Observe the remains of lavaflooded crater Eddington
Understandably, the most famous and popular lunar features are those which are presented almost face-on to us.
The ray-sploshed craters Copernicus and Kepler; the curved, bony spine of the Appennine Mountains and the great vertical crater chain of Ptolemaeus, Alphonsus and Arzachel are all very central on the Earth-facing side of the Moon and so are observed, photographed and sketched much more often than other craters and mountains closer to the limb of our planet’s natural satellite.
However, if you can drag your eyes – and swing your telescope – away from the centre of the Moon you will be rewarded with some fascinating and intriguing sights as you gaze across features foreshortened by their proximity to the Moon’s curved edge.
One such feature is the crater Eddington – or, rather, what is left of the crater Eddington. Many millennia ago Eddington (named after famous astrophysicist Arthur Eddington, who was born in the Lake District town of Kendal, less than a mile from where I’m writing these very words!) was a crater like countless tens or even hundreds of thousands of craters on the Moon – the ancient scar left behind when an asteroid slammed into the Moon and blasted a hole out of its rocky crust. It probably remained like that for a long time, but then something very dramatic happened that changed Eddington forever…
A huge piece of space debris slammed into the Moon somewhere close to Eddington, melting a vast area of the surface into molten lava. This lava flowed across the surface of the Moon towards Eddington – then rolled up and over its southern walls and overwhelmed them, flooding the interior of the crater and smothering any features on its floor and on the lower reaches of its walls.
When this glowing, bubbling tsunami of melted rock eventually settled and cooled, the southern part of Eddington was gone, hidden beneath kilometres of cooling, darkening rock. If it had once had a central peak standing on its floor, that was lost from sight too. All that remained of the crater was the semicircular arc of its northern rim and a few smaller, lower ‘islands’ of rock to the south – the highest parts of its southern walls. Eddington was now a ‘crater remnant’, the ghost of a once proud feature.
If Eddington had been blasted out of a region of the Moon more central on the Earth-facing side, even after its heart had been flooded with lava it would have been easily visible from our planet through a pair of binoculars as a bright, roughly semi-circular feature with a dark floor and smaller satellite craters dotted around it.
However, as it was created near the north-western limb of the Moon it appears very foreshortened to us, little more than an oval, and a telescope is needed to see it. Even then all you will see is the arc of its northern edge, which makes the crater remnant look more like the front end of a pair of tweezers than a semi-circle. If you view Eddington at high magnification you will be able to see a few tiny craterlets dotted across its dark, almostflat floor to the north west, and to the south east you’ll see two tiny arcs. This is Eddington P – a smaller, lava-filled ghost crater on the floor of the ghost crater of Eddington itself.
At the start of our observing period Eddington cannot be seen; you’ll have to wait until 23 October for your first glimpse of it as the terminator – the line between lunar night and day – sweeps over it and its eastern side is kissed by the Sun’s rays again. For the following three or four days Eddington will be as well placed for observation as it ever gets: the Moon’s libration – its slow wobble around its axis – will tip the crater towards us during that time, before pulling it away from us again. Eddington will vanish from our view on 4 November when the terminator slides over it again, plunging it back into darkness, and it will not be visible again during this month’s observing period.