Plan for optimal conditions for this year’s Perseid meteor shower early in the mornings of August 12th and 13th.
“Stars fell like weaving in the south, unceasingly through the night.” So a city gazetteer printed in Shanxi, China, described the sky above Fenyang on August 10, 1862. Calculating backward, scholars have determined that the “weaving stars” witnessed by the townspeople were in fact Perseid meteors, falling at a time when the shower’s radiant, the point from which the meteors appear to emanate, lay low in the sky.
The Perseids are associated with the short-period Halley-type comet 109P/Swift-Tuttle, which was independently discovered by American astronomers Lewis Swift and Horace Tuttle as it approached perihelion in July 1862. When Earth crosses Swift-Tuttle’s orbit, bits of dust and rocks left behind by the comet hit the planet’s atmosphere, creating the light show we know as the Perseid meteor shower. In years when Swift-Tuttle reaches perihelion, the number of visible meteors significantly increases, which explains the dramatic display of August 1862. Since the comet has a 133-year orbital period and last visited the inner solar system in 1992, we’ve got another 107 years to go before that possibility arises again. Occasional bump-ups in the number of detected meteors do occur when Earth passes close to dust trails left behind during even earlier visits. However, the next such major outburst isn’t predicted until 2028. Even so, the “ordinary” Perseids make for a strong shower and an impressive spectacle. Under ideal, dark skies, viewers may see more than 50 meteors in an hour.
The extant Chinese records for the 1862 event all come from northern China, suggesting unfavorable weather in the south that year. Here and now in 2018, we hope for good weather for the summer’s greatest shower, because viewing conditions should otherwise be perfect. The shower’s predicted peak falls on the evening of August 12th, soon after new Moon (9:58 UT August 11th). Weather and light pollution (and possibly mosquitoes) should be the only impediments to a good show. The best viewing will certainly be early on the mornings of August 12th and 13th, but don’t wait for the predicted peak to go outside. Perseids begin streaking across the sky in mid-July, when the radiant is still in Cassiopeia, and the odd shower meteor will continue to be visible until around August 24th.
The easiest way to view a meteor shower is to kick back with a lawn chair and a sleeping bag. All you really need to do to see the show is look up, stay warm, and stay awake. Find the darkest sky you can, away from street lights. For most viewers in the Northern Hemisphere, shower meteors can appear any time after evening twilight ends, since the radiant in Perseus is circumpolar (i.e., up all night). So start looking as soon as evening twilight ends. These early evening space streaks will be long, showy “Earthgrazers” that skim Earth’s upper atmosphere. As the radiant moves higher in the northeast throughout the evening, the meteors will become more frequent and appear all over the sky. The later you observe, the more meteors you’ll see, and the best hours fall between midnight and dawn.
The most suitable equipment for watching a meteor shower are desire and dark skies, but if you’d like to document your observations more formally, the International Meteor Organization (IMO) recommends that visual observers track sky activity for at least 1 hour, with reports broken into short intervals of no longer than 15 minutes. For more detailed instructions regarding recording and reporting, visit the IMO Visual Observations page. Earth may encounter a Perseid filament — a relatively young accumulation of meteoroids — on August 12th around 20h UT. There’s also a possibility of our planet meeting an old Swift-Tuttle dust trail on August 13th around 1h 27m UT. Accurate observation records may help detect activity connected to filament and meteoroid clump.
This article originally appeared in print in Sky & Telescope's August 2018 issue.