Popular Astronomy

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Mighty lunar cliffs (SPA News Circular 272)

On 20 June the Moon’s sunrise terminator will have rolled back to reveal a remarkable feature near the southeastern shore of Mare Nubium (the Sea of Clouds) — one of the Moon’s most magnificent cliffs, Rupes Recta, the so-called ‘Straight Wall’, a feature caused by crustal faulting. Tension in the Moon’s crust following the cooling of the lavas which filled Mare Nubium around three billion years ago pulled the crust apart. The crust eventually cracked under such stresses, and the force of gravity produced a horizontal displacement between the two separated blocks of crust, the crust in the west dropping down relative to the eastern side of the fault; the exposed edge is a cliff known as a fault scarp.

Rupes Recta is by far the boldest and most clear-cut example of a normal fault on the Moon. The fault scarp has a rather gentle gradient of around seven degrees and runs north south for 126 km from the Stag’s Horn Mountains to the small crater Birt D, in a very slight curve. The Straight Wall is best seen after First Quarter phase, as on 20 June, when it throws a prominent broad shadow westward onto the relatively flat mare floor, so bold as to be easily visible through binoculars. The cliff’s shadow gradually recedes over the next day or two and is virtually invisible at high illuminations when no shadows are cast. However, around Last Quarter phase as the evening terminator approaches Mare Nubium, the cliff face brightens and shows up clearly as a bright narrow line. 35 km west of Rupes Recta, and lying parallel to it, is a narrow cleft — a channel carved by running lava — known as Rima Birt which connects the small craters Birt E and Birt F. An 80 mm refractor will just about resolve Rima Birt on a fine night and under favourable conditions of illumination.

Another example of faulting on the Moon can be found in Mare Tranquillitatis (Sea of Tranquility), near the bowl-shaped crater Cauchy (12 km); to its north is the Rima Cauchy, a 210 km long fault valley that runs northwest-southeast across the plain in a slightly curved path. To the south lies Rupes Cauchy, an odd combination of fault-scarp (120 km long) and rille with an overall length of 180 km. Rupes Cauchy is also slightly curved, but in an opposite direction to Rima Cauchy — the pair have been referred to as ‘hyperbolae’. As with Rupes Recta, the scarp face of Rupes Cauchy faces west and casts a shadow during early morning illumination, a favourable illumination occurring on 16 June.

Above: Walter, Werner and Aliacencis, large craters in the Moon’s heavily cratered southern highlands, captured in pencil by Graham Sparrow in this observational sketch made on 22 April from 20:30 to 21:20 UT. Graham used a 200mm Newtonian and a magnification of 70.

Recent visual observational drawings have been received from Peter Grego, Dale Holt, Rob Peeling, Sally Russell and Graham Sparrow; lunar images have been received from Anthony Ayiomamitis, Mike Brown, Maurice Collins, Dave Finnigan and Brian Ritchie.

Added by:  Graham Sparrow