image by KC Pau, Hong Kong

Concentric rilles are often the largest and most impressive on the Moon. The best examples are the Hippalus Rilles that arc around the east side of Mare Humorum. KC's regional view shows three or four of these big rilles at bottom left. To the northeast of Hippalus an odd thing happens - the rilles bend around and become concave to the east. The most conspicuous of these right-opening features is named the Agatharchides Rille, but it is really (rilly?) part of the Hippalus family. KC's image also reveals the infrequently seen shallow and fainter rilles near Agatharchides A that are roughly concave to the east. These rilles must be centered on the poorly documented Nubium Basin, suggesting that it's mare fill caused subsidence and the tensional fracture rilles near Agatharchides. Another likely casualty of the basin center sinking is the lost western rim of the old crater surrounding the Straight Wall. At the central bottom edge of this image are the two non-identical twins, Mercator and Campanus. I had never noticed before (at least my faltering memory doesn't recollect if I have) the hint of a rille crossing the mare-covered floor of Mercator - is it real?

Chuck Wood

Technical Details
Jan 6, 2008. 250mm f/6 Newtonian + DMK31AF03 camera. Lat.: 22º 15′ N, Long.:114º 10′ E; elevation is 0 meter.

Related Links
Rükl plate 53


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