Perseids Are Coming, Full Moon or Not

Skywatchers are fond of alignments but not those where a bright Moon coincides with the peak of a major meteor shower. In this year's roll of the dice the annual Perseid maximum falls just a day after the full Sturgeon Moon.

Despite the light bath, there's still hope for a good show for a couple of reasons. First, the shower is prolific — Perseids flash by at the rate of more than one a minute. Even if you reduce that by more than half due to moonlight, that's still around 25 per hour. Second, this famous annual shower is the undisputed king of fireballs, according to Bill Cooke at NASA's Meteoroid Environment Office. The American Meteor Society (AMS) defines a fireball as a meteor "generally brighter than magnitude -4" or the equal of Venus or brighter.

Great balls of fire

Cooke and team monitored the shower between 2008 and 2013 and recorded 568 fireballs compared to the second-place Geminids with 426. The greater percentage may have to do with the considerable size of the Perseids' parent body, Comet 109P/Swift-Tuttle, which has a diameter of around 26 kilometers. Bigger comets like 109P release more material when baked by the Sun, resulting in a greater number of larger pieces. And larger meteoroids typically produce brighter flashes when they enter the atmosphere.

"The average peak magnitude for a Perseid observed by our cameras is –2.7; for the Geminids, it is –2,"explains Bill Cooke. "So on average, Geminid fireballs are about a magnitude fainter than those in the Perseids." Most Perseids are the size of sand grains with a few "chocolate chips" thrown in for good measure. In a 2004 paper that appeared in the Monthly Notices of the Royal Astronomical Society, a team of Canadian astronomers estimated that it only takes a comet shard about 30 mm (1.2 inches) across to create a –6 magnitude fireball.

Comet 109P/Swift-Tuttle makes a pass through the Sun's hot kitchen every 133 years. Each time, it loses a fraction of its mass as dust and rocks that fans out into a broad stream of debris. Earth makes contact with the stream's fringes in mid-July and doesn't exit until the end of August. In fact, during the recent Southern Delta Aquariid shower, which peaked on July 30th, I counted more Perseids (five) than Delta Aquariids (three) in an hour, proving that Earth was already knee-deep in the flow nearly two weeks before maximum.

Viewing opportunities

The real action happens when we speed through the stream's core which occurs on August 13th around 1:00 UT (August 12th, 9 p.m. EDT). For North American observers that puts the peak of the shower on Friday night August 12–13. If the weather forecast looks grim, the previous evening (August 11–12) should be nearly as good. Alternatively, you can avoid moonlight altogether for one precious hour on Wednesday morning August 10th beginning around moonset at 3:30 a.m. local time until the first blush of dawn.

That brief window of darkness will also be an ideal time for meteor photography. I like shooting 30-second exposures wide open at f/2.8 with a wide-angle lens (16- to 20-mm) at ISO 1600 or 2500. With the full Moon out, photos are still possible, but you'll need to reduce exposure time and lower your ISO, necessarily capturing fewer "shooting stars."

Perseid streaming service

It's tempting to visualize the comet's debris trail as a Saharan sand storm. But the 2004 paper cites an earlier study that determined that there are just 90 ± 16 meteoroids per 1 billion cubic kilometers of space capable of producing meteors brighter than magnitude 6.5. For reference it takes our planet about 15 minutes to sweep across a volume of that size.

The combined motion of the Perseid stream and Earth's orbital hurry keep the meager supply of visible meteoroids coming and help us to form a mental image of the space we must cover to see even a single Perseid, let alone 20 or 30. Play with the interactive visualization of the shower (above, by Peter Jenniskens and Ian Webster) to get a real, 3D-feel for the event we're about to witness.

The emissaries of Swift-Tuttle may only be as big as sand grains or at best one of those shredded wheat pillows, but they're traveling at more than 200,000 kilometers per hour when they encounter Earth's atmosphere.

Ramming home the point

Ram pressure — the rapid compression of air in front of the speeding particle —quickly heats it to more than 1650°C (3000° F), much like the heat shield of a spacecraft reentering the atmosphere. That's hot enough to ablate the incoming meteoroid, ionizing its atoms as well as the air molecules along its path. This creates a dense, glowing channel of plasma many times the size of the original particle. In a word — a meteor. I find it nothing short of astonishing to witness the transformation of an insignificant crumb into a blaze of light that makes you want to raise a fist and shout "YES!"

You can begin watching for Perseids as early as 9:30–10 p.m. local time, when the radiant has barely risen in the northeastern sky. That's the ideal time to spot meteors arriving nearly tangent to the upper atmosphere called earthgrazers. Skittering through the thin air like stones skipping on water, they travel long distances across the sky and sometimes glow for many seconds. The radiant itself is just an optical illusion, a vanishing point like parallel railroad tracks or crepuscular rays that appear to converge in the distance. Like those tracks and rays, Perseids enter on separate parallel paths as Earth lunges into Swift-Tuttle's detritus.

Citizen science

The higher the radiant climbs the more meteors you'll see, with the best hours from about 2–4:30 a.m. No matter the hour, invite a friend or family members and set up somewhere with a wide-open view of the sky. Sit halfway up in a reclining chair for the best view. Perseids will appear anywhere in the sky, but given torrential moonlight, your best bet is to face north with the Moon at your back and the radiant off to your right. Perseids are distinguished from random or sporadic meteors because they point back to the radiant in Perseus.

If you'd like to record your meteor observations for personal use, download this blank form from the AMS. You can also contribute that data to the International Meteor Organization (IMO), which partners with the AMS, by first registering on the IMO website (it's free). Each meteor you see provides a nugget of useful scientific data to improve our understanding of the year's most popular shower.

If you're utterly clouded out or otherwise unable to see the shower in person, astronomer Gianluca Masi will live stream the Perseids on August 10th starting at 01:00 UT (9 p.m. EDT, August 9th) on his Virtual Telescope site. Also, the Spanish language Sky-live.tv will broadcast the Perseids live from the island of Madeira and an observatory in the Canary Islands on August 13th starting at 01:00 UT (9 p.m. EDT, August 12th).

I share your hope for clear skies — enjoy the show!

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