To whet your appetite for the upcoming eclipse, nature has an opening act performing this weekend. It’s the annual Perseid meteor shower, a pleasant “rain” of fiery flashes coming to a sky near you. Each year in mid-August, Earth zips through the stream of grit and ice boiled off the nucleus of comet 109P/Swift-Tuttle during its once-every-133-year-journey around the sun. Particles that range in size from sand grains to Grape-Nuts and even a few pebble-sized pieces slam into the atmosphere at speeds in excess of 133,000 mph (59 km/sec) and vaporize in a flash of light.
While still out in space, clueless of the approaching Earth, the cometary dribs and drabs are called meteoroids. When they strike the atmosphere, they’re meteors, and if by some rare chance one is big enough to survive the plunge and land on the ground, it’s becomes meteorite. Since meteor shower material is almost always small and crumbly, not a single meteor shower meteoroid has ever been recovered as a meteorite.
I love the Perseids. They’re so easy to watch. August evening temperatures are comfortable, and many of us are outside anyway at reunions, parties or just out for a walk to cool down. Like cosmic spitballs, they shoot earthward from a point in the sky called the radiant, located in the constellation Perseus, from which they get their name. The radiant mimics the effect of looking down a set of railroad tracks into the distance. When you’re standing between the tracks, they run parallel on either side of you, but if you look ahead, they appear to converge in the distance. The Perseids shoot through the sky parallel to one another just like the rails (multiple rails!) and likewise appear to converge in the distance at the radiant. It’s all a perspective effect. Have you ever seen sunbeams bursting through the clouds? Those beams are also parallel and centered on the “sun radiant.”
Earth enters the Swift-Tuttle’s stream in mid-July and departs it in September, but the best time for viewing is when we’re in the thick of it on the night of Aug. 12-13. That evening and into the pre-dawn, we might see up to 80 meteors an hour from a moonless, dark sky location. Ah, but there’s the rub! We won’t be moonless, and therefore the sky won’t be very dark.
Fear not. This should still be a pretty good shower. The 70% waning gibbous moon won’t rise till around 11 p.m. local time, so most of the sky will be dark or reasonably so until around 11:30 p.m. You can catch early, pre-peak Perseids, when the radiant is just coming up below the W of Cassiopeia in the northeastern sky. Alluded to earlier, Perseids can appear anywhere in the sky, but their trails all lead back to the radiant.
As the moon climbs higher and spreads it light, the radiant climbs, too and meteor counts increase through the night to a maximum around 3-4 a.m. Sunday morning the 13th. With the moon up, expect closer to 30 meteors per hour instead of 80. I’m not complaining.
I like to face east or south on a comfy reclining lawn chair for the Perseids. Bring a blanket in case it’s expected to cool down and a snack, too. No other equipment is needed — just your eyes. Friends are great to share the shower with. You’ll see many more with four eyes and keep each other from falling asleep. While watching the shower, try to avoid staring at the moon, so you can preserve your night vision.
Perseids are swift meteors and known for the occasional brilliant fireball as well as trains. Trains are trails that continue to glow for a second or two after the meteor has disappeared. The meteoroid’s rapid speed heats and strips air molecules of some of their electrons. When they regather and return to normal, light is released, helping to create both meteor’s flashy streak and the train.
Even though the peak’s Saturday night, you’ll see Perseids all weekend. Relax and enjoy!