August 10, 2015, by Lindsay Brooke

The Perseid meteor shower – for the deckchair (or hot-tub) astronomer

Michael Merrifield, Professor of Astronomy in the School of Physics and Astronomy hands out some top tips on how to make the most of this summer’s Perseid meteor shower.

This month sees my absolute favourite annual astronomical event, the Perseid meteor shower.  There are around a dozen predictable meteor showers every year, but this one is definitely the winner because it produces nice bright shooting stars pretty much every year, and because it happens in the middle of August so staying outside for the half hour necessary to become fully adapted to the dark is less likely to cause hypothermia than, say, the Geminid shower in December.

The other nice thing about meteor showers as astronomical phenomena is that the equipment required to observe them is pretty minimal: no telescopes or even binoculars; ideally just a deck chair and a duvet.  The show goes on for several weeks, so even if you miss the absolute peak of the shower, you should still be able to pick up a good number of shooting stars.  Head out as late into the night as you can bear to somewhere with a clear, dark northern horizon (so usually north of the town or city in which you live), set up your deckchair facing north, and settle in under the duvet to wait.  Actually, deckchair and duvet are optional, and there are some appealing alternatives – my favourite ever Perseid watching was from a hot tub in Cornwall, although that did create the interesting additional challenge of trying to stop my glasses from steaming up for long enough to watch the show.

Near the peak of the Perseids, you should see more than one bright shooting star per minute flying across the sky once your eyes are adapted.  The shooting stars associated with the Perseid shower all seem to emanate from a single point in the sky, the “radiant point,” which lies in the constellation of Perseus (hence the name).  Perseus is just north of the easier-to-find constellation of Cassiopeia, which is shaped like a W in the northern sky, but, to be honest, it doesn’t matter that much as the shooting stars fly out in all directions right across the sky, so you don’t need to be staring too hard in any specific direction to see them.

The reason for the single radiant point is that the shooting stars aren’t so much flying into the atmosphere, but more that we are flying into them.  Meteor showers occur when the Earth ploughs through the debris left behind in the wake of a comet, in this case one called Comet Swift-Tuttle (after its 19th Century discoverers).  At pretty much the same time every year, the Earth crosses the orbit of this comet, and collides with the tiny particles of dirt it has left behind; as these particles burn up on entering the atmosphere, they create the firework display of the meteor shower.  And, just like bugs splatting on the windscreen of your car, these collisions occur primarily in the direction in which the Earth is moving relative to the comet debris, which happens to lie in the constellation of Perseus.

For observing the Perseids this year, there is both good news and bad news: the good news is that the peak, on the night of August 12-13, occurs very close to new Moon, so the sky will be suitably dark (although Perseids can be bright enough that this isn’t really an issue – I have sometimes seen them from my back garden in the middle of Nottingham over all the city lights); the bad news is that the Earth will pass through the densest part of the Swift-Tuttle debris trail, when the peak of activity should occur, after dawn on the 13th here in the UK, when we won’t be able to see them.  So, your best bet is the pre-dawn hours of the 13th, although there should still be plenty around for a few nights either side of the maximum.  Get out and enjoy a free firework display, but don’t forget your duvet (or your hot tub).

Posted in Science