Moon tour

We visit a large semi-hexag­o­nal crater in the Moon’s cen­tral high­lands, Al­bateg­nius

All About Space - - Contents -

There are many fea­tures on the

Moon that have ac­quired ‘celebrity’ sta­tus be­cause they are gen­uinely im­pres­sive. Other fea­tures aren’t given the same credit or at­ten­tion be­cause they are over­shad­owed by their more im­pres­sive neigh­bours. One such fea­ture is a walled plain called Al­bateg­nius, which can be found al­most in the cen­tre of the Moon’s face as we see it from Earth.

If you’re not fa­mil­iar with this fea­ture that’s no huge sur­prise. Al­bateg­nius is over­shad­owed by three huge and very fa­mous fea­tures di­rectly to its west: Ptol­maeus, Alphon­sus and Arzachel. Linked to­gether, and a strik­ing sight in both a small pair of binoc­u­lars and a large tele­scope, th­ese three craters are very pop­u­lar ob­serv­ing tar­gets, which is why poor Al­bateg­nius, just to their east, is usu­ally over­looked. It’s a shame, be­cause it is a fas­ci­nat­ing and re­ward­ing fea­ture.

Hav­ing said that, not ev­ery­one has ig­nored Al­bateg­nius. In 1610

Galileo ob­served it through his first tele­scopes and was so im­pressed by its ap­pear­ance that he drew it, in­clud­ing it on his fa­mous sketches of the Moon. In mod­ern times Al­bateg­nius has been ob­served and pho­tographed in rather greater de­tail by many lu­nar probes, and in 1972 the crew of Apollo 16 took some beau­ti­ful im­ages of it as they or­bited the Moon on the fifth and penul­ti­mate Apollo mis­sion to land on the sur­face of our planet’s fas­ci­nat­ing nat­u­ral satel­lite.

Al­though Al­bateg­nius looks like a large crater at first glance it is ac­tu­ally classed as a ‘walled plain’, so it is more like a small sea sur­rounded by high walls than a sim­ple crater. It is ap­prox­i­mately 130 kilo­me­tres (81 miles) across, sur­rounded by jagged walls that tower more than four kilo­me­tres (2.5 miles) above the lu­nar sur­face, and has lots of smaller craters spat­tered across its deep floor, around 40 of them.

To the north of the crater floor a tightly clus­tered trio of th­ese craters runs from west to east, it­self pre­sent­ing a very in­ter­est­ing sight through a small tele­scope. Al­bateg­nius also has an­other ma­jor crater in­side its walls. Look down to the south­west and you’ll see the crater Klein, a 43-kilo­me­tre (27-mile) steep-walled pit.

In com­mon with many large craters, Al­bateg­nius has a cen­tral peak, a moun­tain that stabs up from its floor. Its sum­mit is over 1.5 kilo­me­tres (0.9 miles) above the floor and topped with a small crater of its own. Some ob­servers think the moun­tain range has the shape of a ghost or an an­gel – you’ll need to look at it through your own tele­scope to de­cide if you agree…

Al­bateg­nius’ walls are, like those of most large craters on the Moon, very com­pli­cated fea­tures in their own right, with mul­ti­ple ter­races and ledges break­ing them up in ev­ery di­rec­tion and criss-crossed and cut into by valleys and gorges here and there.

So, when can you see this in­trigu­ing if over­looked fea­ture for your­self?

As our ob­serv­ing pe­riod opens Al­bateg­nius is in­vis­i­ble, still deep in shadow. It doesn’t emerge from the dark­ness un­til 16 Septem­ber when the ter­mi­na­tor sweeps over it, sur­ren­der­ing it to the sun­light. A day later the plain will be fully vis­i­ble, its high walls stand­ing out starkly against the sur­face with the Sun’s rays strik­ing them at a low an­gle. By the time the Moon is full on 24 Septem­ber, with the Sun blaz­ing di­rectly over­head, the plain will have been re­duced to a mere dark patch. As the days pass and the ter­mi­na­tor creeps back to­wards it from the east Al­bateg­nius will be­come more and more prom­i­nent again, un­til it is swal­lowed up by the dark­ness on 2 Oc­to­ber and is lost from our view.

Why not take a look at Al­bateg­nius this month? True, there are larger and more dra­matic fea­tures around it, but if you can drag your eyes away from those and dare to stray from the well-worn path you usu­ally fol­low across the Moon, you’ll find Al­bateg­nius a very re­ward­ing ‘off the beaten track’ des­ti­na­tion.

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