Time for the Perseids, the Year's Most Enjoyable Meteor Shower — Peaks Aug. 11-12
I really enjoy the Perseids. Nice weather makes them easy to watch, and there are always enough meteors to go around. While the Geminids of mid-December boast higher hourly numbers of you have to deal with Jack Frost nipping at your nose...
I really enjoy the Perseids. Nice weather makes them easy to watch, and there are always enough meteors to go around. While the Geminids of mid-December boast higher hourly numbers of you have to deal with Jack Frost nipping at your nose and toes. The Perseids are active between July 17 and August 26 but peak on the night of August 11-12when up to 100 meteors per hour might be seen — under ideal conditions from a dark-sky location.
Because light-pollution is a fact of life for many of us, and the moon will interfere a bit, you're more likely to see about 30-40 per hour. In my years of Perseid-watching I've never done better than that. Nor have I ever walked away from a Perseid shower with anything but gratitude.
Perseids get their name from Perseus. You can track all shower members back to a point in the sky called the radiant, which is located in Perseus the hero just below the W of Cassiopeia in the northeastern sky. Any meteor that points back in a different direction is a random or sporadicmeteor.
Perseids originate from Comet 109P/Swift-Tuttle, discovered in 1862 by American astronomers Lewis Swift and Horace 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 (5.9 billion km) away, far beyond Pluto — but will return to the inner solar system in 2126. That year the comet will become a bright naked-eye object much like our friend NEOWISE this summer.
Despite its current remoteness, each time Swift-Tuttle returns to the inner solar system, solar heating vaporizes 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 Swift-Tuttle's orbit.
Every mid-August, the Earth plows through the trail of comet crud and the particles (called meteoroids) slam into the atmosphere at around 135,000 miles per hour (216,000 kph). The friction created when they rip through the air at that speed produces a brief but intense heat that vaporizes the grain and excites air molecules to glow. When we see a meteor, part of the light comes from the heated particle but much of it from the glowing air. The streak itself is a column of light less than three feet (1-meter) across but tens of miles (km) in length.
Nearly all comets leave dust trails, but we don't get a meteor shower unless the comet's orbit and that of the Earth cross at some point. Because Comet NEOWISE crosses the plane of the solar system (defined by Earth's orbit around the sun) inside the orbit of Mercury any meteoroids it produces will never get close enough to Earth to produce a meteor shower.
Let's return to the radiant for a moment. It seems like some sort of magical place in the sky, but it's just a beguiling perspective effect identical to that seen in a set of railroad tracks. Stand in the middle of the tracks (thought experiment only — not recommended in real life) and the rails run to your right and left sides. Now look into the distance and they'll appear to converge and "touch" at their vanishing point. We know it's impossible for parallel rails to touch but it sure looks that way.
When Earth moves through Swift-Tuttle's flotsam and jetsam, meteors appear to converge at the radiant exactly the same way two rails "meet" in the distance. In reality, the meteoroids and the meteors they produce arrive at the Earth on parallel paths like rails or runners in separate lanes in a 100-meter dash. Kind of amazing to think about. Because of the convergence effect meteors near the radiant (vanishing point) are short — they're coming almost straight at you — while those off the side of the radiant have long trails.
How and when to observe
Find the darkest sky available and relax in a reclining chair tilted so that you're facing about halfway up the sky. I usually point the chair east or a little southeast with the radiant off to one side. That way I get a mix of short- and long-trailed meteors. You can also face north for the same effect. As far as timing, if you go as soon as darkness falls (around 9:30-10 p.m.) the radiant will still be low in the northeastern sky. You'll see fewer Perseids because meteors shooting out below the radiant will be blocked from view by the horizon.
Still, you'll see some, and that's the most likely time young kids will get a chance to enjoy the shower ... unless you get them up in the middle of the night. The later you observe the more meteors you'll see with the peak occurring on Wednesday morning from 2-4 a.m. when the radiant stands highest.
But there's a slight problem — the moon. It comes up around midnight just shy of half and will brighten the sky just enough to blank the fainter meteors from view. Depending on your circumstances, pick a favorite interval. Right now, I'm thinking of watching from 11 till 1 a.m. before the moon grows too bright. If bad weather gets in the way, the shower will still be at half-strength from August 10-13. You can also watch the show live online at Gianluca Masi's Virtual Telescope Project website starting at 22:00 UT (5 p.m. Central Time) August 11.
Take in the sky, the Milky Way, the bright planets and enjoy every meteor you see. Sometimes I'll set a goal of say, 25 sightings, 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.