|Robin Casady's CCD mosaic of Grimaldi.|
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.
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.
|Chart 38||Moon-Lite Atlas for chart 39||Chart 40|