This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Wherever you are this weekend, if you get the chance, don’t forget to look up.
The Perseid meteor shower was first seen two thousand years ago, and is visible every year from mid July to the end of August. At the peak of the shower, there can be up to 100 or so shooting stars every hour — more than one a minute. This year, the peak of the shower is between 10 and 12 August.
Perseid meteors originate from the comet Swift-Tuttle, which was discovered in 1862 and has a rocky core nearly 17 miles across. The comet is locked into an orbital resonance with Jupiter -- for every 11 times Jupiter completes an orbit of the sun, Swift-Tuttle goes 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.
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When the Earth passes through this stream we get a meteor shower. As the particles enter the atmosphere, they travel fast enough to make the air in front of them 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.
The fast streak of light we see is 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. That's 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 and just to the side.
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, increasing the number of meteors in the sky.
Phil Plait over at Bad Astronomy has a good guide with more meteor spotting tips. Check out the International Meteor Organisation for rates of meteors already seen this year.
Wherever you are this weekend, if you get the chance, don’t forget to look up.
This post in an updated version of one that appeared on this blog two years ago.