Watching the Moon last night, at about 90% full, we crossed a boundary and saw the far side of the Moon.
How so? Well, largely by accident. We were following the ray patterns towards the south pole from the Tycho crater when we came across a pattern that was both striking and strange. We saw a long elongate scar beneath a pretty quartet of perfect lunar craters. Beyond these lay a Mordor rim of battlements and then a crater, more a rampart-bound plain, vast and still as it slid over the edge of vision into darkness.
We were looking at the Bailly Crater. It lies on the rim, in fact over the rim, of the southwestern lunar limb as it faces us. From Earth, Bailly is always seen at an oblique angle that foreshortens the appearance of what is in fact one of the largest craters on the lunar surface, 183 miles across and with crater walls as high as 14,000 feet in places.
It lies on the edge of vision.
Bailly is a place of rock and walls, silence and above all shadows. It is a field of ruins punctured by smaller craters. The largest of these – we saw one, overlapping the outer walls – are named Bailly A and Bailly B, and their presence tells us that the main Bailly crater is old enough to have been subjected to bombardment itself. It is one of the oldest visible lunar formations, dated to the Nectarian period of lunar geology.
Bailly is on the edge of history as well as sight.
How is it possible even to see Bailly? It is a will-o’-the-wisp, hovering between the near and far sides of the Moon. It is not always possible to see. This may seem a strange idea, because they face of the Moon is so familiar, but our view of the Moon is not in fact quite constant. It is possible to see not the obvious 50% but as much as 59% of the lunar surface over time. This is a result of what are termed librations.
Libration. The word is derived from the Latin libra, meaning scales, and libration refers to a motion comparable to the swaying backwards and forwards of weighing scales. The British astronomer John Flamsteed was the first to use it of the Moon in around 1670.
Planetary scientists now distinguish three kinds of libration by which the Moon sometimes shows more of itself. The proper causes of librations are: eccentricities in the Moon’s orbit of the Earth, the inclination between the Moon’s axis of rotation to the plane of its orbit and the effects of the Earth’s rotation and our place on the planet’s surface. We can distinguish more simply, perhaps, between optical and physical librations – those to do with the Moon and those to do with where we are looking from. The effect is that the Moon's south pole tips steeply towards us. We can glimpse round the corner of the Moon.
The face of the Moon that is permanently turned away from Earth was first photographed by the Soviet Luna 3 probe on 7th October, 1959. Its eighteen original images were augmented a year later by a further 25 from the camera on board another Soviet craft, Zond-3. Frank Borman, James Lovell and William Anders were the first humans to look directly upon its surface in December 1968. Other Apollo missions and many unmanned probes have passed across the far side since.
It is perhaps suitable that this vast but evanescent crater, sometimes there and sometimes not, should have acquired the name of an astronomer whose fortune swung from fame to infamy. Jean Sylvain Bailly was an orator and scientist, a leading light of eighteenth century Parisian salon intelligentsia who wrote knowledgably on the satellites of Jupiter and creatively about much else. He was one of the commissioners at the trial of mesmerism, which was convicted as imaginary pseudo-science. In the early years of the French Revolution, he was President of the Third Estate and then Mayor of Paris, a political achievement for which he was guillotined on 12th November 1793. He is one of very few victims of the Terror to find immortality upon the Moon.
Why, finally, does the Bailly Crater matter? Because the edges of the things and the unseen have always seemed to matter more than the middles and the obvious. The far side of the Moon has been a canvas for human imaginings for a long time. Is that now lost? WH Auden expresses something close to mourning in his poem Moon Landing:
All we can pray for is that artists,
Chefs and saints may still appear blithe to it.
I think the moon landings and the science of our near neighbour do not strip the dreams at all. They make them richer. This is David Hart in his poem On The Moon:
I have left my best socks on the moon
and my shoes and my timbre.
I was paddling in Mare Nectaris
and was sweetened close to the still point
of erasure, mind-blown to extinction almost,
packed tight with crazy rare air.
This is a vision of the Moon's unfamiliarity but also of encounter, not that of the astronaut but the poet. We have met with the Moon and it is a tutor in strangeness. The recent Lunar Reconnaissance Orbiter images of the Apollo landing sites show how small they were, how tiny in the lunar scale, but also how persevering. You can still see the tracks. (The last Apollo astronauts parked their buggy with the wheels turned to the left.) Moon science still poses many questions, and knowledge of what we saw through the telescope increases the sense of wonder. Only ignorance, it seems to me, shrugs and dismisses.