Astronomical Society of Coonabarabran

Einstein! a visit with the Master

by Harry Roberts

Taking a quick look recently at the nearly full Moon showed a particularly favourable libration for western limb formations. I was happy to have my best view yet of multi-ring basin Mare Orientale, and made several detailed sketches. Before finishing for the night I took a look along the west limb northwards: and near flooded basin O. Struve (I was struck by how round it was under the extreme libration) I found a huge crater, nearly filled with shadow. The crater lay on the terminator southwest of Struve, and I was fairly sure I'd never observed it before. It was a dramatic sight, with a huge shadow filled basin, in the centre of which lay another large shadow filled crater, resulting in a multi-ring appearance like a small version of Orientale. It proved very difficult to sketch, and the rapid advance of lighting continuously changed the appearance of the formation (Fig 1). The feature seemed not to be plotted in Rukl[1], until I consulted the Libratiion Zone Map VIII, when I found the crater was 170 km. diameter Einstein. I then realized Einstein IS plotted on Map 17 as a shallow depression on the west limb, clearly I was seeing a VERY favourable libration!

Checking my few Moon references (Wood[2], North[3] and Harold Hill[4]) I found no images or mentions of Einstein, though there were views of nearby Otto Struve. The Lunar Orbiter site had several aerial views of the crater (Fig 2), as it does of most far-side features. There seems not to be any image of Einstein in the Consolidated Photographic Atlas. Obviously views of Einstein are not common.

The explanation is that the 90ºW meridian of longitude passes through the rear half of crater Einstein, and mostly it is unseen from Earth, or is seen only side on. Its walls must be high as the deep shadow within made the crater stand out sharply in its lower surroundings. Libration Map VIII gave me the names of the many adjacent craters. Immediately left (south) of Einstein and seen as a dark edge-on depression, is Bohr, Einstein's old sparring partner, the Danish physicist. Above Bohr and shadow-filled is strange Vallis Bohr, which seems from Orbiter imagery to be a catena or collapse feature radial to the huge Einstein crater. This whole area is in very rough, far-side, highlands. Front left is large crater Vasco da Gama, mariner explorer, and immediately in front (east) of Einstein is Dalton (John, the English chemist and physicist.)

Interestingly, the Orbiter image showed the crater floor sharply folded about a NS line, and my sketch seems to confirm this, with the eastern half of the crater floor well lit, while the rear (west) half is mainly in complete shadow. The large central crater Einstein A seems to lay mostly on the east side of this fold-line, and hence we see well into crater A, partly shadow filled, and partly we see its brilliantly lit high western wall. The extended right side of crater A is a second shallower crater in contact with A. Two very thin bright craters can be seen to left and right of crater A, on the floor of Einstein. Behind crater A, emerging from the blackness of space we see several bright parts of Einstein's crater wall, on left and right sides they form a series of headlands receding westwards. As I watched, two very thin brightly lit ridges, apparently on the western floor of Einstein, began to catch the rising light, glowing like incandescent wires. Interestingly, the central crater Einstein A has eliminated the main crater's central peak, an unusual aspect that adds to Einstein's uniqueness.

Sightings of Einstein may be quite rare, needing a favourable libration and suitable illumination, as well as a clear sky. Seeing was "poor" to "fair" throughout my session, and fine detail was invisible. In spite of this, I closed down with a sense of satisfaction, having seen a rare formation, and thinking that Einstein would be gratified with the majestic crater that memorialises his work.

Enjoy the Moon.

[1] Rukl, Anton, "Atlas of the Moon" Sky Publishing.
[2] Wood, C. "The Modern Moon - A Personal View" Sky Pub. Corp. 2003.
[3] North, G. "Observing the Moon" Cambridge Uni Press. 2000
[4] Hill, H. "A Portfolio of Lunar Drawings" Practical Astronomy Handbooks 1. Cambridge Uni Press. 1991. Hardcover. Out of print.

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