This post in an updated version of one that appeared last year on the previous incarnation of this blog.

Once again, by chance, I'm away from the bright lights of the city during a yearly event that is best witnessed in an area with little light pollution and a clear sky. Unfortunately, this year, the moon will make viewing conditions less favourable, though not completely hopeless.

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 can 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 other factors that affect visibility. This year, the peak of the shower is early in the morning on 13th August. Unfortunately, the 13th August is also the date of the next full moon, and its light could drown out all but the brightest meteors.

Perseid meteors originate from the comet Swift-Tuttle, which was discovered in 1862 and has a solid nucleus that’s nearly 17 miles across. 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 meteoroid1 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 1500ºC. 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. To find Perseus in the sky, it may be easier to first look for Cassiopeia — a W shape — which is much brighter.

The best time to see the Perseids 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.

Last year the Perseids put on a better show than usual. The peak of the shower came only two days after a new moon, so there was only be a little moonlight around to spoil the view. But this year the conditions are less favourable, with the peak of the shower coinciding with a full moon. It may actually be better to go out before the peak, if you can find a time after the moon sets but before the sun rises, to reduce the amount of natural light pollution that will obscure your view. (This website might help you find such a time). Although, according to the Met Office, Saturday may have the clearest skies if you're in England.

Many Perseids have already been spotted. The International Meteor Organisation is recording the rates of meteors seen. The maximum their volunteer astronomers have recorded so far is 20 per hour, and this is expected to increase to at least 40 over the next few days. This is the number of shooting stars that you would see under a very dark sky with the Perseus constellation at its highest point, but even in a town or city, with less favourable conditions, a few an hour should be visible.

Wherever you are in the next few days, if you get the chance, don't forget to look up.

  1. 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).

Post title courtesy of Patrick Wolf.