Astro Bob: Best times to watch this week's Perseid meteor shower
Everyone's favorite meteor shower peaks on Friday night, Aug. 12-13.
DULUTH — I'm feeling optimistic and predict that you'll see a meteor sometime this week. It shouldn't take too much effort because we're approaching the peak of the annual Perseid meteor shower, the richest of the year after December's Geminids. Under a dark moonless sky, observers can see upwards of 80 meteors per hour in the early morning hours before dawn. From more light-polluted locales, about half that many.
Every year, the Earth slices across the stream of dust and debris sloughed from Comet 109P/Swift-Tuttle. Each time the 16-mile-wide (26 km) hulk of dirty ice passes through the inner solar system — once every 133 years — the sun vaporizes a portion of its dust-laden ice. The pressure of sunlight pushes those bits and pieces away from the comet to form the tail, which deposits the material along the its orbit like Hansel dropping breadcrumbs in the fairy tale Hansel and Gretel.
Ramming it home
We first encounter those breadcrumbs in mid-July, pass through the stream's core on Aug. 12-13 when meteor numbers peak, and exit the trail at month's end. The combined speed of Earth's orbital motion (18.5 miles a second) and the Perseid stream increase the velocity of the incoming particles to a zippy 37 miles (59 km) per second.
When they strike the upper atmosphere, each sand-grain to apple-seed-sized nugget begin to glow both from friction and ram pressure. Ram pressure occurs when a speedy object compresses the air in front of it. Compressed air heats up and cooks the material to temperatures around 3,000° F (1650° C), hot enough to strip it apart atom by atom. These atoms give off characteristic colors that we can use to identify what the grit's composition — typically iron, magnesium, silicon, calcium and other elements.
The extreme heat also ionizes the air around the speeding particle, creating the moving streak of light we call a meteor or "shooting star." Perseid meteors appear to radiate from a point in the constellation Perseus a short distance below the W of Cassiopeia. You can trace the path of any shower member back to the radiant, whereas random meteors called sporadics originate from anywhere in the sky.
The radiant is an example of a vanishing point, where mutually parallel lines (think railroad tracks) converge at a point in the distance. Just like those tracks, Perseid meteors arrive on parallel paths and only appear to merge in the distance in the direction of the constellation Perseus.
Hopefully, I've whet your appetite to go out and watch the shower. Here's the bad news. The moon will be full during the best viewing time. Moonlight brightens the sky, dimming the stars and wiping out the fainter meteors. Instead of 40 per hour from a moderately light-polluted site, you might see 20 instead. Sorry about that. But don't let the moon steal your joy. Getting out there will be worth your while. Here's a few reasons why:
- Perseids display more fireballs than any other shower, beating out the Geminids 568 to 426 in the five-year period from 2008 to 2013. Fireballs are meteors as bright as Venus or better and are unaffected by moonlight.
- Perseids are prolific. The moon can put a dent in their numbers but can't close the curtain on the show.
- Surprises can happen. Just last year, an unexpected outburst of Perseids occurred over North America on August 14, the morning after the peak. I happened to be out, and it was truly a wow experience.
Best times to watch
Option #1: The peak occurs Friday night / Saturday morning Aug. 12-13, when the moon will be out all night. You can start watching as early as 10 p.m. local time. Activity will be modest because the radiant is still low in the sky, and many of the meteors that shoot from it will be cut off by the horizon. The later you stay up the more flashes you'll see, peaking between about 1 and 4:30 a.m. local time.
Option #2: If the weather forecast looks poor for Friday night, plan to observe the previous night, Aug. 11-12. Activity will still be high, plus the full moon and bright Saturn will pair up in conjunction for a pretty sight.
Option #3: We'll have about one hour of precious darkness on Wednesday morning, Aug. 10 from moonset (around 3:30 a.m. local time) until the start of dawn. While that's two days before the peak, activity should be very good with dark skies to boot. This will also be an ideal time to try and capture meteor trails with your camera.
Attach it to a tripod, set the lens f/stop to let in the maximum amount of light (f/2.8, f/3.5, f/4), and expose for 30 seconds at ISO 1600. Before you take your first picture carefully focus on a bright star using the camera's live view feature. Once focused you're set for the night.
To soften the moon's blow and see the maximum possible number of meteors possible, turn your reclining chair to face north so you can keep the moon at your back with the radiant off to the right. That way you'll preserve your night vision. Perseids fly all over the sky, so no matter what direction you face you'll see meteors.
Good luck and feel free to share your views either by email at firstname.lastname@example.org on my Facebook page . Clear skies!