Pleasant temperatures, family time, and the annual Perseid meteor shower come together to make this week an astronomical highlight of the year.
The Perseid meteor shower is like no other. Every August it delivers up to 100 meteors an hour in pleasant weather conducive to getting outside and staying up late. What's more, most kids still aren't in school, making it possible for the entire family to enjoy the event. The shower's unique combination of richness and accessibility makes it the one best known to the public.
Nevertheless, this year's Perseids may put skywatchers in something of a pickle. The shower peaks on Tuesday night August 11–12 with the maximum occurring between 2–4 a.m. on the 12th. But just as things start heating up, the last-quarter Moon makes an appearance, rising around midnight. While not a shower-killer, it will filter out the fainter Perseids and lower meteor counts.
One hundred meteors an hour is an idealized maximum from a dark sky on a moonless night with the radiant overhead. But given that light pollution is a fact for many of us, a more reasonable expectation would be 30–50 meteors an hour. The number of meteors you'll see depends a lot on the altitude of the radiant, the point in the sky from which these javelins of light appear to emanate. At nightfall the Perseid radiant hunkers low in the northeastern sky. Early evening activity is low because the horizon cuts off most of the meteors that flare below the radiant. But around midnight, when Perseus has climbed to around 30° altitude, meteor counts rise and continue climbing until dawn.
If you're scratching your head wondering when it's best to sacrifice your precious sleep, consider these possibilities:
- Begin at nightfall. Although the radiant will be very low this is the best time to catch sight of Perseid earthgrazers — long, slow meteors that graze the atmosphere at a glancing angle like a stone skipping across a pond.
- Start around 11 p.m. and watch until 12:30 a.m. (a little past moonrise) to see a modest number of meteors in a dark sky.
- Catch the peak from 2–4 a.m., trusting that lunar glare won't compromise the shower overmuch.
No matter which option you choose (maybe you're lucky enough to consider all three!), observe from as dark a location as possible while settled in a comfortable reclining chair. There's no need to face the radiant directly as meteors will appear all over the sky. I like to keep Perseus off to one side, facing either north or east, the better to see a mix of short- and long-trailed meteors.
Each sudden flash of a meteor will come as a pleasant surprise. Sometimes I'll set a goal of spotting 25 or 30 Perseids, but I always stay longer and see more because I love the sense of anticipation and the guarantee of a prize. Makes me feel like a kid again.
Should clouds interfere during the shower's peak, don't despair: It will remain active at roughly half-strength from August 10–14. You can also watch the shower live online on at Gianluca Masi's Virtual Telescope Project site on August 11th starting at 22:00 UT (6 p.m. Eastern Daylight Time). See also feeds from the Instituto de Astrofísica de Canarias, which contain comments and infographics (in Spanish) or only music, beginning August 12th around 7:15 p.m.
What is a meteor anyway?
Perseids originate from Comet 109P/Swift-Tuttle, discovered in 1862 by American astronomers Lewis Swift and Horace P. Tuttle. The comet takes 133 years to orbit the Sun and made its most recent pass by Earth in 1992. Right now, it's extremely distant — more than 3.7 billion miles (6.0 billion km) away, currently far beyond Pluto — but will return to the inner solar system in 2126 as a bright, naked-eye object.
Despite its current remoteness, each time Swift-Tuttle returns to the inner solar system, solar heating sublimates some of its dirty ice. Particles locked in that ice, ranging from sand-sized grains up to mini-marshmallows, are released and deposited as strands of debris along the comet's orbit.
Each August, Earth crosses the comet's path and hurls headlong through the flotsam and jetsam the same way you'd drive into a sudden snowstorm on the freeway. But instead of snowflakes striking a glass windshield and peeling off to the sides, cometary grit slams into the planet's atmosphere at more than 215,000 kilometers an hour and vaporizes in the heat of friction some 100 kilometers overhead.
The intense heat generated by the particle's passage ionizes air molecules along its path. When those ions recombine they release energy in a brief burst of light. This light combined with that of the vaporizing meteoroid create the shooting-star effect. In reality a meteor is a narrow a column of light less than a meter across but tens of kilometers long.
The amount of dust and rock fragments sloughed off by Comet Swift-Tuttle varies with each revolution to create debris strands of different density and width that account for variations in the strength of the shower over time.
Future and past
Because the comet's perihelion lies just inside Earth's orbit it makes frequent close approaches to our planet. That and its large size inspired radio astronomer Gerrit Verschuur to describe it as "the single most dangerous object known to humanity" in his 1997 book Impact!: The Threat of Comets & Asteroids. Good thing we get to enjoy it safely — one grain at a time.
Astronomers were unaware of the Perseid shower's existence until the mid-1830s when two American scientists — Edward Herrick and John Locke — along with the Belgian statistician Adolphe Quetelet noticed an unusual number of meteors on the night of August 9–10. Their subsequent research confirmed the existence of the shower with the radiant in Perseus.
But that wasn't the end of the story. In their studies Herrick and Quetelet chanced upon a reference in an English farmer's almanac describing how the "burning tears of St. Lawrence" appeared in the night sky every August 10th. For ages, Catholics in Germany and England had been commemorating the date of his martyrdom on August 10, 258, when Lawrence was apparently burned in a gridiron over hot coals. What had been dismissed as a folk tale turned out to be evidence for the Perseid meteor shower's ancient roots. For the full story click here.
As a personal aside, my mother passed away two years ago on August 12th during the Perseid peak. Each year when I see the "tears of Lorraine" streak across the sky I sense her presence and remember her goodness.