a SkyWatcher 10″ and a TeleVue Nagler 3-6 mm zoom. Seeing 4/5. South is up.

370In 2009 the Japanese Selene mission found what might be a sky light to an underground lava tunnel in the Hills. The Marius Pit is located close to the rightmost dome in the sketch, but since it is just 65 meters in diameter it is well beyond the resolution of my scope. A fly-over movie from JAXA can be found here. Lava tunnels might be an option for a future lunar base, and apparently there is advanced plans for a private moon mission to target the Marius Pit.

L95: Procellarum Basin

The Procellarum Basin draws it’s name from Oceanus Procellarum (Ocean of Storms) and is the unofficial name for a hypothetical impact crater covering a large part of the moon’s nearside. There seems to be at least two alternative explanations to the features associated with the basin, and I will get back to them below.

Working through the Lunar 100 the Procellarum Basin is something of an enigma. What exactly are we supposed to observe? If it is the circular shape that can be traced in the nearside maria, why is it inserted at the difficult end of the Lunar 100 list, and not right after L3: the Mare/Highland dichotomy? So far I haven’t managed to find any information on the web that might shed light on these questions.

Some days ago I had my rarely used 120 mm refractor set up at the kitchen table, doing some maintenance. When done I looked out through an open window and noticed the moon rising over a nearby cliff. Still seated I swung the telescope over and was treated with a really nice view. The indoor/outdoor temperature difference didn’t seem to affect the image, at least not at low power. Comfortable seated I decided to sketch the (almost) full moon, and to try to outline the Procellarum Basin as well as some of the more prominent ejecta ray systems. As the moon would eventually wander out of sight, I had to work quite swiftly, and the sketch was done in about 45 minutes.

Moon observed 28 August 2015, UT 20:30, using a Celestron Omni XLT 120 mm, and a noname Plössl 24 mm (42x). Illumination 98,6 %.

Anyone who have tried to sketch the full face of the moon, know that it is quite challenging. The amount of detail is overwhelming, and it is easy getting stuck somewhere in the highlands, sketching away at the many craters. I decided to go for a rather crude sketch, just outlining the maria and sketching only a few of the craters. I didn’t even bother to trace the terminator — the moon was about a day from full — I just pretended that the moon was a perfect circle.


In the image to the right I have tried to mark what I believe is recognized as the outline of the Procellarum Basin [Edit: see new image at the end]. The maria seems organised in a circular pattern, most notably to the north and southwest, and in accordance with one interpretation it might be traces of an ancient and enormous impact crater. If this is correct the energy of the impact might also have been enough to create the highlands on the far-side of the moon.

IMG_heatingA later and opposing view do away with the impactor, instead claiming that the Procellarum region is the result of internal processes active 3 to 4 billion years ago. Data from the Gravity Recovery and Interior Laboratory (GRAIL) mission show evidence of remnants of lava-filled rifts forming a squarish pattern (red in the image to the right, original image here). It is suggested that as the original lava oceans that covered the young moon solidified, they also contracted. Molten lava then rose at the edges creating the rifts.

If the latter interpretation is true there is no Procellarum basin, which of course again raises the question of what we are suppose to observe. Hopefully I have done enough to tick off L 95 from my list. Come to think of it, and since the sketch also depicts the mare/highland dichotomy, I will tick off L 3 as well.

Edit: After a short exchange with Jef De Wit at Cloudy Nights’ sketching forum, I realized that my depiction of the basin needed to be adjusted. In the paper that suggested the Procellarum Basin (linked in text above), E. A, Whitaker found evidence of three concentric rings associated with the hypothetical impact. “The rings have surface diameters of 1700, 2400 and 3200 km, and are centered at about 23º N, 15º W, near the crater Timocharis.” In the image inserted below I have drawn these circles onto my sketch.

Basin rings

The original Lunar 100 list only mentions the 3200 km ring. And since only a small part of that ring can be clearly seen, and since the impact as such is contested, it might explain why the Procellarum basin ended up on the difficult end of the list.

L41: Bessel Ray

When a lunar crater is formed by an impact, material from the depth of the crust is forced up and catapulted into the surrounding landscape, forming radial ejecta rays. The ejecta material has a higher albedo than the dark maria, making the rays plainly visible. Many lunar craters are associated with ray systems, most notably Copernicus, Kepler, Proclus and Tycho.

The Bessel Ray is a single ejecta ray running almost north-south through the middle of Mare Serenitatis. On it’s way it touches upon the Bessel crater (in the middle of the sketch), giving it it’s name.

Sketched 26 April 2015, using a SkyWatcher 10″, and a TeleVue Nagler zoom 3-6 mm. North is down.

Apparently there is some debate concerning the originator of the Bessel Ray: some suggest that it is associated with the nearby crater Menelaus (at the top of the sketch), others that it is part of the extensive ray system emanating from Tycho, far down south.

In an image from the Galileo spacecraft, the ray systems of Copernicus (left), Kepler (far left), Proclus (right)  and Tycho (bottom) are prominent. Mare Serenitatis, with the Bessel Ray, can be seen near the center. Judging from the image the Bessel Ray seems to connect both with Menelaus (the bright spot on the southern shore of the mare) and with Tycho. I am not sure what to make of it, but if think I put my money on Tycho. 


L39: Schickard and L43: Wargentin

The walled plain of Schickard and the odd looking crater Wargentin can be found near the limb in the moon’s southwestern quadrant. Schickard has been partially flooded with lava and some high terrain is still visible close to the southwestern rim. In contrast the much smaller crater Wargentin has been flooded to the rim with lava, giving it the appearance of a plate turned up-side-down.

I observed this region on a cold and very windy evening. The seeing was average and the high winds didn’t help, so the sketch was quite a challenge to make.

Sketch made 30 May 2015, using a SkyWatcher 10
Sketch made 30 May 2015, using a SkyWatcher 10″ and a Televue Nagler 3-6 zoom eyepiece. North is up.

Schickard — to the left in the sketch — dominated the view. It showed some smaller craters on it’s floor and hints of the higher terrain. I couldn’t make out any albedo features on the plain. The spike protruding into the terminator seems to be part of Schickard’s rim. The oval shape of the plain came out a bit thick in the sketch, and it should have had a somewhat slimmer outline.

Wargentin is the small crater rising from the terminator to the lower right of Schickard. A thin shadow marked the part of the rim still sticking up through the lava. On the crater floor the wrinkled ridges that forms the ‘bird’s foot’ were quite distinct. The two craters to the right of Wargentin is Phocylides (rightmost) and Nasmyth.

L28: Hipparchus

This is a large and degraded crater southeast of Sinus Medii. Since it’s floor has been flooded with lavas it is technically a walled plain.

At the eyepiece this was a rich and very interesting object. The angle of the local sun was perfect for catching a lot of detail, as for example the string of bright peaks to the southwest (bottom in sketch) and what appears to be a ghost crater to the west. The small crater on the northern rim is Horrocks, and the one on the southern is Halley.

Sketch made on 25 May 2015 (lunar age 7.5) using a SkyWatcher 10″ and a TeleVue Nagler zoom 3-6 mm. North is up.

The Lunar 100 list states that the significance of Hipparchus is due to it being “Subject of first drawing of a single crater”. Lacking context this at first struck me as a rather odd statement, but after some googling I found the story behind it: There has to be a first time for everything, including sketching single lunar craters. When Robert Hooke published his Micrographia in 1665 it contained — among drawings of the many wonders found at the microscope — a detailed sketch of Hipparchus. Lunar maps had been around for quite some time, but apparently this was the first sketch published depicting a single crater.   

Hooke's MoonComparing my sketch with Hooke’s the similarities (obviously) are striking. His shadows are a tad bit shorter so his sketch must have been made under a somewhat older moon. Hooke also seems to have found traces of the ghost crater described above.

Finally it can be mentioned that when Tintin lands on the moon in Explorers of the Moon they set down their spacecraft on the floor of Hipparchus.