Full Moon on a Salt Pan
11 May 2018
I have always found that stargazing allows you to separate yourself from the hundreds of extraneous influences we deal with every day – the noise and clutter of day-to-day life, the trials and tribulations of all our complex relationships and the constant challenge of dealing with an increasingly busy world. It was never more so than last night.
I set the 10” Dobs up out on a remote Kalahari salt pan; it was a magical experience to watch the full Moon rise above the distant sand dunes, flooding the vast empty pan with its lovely light, and turning the salt deposits into a silvery shimmer; the only sound the occasional cries from a bat-eared fox echoing faintly across the void.
Just me, my dog and the Moon. Nirvana.
Most people think that the worst time to observe the Moon is when it’s full. Au contraire! Quite apart the fact that you can hold the entire Moon in your eyepiece and look at a place where billions of years of Solar System impact history are preserved, it is magnificently full of wonder… the dazzling albedo difference between the dark seas and the light highlands; the complex shades of grey in the majestic stretches of the lunar seas; delicate albedo features inside the craters; a completely new and fascinating perspective of the craters themselves; subtle tints of colour – bluish-grey; olive-grey, yellowy-grey, brownish-grey, reddish-orangish-grey…
…But most incredible of all are the beautiful ray systems that are seen in their full glory at full Moon.
They are dramatic, incredibly complex and absolutely fascinating. It is a mind-bender to look at the rays and try to appreciate the force behind an impacting projectile that scattered boulders, rocks and pulverized ejecta hundreds of kilometres across the surface of the Moon. It also is a mind-bender that over the next billion years they will have disappeared – pelted into obscurity by myriad micrometeorites.
I spent some happy hours examining the extraordinarily diverse splash rays…
Tycho
The most spectacular ray system of all is Tycho’s. Its stupendous rays rocket off in a butterfly pattern toward the northwest, east, and southwest, partially encircling the full Moon, and draping themselves over all sorts of lunar features. One incredible ray extends all the way across Mare Serenitatis… and keeps on going….
And talk about rays being a lesson in ballistics! Toward the west of the crater there is a large zone that is empty of rays, and the large nimbus of white material surrounding Tycho is twice as wide to the east as to the west. This is where the impactor came from. I cannot imagine the force of the projectile that came skimming in low over the Moon’s western horizon at a fantastically high speed, and scattered boulders 1,500 kilometres across the surface of the Moon. (Although it also makes you think that the descriptor ‘splash’ understates the ferocity of the event just a little.)
Tycho also has a very curious ray that raises a ballistics question: it emanates from the southwest edge of Tycho and doesn’t lead back to the centre of the crater, but is tangential to it. What happened at the time of the impact to have sent the debris on such an inexplicable flight path?
Another unusual feature of Tycho’s that is brought out by the overhead sun is the beautiful dark grey collar of impact melt that encircles the rim like a halo. No other rayed crater has a surrounding nimbus that is so dark; like the splash rays, it too will be obliterated by the myriad micrometeorites that pelt the Moon.
Hell Q – Cassini’s bright spot – just to the north of Tycho, may or may not be related to Tycho’s ejecta, and it isn’t much to see when it’s close to the terminator, but under the overhead sun it is a brilliantly bright white scrap of dazzle, one of the brightest features on the lunar disc.
Copernicus
One of my most sublime lunar experiences was once watching the sun rise over the great crater. Now with the sun overhead, its conspicuous features have been washed out but its patterns of rays and ejecta are incredibly bright, complex and fascinating.
I find it very curious that Copernicus is a much larger crater than Tycho and was created by a more powerful explosion, yet unlike Tycho’s rays that fly out across the face of the Moon, Copernicus’ rays are short, feathery and plume-like, some of the rays doubling back on themselves to form oval loops. It is difficult if not impossible to sort the rays out; what a gloriously convoluted sunburst-shaped splotch of ejecta.
Pytheas
Just north of Copernicus lies the crater Pytheas – it’s not a rayed crater itself, but a tremendous place to see secondary craters that have landed within a splash ray; now with the overhead sun, the line of craters have disappeared, but the splash ray can be seen clearly as a gorgeous bright arc.
Kepler
To its west, Copernicus’ rays intertwine with those of Kepler whose rays are also dramatic and conspicuously visible under the overhead sun. But most interestingly to me is the way that Kepler’s longest western ray points directly to one of the most famous of the lunar swirls on the Moon… Reiner Gamma. Lunar swirls are fascinating and enigmatic – absolutely flat features whose whorl-like shapes look as if they have been finger- painted on the surface of the Moon. Under the overhead sun the swirl looks like a tadpole-shaped smear of lunar lightness.
Aristarchus
Nothing on the Moon compares to Aristarchus’ dazzle, and the way its asymmetric rays fan out from the dazzle in a complex and widespread fashion is fascinating; but the region was more fascinating for the large subtle reddish-orangish-grey patch of colour on the Aristarchus Plateau to the northwest of the crater.
Proclus
Always a stunner. Its asymmetrical rays display an incredibly bright and dramatic butterfly-wing pattern – indicating it’s one of the low angle impact craters, where the impact angle is 15º or less, and the impactor had an almost grazing impact with an approach from the southwest. The longest ray runs northeast, right across Mare Crisium.
Mare Fecunditatis’ ray craters
And staying with low angle impact craters, one of the most extraordinary ray systems is Messier A’s – two gorgeous long bright rays of ejecta, looking like a comet flying over the centre of Mare Fecunditatis.
The mare is also adorned with the ray systems of three other craters: Langrenus, lying on the on the eastern shore of Mare Fecunditatis has a large but very pale system of rays – a lovely fuzzy-ish looking corona of rays that wash out across the surface of the dark mare.
Petavius B, located to the southeast of Mare Fecunditatis, has a bright asymmetrical ray system that stretches out over a large area of the mare, and fans out brightly to the east in a gorgeous array of rays.
Taruntius, lying on the northwestern edge of Mare Fecunditatis, has a very faint nimbus, with faintish rays that show up beautifully on the mare’s dark surface.
Stevinus A and Furnerius A
The two craters form a close pair of small craters with brilliant rays systems. They are popularly called The Headlights… and for good reason! They are the brightest rays in the Moon’s southeast quadrant, bracketing Stevinus, and their rays radiate out dazzlingly.
One extraordinarily long ray stretches across the west side of Mare Nectaris – it is hard to tell from which tiny crater it emanates, but regardless, one wonders at the force that splattered a tiny crater’s debris out so far.
Menelaus and Manilius
Side-by-side Menelaus and Manilius are extremely bright at full Moon. Manilius is surrounded by a bright halo and beautiful bright rays. You can follow one of its rays all the way to crater Bode in Sinus Aestuum.
Menelaus exhibits the distinctive ray pattern that results from the oblique impact angle of the impactor that formed the crater. It is traversed by a long ray from Tycho… the Bessel Ray that runs through the middle of Mare Serenitatis, and on its way touches upon the diminutive Bessel crater, giving it its name. It really is a gorgeous sight!
Dionysius
The crater is very bright at full Moon, and has the most stunning ray system – not for being bright and obvious but for a very subtle and unusual feature… dark rays. The crater is surrounded by a bright halo close to the outer rim. Not visible at first glance, but revealed under careful observation are an unusual display of radial dark rays beyond the bright halo, giving it an extraordinary radial zebra stripe appearance.
Anaxagoras
This crater and its rays are a beautifully bright and prominent feature when the Moon is full; the frosty white rays, streaming out from the crater’s rim look like a gleaming northern polar cap. Really lovely. One can follow one ray southwards all the way to Plato.
Aristillus and Autolycus
Aristillus’ delicate rays are lovely on the dark mare surface, in the south they disappear among the rays of Autolycus, which itself if is circled by a delicate nimbus and has a lovely ray system radiating outwards; with four or five prominent rays extending toward Archimedes and overlaying that crater’s flooded floor.
It’s always worth having a look at the bright region between Archimedes and Autolycus, for it was here that Luna 2, the first probe to reach the Moon, crashed on September 14, 1959. I wonder what its resultant crater and ejecta look like?
Timocharis
The crater is surrounded by a pale irregular nimbus and an asymmetrical arrangement of faint stubby rays, those to the east being the most prominent.
Mädler
Mädler is a bright crater lying amidst the wreckage of Theophilus’ mighty impact, and it has a lovely ray system that fans out into Mare Nectaris. One bright ray runs across the (now invisible) ghost crater Daguerre.
Geminus C
The small crater has bright and conspicuous rays; they appear as a bright broad swathe of brilliant lightness.
Lalande
The bright crater has a large bright nimbus and beautifully obvious rays that radiate out from it. There is a lovely intermingling of its rays and those of Copernicus.
Euclides
Euclides’s bright and conspicuous nimbus makes it one of the brightest spots on the Moon, and it is surrounded by stubby, streaky-looking rays.
Byrgius A
Another bright little ray crater, Byrgius A’s brilliant rays radiate out in a beautiful splotchy shape; it looks rather like a lunar flower with bright petals.
Glushko
Glushko (formerly Olbers A before being renamed by the IAU in 1993) is responsible for a bright ray system emanating from the western limb of the moon. You can follow one long bright ray to the northeast, grazing the crater Seleucus as it passes by.
Birt
A lovely crater under low illumination, under the overhead sun it is a bright little crater with a noticeable brightish ray system; two prominent rays splash out westwards, and to the south its fainter rays become mixed with those from Tycho.
A lunar star field
The rays are definitely the most stunning feature on the full Moon, and absolutely fascinating to study, but most gorgeous of all to me is the multitude of tiny bright craters that carpet the full Moon under the overhead sun.
The large swathe of them that lies in the highlands running from Tycho to Menelaus and Manilius – and it looks like a vast glittering star cloud. It is absolutely breathtakingly beautiful!
It was a dazzling night; the only way to have had a better lunar exploration would have been to go to the Moon.
Copyright © Susan Young 2018