Australian Geographic - - Contents -

The Moon’s com­pan­ions

HAVEYOU EVER HEARD of the Lagrange Points? If you’re a reg­u­lar reader of Aus­tralian Geo­graphic, you may have (see AG 91). But it’s usu­ally only as­tronomers and math­e­ma­ti­cians who are fa­mil­iar with Joseph-Louis Lagrange (1736–1813) and the imag­i­nary dots in space that bear his name.

The Lagrange Points are five po­si­tions around an or­bit­ing body where grav­ity and cen­trifu­gal ac­cel­er­a­tion can­cel each other out to leave no grav­i­ta­tional pull. As­tronomers num­ber them L1–5.T wo of them, L4 and L5, are of par­tic­u­lar in­ter­est. They lie in the same path as the or­bit­ing body, but are al­ways re­spec­tively 60 de­grees ahead of and 60 de­grees be­hind it. And they’re sta­ble in the sense that smaller ob­jects can or­bit them. Wait a minute: small ob­jects or­bit­ing… imag­i­nary points? Yes, that’s right.

Per­haps the best way to visu­alise this is to think of the planet Jupiter or­bit­ing the Sun, the L4 and L5 points of which track along Jupiter’s or­bit, ahead of and be­hind the planet. Each has as­ter­oids or­bit­ing it, known as Tro­jan a steroids. To­gether, they num­ber more than 7000.

Now, imag­ine the Earth and Moon. Are there any Tro­jan as­ter­oids in the Moon’s or­bit? The an­swer is no, but there is some­thing else. Back in 1961, as­tronomer Kaz­imierz Kordylewski claimed to have pho­tographed a cloud of in­ter­plan­e­tary dust near the Moon’s trail­ing Lagrange Point – good old L5.

While Kordylewski’s pho­to­graphs were dif­fi­cult to ver­ify, his name be­came syn­ony­mous with the Moon’s Tro­jan dust clouds. And now, he’s been vindi­cated by mod­ern im­agery from Hun­gar­ian as­tronomers .They have con­vinc­ingly pho­tographed the faint L5 Kordylewski Dust Cloud and are al­ready set­ting their sights on its coun­ter­part ahead of the Moon.

If noth­ing else, it’s great to see the dust be­ing blown off old ideas.

FRED WAT­SON is Aus­tralia’s As­tronomer-at-Large.

The Lagrange Points as­so­ci­ated with the Sun–Earth sys­tem.

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