NIGHT WATCH

A STREAK IN THE SKY

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.