Regular solar observer & correspondent Harry Roberts reports on the strange Lunar crater Schiller.
Schiller is one of the strangest formations on the Moon! Located near the lunar SW limb, it is best seen as the lunar phase approaches full, or at last quarter (in early morning). At dusk on June 11, 2003 I had a nice view in the 4” Maksutov and set up the C8 for a detailed drawing (Figure 1 above).
As often happens, dramatic shadows across the crater floor drew the eye to a speck of sunlight on one of the linear central peaks at the northern (left) end of the formation: shadow filled much of the north half of Schiller. It was tricky to sketch since on this date the terrestrial E-W line of drift (one axis of the drawing) lay almost along the formation’s centre. This E-W line moves around a lot relative to lunar longitude due to the moon’s complex orbital motion. In addition, Schiller’s proximity to the lunar limb increased its cigar shape – the whole formation crosses some 6 degrees of lunar longitude and five of latitude.
Schiller is 180 km long and 71 km at the widest. Lunar curvature helped to elongate rim shadows at the northern end. Yet the shadow cast by the eastern rim diminished to zero in the vicinity of crater H (Fig. 1), an odd shaped crater: maybe two superimposed. Presumably H ‘punched’ a gap in the rim wall almost to floor level. Schiller’s rim must be very high north of H, to cause such long shadows: 4km maybe?
At Schiller’s southern (right in Fig. 1) end lies a detached medium sized crater (Rost B?) co-axial with the main formation –connected by a shallow valley it seems? Not labelled in Rukl’s “Atlas of the Moon” – “Virtual Moon” freeware suggests it’s Rost B: Rost is out of the field to the south.
Schiller’s brightly lit inner western wall showed terraces and other slippage features, and cast long shadows towards the lunar terminator. I logged three tiny craters on Schiller’s smooth floor in the good seeing: their diameters are between two to three kms. Interestingly Schiller contains two linear central ranges at the northern end of the formation; one seen clearly, the other a speck of light; as well there is some uneven ground and slippage material from the high scarps on the northern floor.
Formation? The big puzzle about Schiller is to explain how the formation was created. Wood tells us “Impact craters are round because they are essentially point source explosions that throw material out in all directions. So what is wrong with Schiller, a structure 180km long but only about 70km wide? A clue comes from the central ridge at the upper (NW) end of the structure and the coalescence of multiple craters. Similar features are seen in very low angle oblique impacts created in NASA’s labs”. He adds “Perhaps a small asteroid or comet was captured into lunar orbit and while spiralling inward, was torn into pieces…creating overlapping craters” (Kaguya Lunar Atlas, 2011, p132).
The main part of the northern end of Schiller has almost parallel crater walls with the long, linear central ridges. David Gault’s impact experiments (Fig. 2) showed how such craters result from very low angle impacts that create almost ‘linear’ explosions. The Schiller impacts must have been at a very low angle: like Gault’s 2 or 3 degree results.
There are much smaller formations of this kind on the moon, particularly parts of the “Rheita Valley” in the lunar SE, and the Messier craters in M. Fecunditatis. As we have now seen multiple comet impacts on Jupiter – the multi-impact idea no longer seems as implausible as it did before 1994.
Schiller was Julius Schiller, d. 1627. A German monk. Author of an atlas of the sky, in which the constellations were replaced by biblical characters and objects. His system was not adopted. Crater Schiller [51.8°S, 40.0°W] is a very odd ‘creature’: unique on the Moon. Take a close look when the moon’s phase next approaches full. Clear skies.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.