The Lunar Farside

By Duncan Lunan

Last week I described my favourite part of the Moon, down the centre line of the visible disc, highlighted by shadows at Third Quarter.  It has to be emphasised, though, that it is only a small part of the Moon’s total area, which is roughly equivalent to South America’s.  The part least described in most books is the Farside, for the obvious reason that we can’t see it;  but there’s a lot to talk about round there.

To say that we can’t see it isn’t 100% true.  As I previously mentioned, the Moon’s orbit around the Earth is inclined at 5 degrees to the Earth’s orbit around the Sun  (the Ecliptic).  To be strictly accurate, the Earth and Moon both orbit around the barycentre, their common centre of mass, in scalloped paths around the Sun, and they should be considered as a double planet.  We should say that the Moon’s path around the Sun is inclined to the Earth’s path by 5 degrees.  But for most purposes, to say ‘the Moon’s orbit around the Earth’ is accurate enough.

Fig. 1. Regression of the Lunar Nodes and precession of the lunar axis (Duncan Lunan, 1993)

Due to the forces acting on it, including the pull of the Moon, the Earth’s axis wobbles, describing a circle around the pole of the Ecliptic with a period of 26,000 years.  Currently it points to Polaris in Ursa Minor;  at the building of Stonehenge 1 and the Pyramids, it pointed to Thuban in Draco;  13,000 years ago it pointed to Vega in Lyra, and will do again in 13,000 years’ time.  The phenomenon is called the Precession of the Equinoxes.  The Moon has a similar wobble, around Zeta Draconis, with a period of 18.61 years  (Fig. 1).  Due to the pull of the Sun and of the Earth’s equatorial bulge, in the same period the entire orbit of the Moon precesses around the sky, and both solar and lunar eclipses can only occur when the Moon is near the nodes where its orbit crosses the Ecliptic.  For that reason the process is known as the Regression of the Lunar Nodes.  As the Earth’s axis is inclined to the Ecliptic by 23.5 degrees  (currently), every 18.61 years when they add together to the maximum value, the Moon comes to a ‘major standstill’ position where it rises and sets at its furthest north, and at furthest south two weeks later.  9.3 years after that, it comes to a ‘minor standstill’ on the other side of the Sun’s rising and setting positions at the solstices.  (Fig. 2.  See ‘Sighthill Observations’, ON, April 10th 2022.)

Fig. 2. Standstill alignments at the Sighthill stone circle, Glasgow, by Dave McClymont, 1979 (design by D. Lunan)

The upshot of all that is that over the cycle, the Moon wobbles as seen from here, and we can see part-way round into the Farside.  The process is called ‘libration’, and the ‘libration zones’ on the Moon’s surface are the only places there where the Earth can be seen to rise and set  (Figs. 3 and 4).  No human or spacecraft has seen that happen yet, but with the current concentration on sites like Mount Malapert and Shackleton crater at the lunar south pole, we can expect to see time-lapses before too long.  (It has been done from lunar orbit, but you can do that from anywhere.)

As far as we could see, there was little difference in the lunar landscape to the edge of the libration zones, but one big discovery was made by Patrick Moore:  a huge, double-ringed, lava-floored crater named Mare Orientale, because it was then on the eastern limb of the Moon, i.e. the rim nearest our eastern horizon  (Fig. 5).  But as I explained in ‘Compass Points in the Sky’  (ON, 28th March 2021), the convention meant that on the Moon, the Sun rose in the west and set in the east, although the Moon rotates on its axis in the same direction that the Earth does.  The convention was abandoned to avoid confusion during the Moon landings, with the result that Mare Orientale, the Eastern Sea, is now on the western limb of the Moon.

Fig. 5. Mare Orientale from Lunar Orbiter, 1967

It was known that the central part of the visible disc bulges towards the Earth, and that allowed some imaginative writers to suppose that it was one huge mountain sticking up out of a lunar atmosphere which was concentrated on the Farside.  Jules Verne’s travellers in From the Earth to the Moon had intended to land on the Nearside, in daylight, so they passed over the Farside in darkness, and Verne didn’t have to describe what they saw.  Angus MacVicar’s Satellite Seven serial for BBC Scotland’s Children’s Hour went further, providing not only an atmosphere but a glimpse of a valley with a track leading from a cave.  A serial in Young Elizabethan in the early 1950s went a lot further, giving the Farside a human civilisation.  But Patrick Moore’s observations had led him to make a set of deductions about the Farside which gave some idea of what was actually round there.  As noted last week, at Full Moon the larger craters are surrounded by bright ‘rays’ of shattered rock which extend far across the surface:  those from Tycho, deep in the southern hemisphere, extend at least as far as Mare Nubium, near the equator, and were photographed there by Ranger 7, the first successful US Moon probe, on its final approach in 1964.  But at least 13 years before that, Patrick had plotted rays coming over the rim of the Moon from the Farside, and produced a map showing where the larger craters there would be  (Fig. 6;  Patrick Moore, ‘What We Know About the Moon’, in L.J. Carter, ed., Realities of Space Travel, Putnam, 1957.).  It compares very favourably with the image of the Farside obtained from a similar viewpoint by the Clementine probe in 1994  (Fig. 7).

Appropriately enough, Britons’ first sight of the Farside came when Patrick Moore was suddenly handed the images from the Soviet Luna 3, which had come over the wire from Moscow during a live broadcast of The Sky at Night.  He had a moment of near-panic when he could make nothing of them, because the Farside was relatively featureless  (Fig. 8), and the maria which he and Wilkins had predicted did not exist.  But then he recognised the Nearside’s Mare Crisium on the left of the image, and was able to make some sense of the rest.

Fig. 8. Luna-3 Moon Farside, 1959

Hard though it is to believe now, in 1959 the standard method of returning images to Earth was by bringing back the exposed film in capsules.  The US was doing it with its Discoverer satellites, and the off-course fall of one off Spitsbergen was the inspiration for Alastair Maclean’s Ice Station Zebra.  But Luna 3 was still far out in space and didn’t have that option.  The photographs were laboriously printed out on board and then scanned for transmission back, but the quality was poorer than it should have been because the onboard temperature was too high.  Conspiracy theorists were quick to rush forward claiming that they could see brush marks and the photos had to be faked.  Comparison with much better images taken since by Lunar Reconnaissance Orbiter  (Fig. 9)  shows that the photos are absolutely authentic – but still, not everything is at it seemed.

As the discoverers of the features, Soviet mappers claimed the privilege of naming them  (Fig. 10).  At least we were spared Arthur C. Clarke’s nightmare, that it would be done by a bureaucrat with a telephone directory and a pin  (or a US General who was a baseball fan).  Nevertheless there were controversies.  The convention of the International Astronomical Union is that lunar maria should be named either after meteorological phenomena, like Oceanus Procellarum  (The Ocean of Storms), or Mare Imbrium, Mare Nubium, Mare Fecunditatis, and Mare Frigoris  (The Seas of Rains, Clouds, Fertility and Cold, respectively);  or else after mental states, like Palus Somnii  (The Marsh of Sleep), or Mare Tranquillitatis, Mare Serenitatis and Mare Crisium  (the Seas of Tranquillity, Serenity and Crises.  It’s worth adding that ‘the Glen of Tranquillity’ in Gaelic is ‘Glenmorangie’.)  But the Soviet compilers wanted the only true mare on the Farside to be ‘the Sea of Moscow’, and after some argument, the IAU conceded that Moscow was a state of mind.

Fig. 11. Aitken Basin, mapped by Kayuga-Selene, 2007-2009

The largest crater on the Moon, on the Farside, is the Aitken Basin, which extends 2500 km from the south pole to the equator  (Fig. 11).  As mentioned in ‘The Earth from Space’  (ON, 7th May 2023), it wasn’t noticed until the passing Galileo spacecraft in 1992 happened to catch it on the terminator at just the right angle.  Aitken’s floor is at least 12 km below the mean surface level, almost the deepest in the Solar System, but the reason why it’s only visible from a distance  (like the Straight Wall, which I described last week), is that it has no dark floor of basaltic lava, like the large impact basins on the Nearside.  Neither do the two impact basins to upper right of it in Fig. 11, though they’re both larger than Mare Orientale, on the right of the image.  There are some dark-floored craters in the Aitken Basin  (Von Kármán crater, below, is an example, and Jules Verne, on the rim of the Basin, is another).  But if there was magma so close to the surface, in the epoch when the big basins were flooding on the Nearside, why didn’t it break through and flood Aitken itself?  

The next largest dark-floored crater to the Sea of Moscow, on the Farside, is the crater Tsiolkovsky, named after the theorist who was first to work out the basic theories of astronautics in the late 19th century.  When it was photographed from overhead by Lunar Orbiter 1 in 1966, it was clear that its central peaks stood high above the lava floor and would contain material from deep within the Moon, deserving future study.  Immediately intriguing was the way that material from the crater walls had flowed across the floor, reburying the lava – possibly the result of huge landslides, but possibly indicating buried glaciers, because they resembled the burial of the Sherman glacier in Alaska by the ‘Good Friday’ earthquake of 1964.  It was enough to get Tsiolkovsky rephotographed by Apollo 8, Apollo 11  (Fig. 13), Apollo 13 and Apollo 15  (Fig. 14).  The presence of ice wasn’t confirmed  (H. Masursky, G.W. Colton, F. El-Baz, Apollo over the Moon, a view from orbit, NASA SP 362, GPO, 1978).  But there are other even smaller dark-floored craters in the vicinity, and the question remains, why only in that part of the Farside, and not in the biggest basins?

Another controversial feature was the white streak running diagonally down Fig. 8 and captioned as  ‘the Soviet Mountains’ in Fig. 10.  Apart from arguments about the name, the plain fact is that it’s another optical illusion, like the Bridge in Mare Crisium.  By 1961 it was plotted as a crater chain, but it’s actually a pattern formed by rays from the crater Giordano Bruno crossing another ray system, and that was confirmed by the first manned mission to orbit the Moon, Apollo 8  (Fig. 15).  The mountains persisted on Soviet maps until 1978, and after their deletion by the International Astronomical Union, the Russians substituted an equally non-existent crater Lipskiy, named after one of the original map-makers.  (N.P. Barabashov, A.A. Mikhailov, Yu. N. Lipskiy, eds., Atlas of the Other Side of the Moon, Pergamon, 1961.)  Western experts didn’t get the joke.  (James E. Oberg, UFO’s & Outer Space Mysteries, a sympathetic skeptic’s report, Donning, 1982.)

(N.P. Barabashov was a well-known planetologist.cited in G. Tikhov, Reaching for the Stars, Foreign Languages Publishing House, Moscow, undated  (post 1958);  and in Patrick Moore, The Planet Venus, Faber & Faber, 1960 edition.  A barabasha is allegedly a psychic entity sharing the less endearing characteristics of vampire and poltergeist, but Russian surnames tend to be more imaginative than ours.  If we had a venerable surname ‘Vampire-Poltergeist’, it would of course be hyphenated.  “Lord and Lady Vampire-Poltergeist…’)

The 20-km crater Bruno is extremely controversial, and not just because it’s been named after the philosopher who was burned at the stake by the Vatican in 1576.  He believed in heliocentric theory and the plurality of worlds, primarily because he was a Neoplatonist, but the real issues which ended his life were his rejection of Catholic doctrines such as the Virgin Birth.  He is generally regarded as a martyr for science, and giving him a prominent lunar crater wasn’t an issue.  Patrick Moore and H.P. Wilkins had deduced its existence from the rays of impact debris, reaching the hemisphere facing Earth  (Fig. 6).   But for such a small crater to make such an impact  (literally), it must be relatively recent;  and in his TV series Cosmos, Carl Sagan backed the suggestion of Derral Mulholland & Odile Calame, that the effects of the impact had been observed by monks at Canterbury in June 1178  (‘Lunar Crater Giordano Bruno’, Science 199, 875-877, 24th February 1978).   The idea was immediately challenged, first with a claim that the Moon wasn’t visible at the alleged time;  but it turned out that it was, though a very thin crescent, which the monks had seen doubled in its northern half  (Fig. 16) – possibly due to impact plumes, after which the Moon ‘writhed like a wounded snake’, perhaps by the formation of a temporary atmosphere, which could deposit ice at the lunar poles, as discussed last week.

Fig. 16. Lunar impacts, 1178, by Sydney Jordan for Children from the Sky & Incoming Asteroid

For what Carl Sagan didn’t know, or didn’t fully appreciate, was that the event happened 12 times or more.  That became easier to understand in July 1994, when Comet Shoemaker-Levy 9, fragmented by a previous encounter with Jupiter, hit the planet in a string of 22 events.  A search of Voyager and Galileo spacecraft images revealed chains of craters on Ganymede and Callisto, apparently formed by similar events.  The Farside of the Moon has two such ‘parentless’ crater chains, darkened by dust and obviously very old.  Giordano Bruno’s appears to be the only recent event there.

Again the account was challenged:  the monks claimed to have seen ‘red sparks’ flying off from the Moon  (Fig.16), and allegedly they couldn’t have seen ejecta at that distance;  yet there should have been a fall of debris on Earth, of which there were claimed to be no records.  Yet there was a major fall of meteorites in England that year, added by an unknown hand to the Chronicon Anglicanum of Ralph of Coggeshall;  and there are accounts, admittedly anecdotal, of larger falls in South America, the Far East and New Zealand.  The crater has since been photographed by Clementine, Kayuga-Selene  (Fig. 17), and in more detail by Lunar Reconnaissance Orbiter.  The Kayuga-Selene images were used to estimate the age of the crater, by the number of smaller craters on the ejecta blanket immediately surrounding it  (one is arrowed in Fig. 17).  It came out at approximately 4 million years, ‘much too old for the [1178] hypothesis’, says Wikipedia.  But if the monks’ account is correct, and there were 12 impacts in rapid succession, then the ejecta from the multiple events would have been superimposed and it would make the main crater look much older. 

Fig. 17. Bruno crater (below) from Selene overhead

I think I’m the first to point that out;  and it focusses attention of the oddity of those multiple events.  While an asteroid or a small comet could have been fragmented in a close pass by the Earth, you would expect it to hit the Nearside – unless it was going very slowly, like Ranger 4, which did crash on the Farside;  but then, you wouldn’t expect the consequences to be visible from here.  The Shoemaker-Levy 9 impacts on Jupiter were spread out over days, but the parentless crater chains on Ganymede, Callisto and the lunar Farside are contiguous or overlapping, so the sequence of impacts was obviously brief.  Nevertheless they weren’t all in the same spot.  The Bruno impacts occurred within 30 minutes, so the Moon’s slow rotation would allow them all to hit within the crater’s 20-km radius, but only if the incoming track was vertical, pointing straight at the centre of the Moon – not consistent with a recent flyby of the Earth.  The lumps and bumps on the crater floor, including a swirl of lava from one of the walls and a strange shelf – a ‘slump block’ on one of them – look as if they were caused by multiple strikes.  It looks purposive.

Carl Sagan thought it was an extraordinary coincidence that such an event should have happened within the comparatively short span of recorded history.  What’s even more extraordinary, for me, is that it happened at the height of the mediaeval mystery of the Green Children of Woolpit  (ON, June 19th and 26th, 2022).  It’s as if whoever was behind it all had staged a demonstration of how the crater chain of the children’s home world had been created  (Fig. 18).  But why would they do that, and why on the Farside of the Moon, where only the effects and not the events could be seen from Earth?  The only explanation I could come up with, in Children from the Sky, was that it was part of an effort to throw a scare into king Henry II, to get him not to interfere more than he already had.  But I’ve never been satisfied with that, to be honest, and my brain is still working on it.

Fig. 18. colony valley impacts, by Sydney Jordan for Children from the Sky

The only landing on the Farside to date was nowhere near any of the features above.  In January 2019 China’s Chang’e-4 landed in Von Kármán crater, within the Aitken Basin, and released the Yutu-2  (Jade Rabbit)  rover  (Fig. 19), which is still going strong.  In May 2019, it was announced that mantle rocks had apparently been found, from deeper within the Moon than before.  Just keeping in touch with Earth through relay satellites for so long is a major achievement, and there have been a couple of false alarms about oddities apparently found and then identified – more about that next time.

Fig. 19. Yutu 2 (Jade Rabbit) at crater, from Chang’e-4, January 2019

Duncan Lunan’s book Children from the Sky, published by Mutus Liber in 2012, includes a chapter on the strange events of 1178.  The book is available from bookshops or through Amazon.

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