Astronomical Society of Coonabarabran

Anaxagorus revisited

by Harry Roberts

Constantly changing libration adds a special challenge to Moon observing; features hardly ever look the same twice. The "Virtual Moon" (c) freeware devised by Christian Legrand and Patrick Chevalley (and others) makes light of a task that must have driven earlier observers mad; the Moon's motions are so complex. But with the software you can search for times when libration is best for a given landform, or just to make a telescopic tour of a region that's seldom seen; it's magic! Google "Virtual Moon".

Anaxagorus is a large, fresh crater, and would be well known if it wasn't at 73º N, in high lunar latitudes. My first view of it occurred with "fair" libration of 2º20' in latitude. Revisited when libration was vastly better at 6º30', in spite of poor "seeing", I was treated to a more detailed interior view. The two views contrasted strongly. The earlier showed an idealize scene with regular rayed walls and uniform rim, the second was much more "warts and all", and showed that Anaxagorus has been substantially degraded.

Most striking of all was the mountain chain that seemed to stretch across the crater from wall to wall. The earlier white peaks were there but now more peaks and lower ridges bisected the crater floor, and the floor looked dark and textured. Like other Copernican-era craters the floor is not lava flooded, but is probably covered with the darker, blistered and ropey, Pahoehoe terrain.

The SE rim was a crescent of dark shadow, but the NE rim was dimly lit and showed many thin dark terraces. Below was featureless grey scree of slippage material, perhaps, above some brighter scarps. The northern (top) edge showed a bright main scarp above a wide grey multi-tiered terrace about 40 km long, that curved around onto the NW face of the crater wall. Here the main scarp has partly collapsed into two parallel terraces that erode back into the surroundings. Below this the original crater wall seems to be well preserved and looks a lot like smaller Aristarchus, with its bright and dark bands.

Below right of the main crater is secondary Anaxagorus A, and the large ancient basin Goldschmidt lies to the right. Clementine images seem to show a thin layer of ejecta overlays Anaxagorus in the NW to SE direction; where did this come from? Anaxagorus has undergone some lunar "weathering", and is no longer a pristine formation.

Run "Virtual Moon" forwards in time to find favourable librations and take a look at Anaxagorus.

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