Hitchhiker's Guide to Rukl Chart 39

[Robin Casady's Grimaldi mosaic] Robin Casady's CCD mosaic of Grimaldi.
Grimaldi (JRF <freeman _at_ netcom.com>)
This dark-floored basin is reportedly visible from the Earth with the naked eye, but I have not seen it. Grimaldi is a "landmark" feature, easily recognized and useful for orientation.
Grimaldi (David North <d _at_ timocharis.com>)
Particularly fun was watching the sunrise on Grimaldi, which seems so much more complex at a low light angle; soon it will be just a big dark spot. Because it is so low, flat and dark, sunrise happens faster there than most places on the moon, and the changes over a short period of time can be fairly dramatic during those lunations when it is showing. A cheap movie if there ever was one.
Riccioli (David North <d _at_ timocharis.com>)
The dark stain of Grimaldi looms next to massive old Riccioli (R 39) with its many slurred interior fractures and folds. It is an ancient crater, and it shows its age like an old face. It is rumpled with wrinkle ridges, slightly cratered, softened by dust and time, with an impressive collection of straight rilles inside, most running rougly north/south.

Riccioli is one of those wonderfully complex features that tends to get overlooked since it shows at its best near full... another reminder that a fat moon is not necessarily a boring moon. Nearby Hevelius shows an even more fascinating collection of crosshatched rilles and cracks.

Together, these two massive plains can be a rille and craterlet hunt of notable proportion, all by themselves. But... it's hard to stop there when just a bit more cranking on the scope (a glance at Rima Grimaldi on the way by...) will get you to the long, straight, easy and downright weird Sirsalis Rille... a perennial favorite of mine (see also chart 50).

It starts as part of an interior rille in Darwin, crosshatched like those in Hevelius though somewhat simpler. There the similarity ends.

Just outside Darwin, it begins a run north that takes it straight through at least three craters by my count (one fairly large) and by several others, all through highland material, eventually almost "draining" into Procellarum. This is odd behavior, even for rilles, which are noted for acting odd. Part of it is the length of the run, something on the order of 300 Km. Part of it is the massive nature of the rille; it can be seen with just about any telescope in virtually any seeing: this is one big snake. And since most rilles are in maria, near and parallel to the edges, the orientation and highland presentation are both odd as well. There's nothing else quite like it on the moon, and I look forward to it each month like an old friend, a parting shot before the full moon arrives.

There is an odd "wall" structure between Riccioli and Lohrmann, both just north of the easily spotted Grimaldi. There are a wealth of rimae in the area (Grimaldi, Riccioli, and especially fine was Hevelius), all doing a line dance. But that is somewhat overshadowed by the incredibly detailed view of Riccioli crater itself. Rumples and bumples and rillets and everything you could imagine. It's wonderful, huge; a night unto itself.

Riccioli-Lohrmann Wall (...Akkana)
[Riccioli Ridge] On 12/31, I noticed a sharp, thin line extending from Riccioli to a nearby crater. At first I thought it was the crater which Rukl marks "B" (Grimaldi B?), but later Bill O'Connell pointed out to me it was really Lohrmann, referencing his video capture of the area, so what I was seeing was in fact the "odd wall" David North mentioned.

It looked a bit like the Straight Wall in some lights, and was so sharp I initially thought it was a shadow, but several hours later, with Riccioli having changed from fully shadowed to fully illuminated, the dark line was still there, though less sharply defined; a shadow would have shortened in that time.

Damoiseau (Matt Tarlach <tarlach _at_ earthlink.net>)
Damoiseau appears as an interesting set of concentric craters, almost like a bullseye. The rings are so neatly arranged that I doubt they could have been created by successive impacts. More likely the appearance is caused by an unusual circumstance of collapse or erosion of a single crater wall.
Damoiseau (Robin Casady)
Damoiseau D appeared as a black teardrop outlined in white, and dripping north on the rim of Damoiseau A.
Sirsalis (Matt Tarlach <tarlach _at_ earthlink.net>)
Nearby is the double crater Sirsalis. (Wilkins and Moore identify the westerly of these twins as Bertaud, but it seems that name didn't stick.) Rukl draws the westerly component (which he calls "A") as the lower and less defined of the two. Wilkins and Moore agree, putting the height of Sirsalis' walls at up to 10,000 feet and those of Bertaud (Rukl's "A") at only 3000 feet. Yet under observation near the terminator, Bertaud ("A") seems the more filled with light even in its more westerly position. My guess at an explanation is that Sirsalis' wall is much higher on the eastern side than the west. The high eastern wall blocks sunlight, keeping the crater floor and the inside slope of the western wall in shadow. The lower western wall of Sirsalis neither rises to catch the sunlight, nor casts much of a shadow upon Bertaud. Sound reasonable?
Sirsalis (JRF <freeman _at_ netcom.com>)
Inconsequential crater, with a major rille (and a few smaller ones) just east. The Sirsalis Rille extends south well onto Rukl chart 50, all the way to the vicinity of Darwin. At the right sun angle, the Sirsalis Rille will make you think there is a black thread stuck to the field lens of your eyepiece.
Sirsalis Rille (David North <d _at_ timocharis.com>)
This is a moderately easy rille to spot, but nevertheless curious in that it is not normally placed for a stress fracture (they are usually radial to a mare; this cuts away from a mare through highland territory for an incredible distance, and deserves as much study as you can afford it).
Rimae Sirsalis (Robin Casady)
Rimae Sirsalis showed most clearly to the south of Sirsalis F down to Crugen A. The surface seemed darker here and contrasted with the rille. Other sections of the rille would come in and out of sight with the seeing.

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