Thursday, August 12, 2010

Tomorrow morning, I'll be heading away from the bright lights of London towards the not-so-bright lights of my home town in the North West of England. By complete chance, my trip away from the city coincides with a yearly event that requires a clear sky and as little light pollution as possible to be fully appreciated. Other than finding the right conditions, all you need to do to witness this event is look up.




The Perseid meteor shower was first observed two thousand years ago, and is visible every year from around the middle July to the end of August. At the peak of the shower, there should be 60 or so shooting stars every hour - meaning anyone looking to the sky can expect to see around one a minute, depending on location and a few other factors that can affect visibility. This year, the peak of the shower is tonight at around 0100 GMT.

The meteor shower originates from the comet Swift-Tuttle, which was discovered in 1862 and has a solid nucleus that's nearly 17 miles across. Unusually, the comet is locked into an orbital resonance with Jupiter, meaning that for every 11 times Jupiter completes an orbit of the Sun, Swift-Tuttle will go round only once. It was last seen in 1992, but we see its debris every year in the form of the Perseids.

Small particles in the comet's tail spread out along its whole orbit, forming something known as a meteoroid* stream. When the Earth passes through this stream we get a meteor shower. As the particles enter the atmosphere, they travel extremely fast (around 20km/s) causing the air in front of them to compress. The compressed air heats up, and both it and the meteor can reach temperatures of just over 1500C. At temperatures this high, the meteor doesn't last long - it burns up in the atmosphere creating shooting stars that we can see. Technically, the fast streak of light we see is called the meteor's trail, and the remnant after the trail has passed is known as the train.

All of the meteors in a shower appear to come from the same point in the sky, a spot called the radiant. This happens because all the meteors are travelling parallel to each other (the same effect causes train tracks to appear to converge in the distance). The meteors in the Perseid shower all appear to be coming from the direction of the constellation Perseus, and this is how the shower got its name.

The best time to see the Perseids, and all other meteor showers, is in the last few hours before the Sun comes up in the morning. As the Earth rotates, the side turning towards the Sun is able to catch more meteoroids, upping the number of meteors in the sky.

This year astronomers are expecting a more spectacular light show than usual. The peak of the shower is coming only two days after a new moon, so there will only be a little moonlight around to spoil the view. Even in urban areas the number of meteors visible per hour could reach between 10 and 20.

Wherever you are, don't forget to look up.



* Before entering our atmosphere, the particles are known as meteoroids. When they're travelling through the atmosphere they are meteors, and if one managed to make it to the ground intact it would be called a meteorite (a Perseid meteor is very unlikely to reach Earth, as the biggest ones are only around the size of a pea).


For details on how best to see the Perseids where you are, take a look here and here.


Post title stolen from Patrick Wolf.