Hitchhiker's Guide to Rukl Chart 68

Vallis Rheita (JRF <freeman _at_ netcom.com>)

This odd valley is best seen at low sun angles.

Vallis Rheita (David North <d _at_ timocharis.com>)

Tracing the entire eastern end of the Rheita Valley is an iteresting puzzle, as some large craters are involved... but it's possible in even the most modest scopes if the sky cooperates.

Give it a shot!

Rheita E (Matt Tarlach <tarlach _at_ earthlink.net>)

To the South, past Reichenbach, I arrived at the curious elongated feature Rheita E. Rheita E looks a lot like normal, approximately circular craters do near the lunar limb, where perspective makes them appear elongated. But Rheita E is not *that* close to the limb, and its neighbors don't show nearly the same degree of elongation. Rukl calls it a "crater valley," which seems a misnomer - it appears much more like a large crater chain. If that identification is correct, another question presents itself: the overlapped walls of the two largest, central craters in Rheita E are absent, leaving a fairly smooth and level valley floor. This is unlike most examples of double craters (Sirsalis comes to mind) where the more recent crater displays a well defined rim, superimposed over the earlier feature. Could this indicate that at least two of the impacts at Rheita E were simultaneous, and evidence a double asteroid?

(One night later) One area I noted on the 27th, and wanted to return to last night, was east of the crater-chain Rheita E. 2 nights ago a large, approximately circular shadow resided there, definitely not cast be Rheita E but by what appeared to be a curved, gently rounded ridge that is somewhat visible in Rukl running near 48 E. Under higher illumination this seems clearly to be the remains of a large, greatly eroded crater, of which nearly all the rims have been leveled and only the contours of the bowl remain. I estimate the diameter at approximately 100 km, centered about 45 E, 32.5 S; it is not named or even apparent (except for the remnant of the eastern rim) on Rukl's chart. Reichenbach M and Z sit on its floor, and Neander H is on what would have been the western rim. Not a very dramatic feature, but with a form more clearly visible than any circular shape I can see to Brenner.

Metius (Matt Tarlach <tarlach _at_ earthlink.net>)

I moved on to Metius, sitting on the terminator with just its rim illuminated. It took me a while to orient myself here, as the peaks and crater rims catching the first rays of sun did not at first seem to correspond well to the map. Arcs of illuminated peaks stretched north and south of Metius, so that it seemed to be perched at the rim of a huge impact site, almost on the scale of Crisium. Yet no such feature appears on the charts. I finally concluded that I must be seeing the rims of Janssen and Brenner, and that some trick of perspective was making me see them as one huge arc rather than two large ones, to the north and to the south of Metius. I am still not really satisfied with this explanation, though - has anyone else seen, or seen a reference to, evidence of an ancient, giant impact in this region, centered perhaps near Janssen M?

The Brenner Triangle (Matt Tarlach <tarlach _at_ earthlink.net>)

Under higher illumination, there was no evidence visible of [last night's] suspected large impact area around Brenner/Janssen. I am more willing to accept that the brightly lit rim I saw on the 27th was really the distorted (by perspective) rim of Janssen, and the irregular raised area that is Brenner. Also, looking again at Vallis Snellius, it was apparent that the shadow seen south of Borda on the 27th is not really aligned with the rest of the valley, which I would estimate terminates at about 48.5d East.

I spent most of the session on Brenner, as several ridges or valleys were visible that are not named on the charts - and I find the "discovery" of objects like this especially enjoyable. The most prominent of these, while unnamed, shows well on Rukl 68 as a ridge running approximately parallel to the lunar axis (from our perspective), from about 37 E, 33 S to 42 E, 41 S, the southern terminus being at the joined rims of Metius and Fabricius. Barely apparent on the map is another ridge quite visible under last night's illumination, even through the poor seeing; this runs again from the western junction of Metius and Fabricius in a W by NW direction to end at about 35.5 E, 40.5 S, to the NE of Brenner F. The third line of shadow, of which I see no evidence on the map at all, begins near 39 E, 35 S and runs past the SE wall of Stiborius A, ending near 36 E, 39 S.

These three markings together just fail to enclose a neat triangle, the only gap being S of Stiborius A. They were all quite easily visible in the 6" even through last night's average seeing, and show fairly well in a published photograph I have of the region (not in Rukl, unfortunately).

more on the Brenner Triangle (Bill Arnett)

ust west of Vallis Rheita, starting between craters Metius and Fabricius is a linear feature running straight at the heart of Fracastorius. I *must* :-) be associated with Nectaris somehow. It is shown on Rukl but not named.........I should have checked HGTTM first! What I'm seeing is obviously the eastern side of Matt's "Brenner Triangle"; the other two sides were past the terminator.

Also the region just east of Neander was right on the terminator. It made it look like a HUGE valley just east of Neander. Is that part of Vallis Rheita?

BTW, the way Janssen is marked on Rukl 68 is very confusing. It makes it looks like Janssen is only half its real size. The labeling on page 67 is much better.