NIGHT WATCH

A STREAK IN THE SKY

go! - - In Brief Upfront -

sk any­one to name a comet and they’ll most likely say Hal­ley’s. It’s by far the most fa­mous one, largely be­cause it puts on some of the most im­pres­sive dis­plays and also be­cause it’s the comet that made Sir Ed­mond Hal­ley re­alise that comets fol­low an el­lip­ti­cal or­bit around the sun. Hal­ley’s Comet takes 75 years to com­plete an or­bit, but oth­ers make the jour­ney in a much shorter time. Chances are good that we’ll get to see one such comet dur­ing Septem­ber: Comet 21P/Gi­a­cobiniZin­ner, which takes 6,5 years to or­bit the sun. French as­tronomer Michael Gi­a­cobini dis­cov­ered the comet in De­cem­ber 1900 and cal­cu­lated its or­bital pe­riod at 6,8 years. When, 6,5 years later, his fel­low as­tronomer Ernst Zin­ner from Ger­many dis­cov­ered a “new” comet, peo­ple at first thought it was a dif­fer­ent one. It wasn’t, hence the name. It’s very dif­fi­cult to pre­dict how bright a comet will be, but all signs in­di­cate that Gi­a­cobini-Zin­ner might be vis­i­ble to the naked eye on 10 Septem­ber when it’s clos­est to earth. And even if it’s not very bright, you should be able to find it with binoc­u­lars. When a comet passes close to the sun, it warms and be­gins to re­lease gases that pro­duce a vis­i­ble at­mos­phere and some­times a tail. Use your binoc­u­lars to see the puffed-up “body” of Gi­a­cobini-Zin­ner; it also showed a short tail when it made its pre­vi­ous ap­pear­ance in 2012. The il­lus­tra­tion shows the comet’s or­bital path through the sky dur­ing Septem­ber so you’d know where to look. You’ll have to set your alarm clock: It will only be vis­i­ble shortly be­fore sun­rise, when it rises in the east.

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