moon tour

Ob­serve the re­mains of lavaflooded crater Ed­ding­ton

All About Space - - Contents -

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.

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