From the June 2015 issue

Mystery men of the Perseids

Fun facts about the most famous meteor shower.
By | Published: June 29, 2015
Meteor showers are like nothing else in life. On what other occasion do you wait around for something sudden to happen? And why do brief dim streaks make people shout?

This month, the most famous meteors, the Perseids, unfold under ideal dark conditions, the best in five years. A hair-thin crescent Moon won’t rise until morning twilight.

You know the rules. Get away from city lights to an unobstructed sky. Unlit school sports fields and hay meadows offer open sweeps of the heavens. Cemeteries are good if there aren’t trees. Don’t keep glancing at the people you’re with. Keep your eyes glued upward. Start watching around midnight on August 12/13. You’ve heard all this a million times.

So let’s explore a bit deeper. Start with the obvious: the best direction. Because the meteors radiate from Perseus, which rises in the northeast around 11 p.m. local daylight time, you’re usually told to face that direction.

That’s true early in the night when you mostly see meteors that zoom upward from the horizon. But at 1 a.m. the radiant has risen and “falling stars” streak not just upward but also to the left and right of the radiant. Fast forward to 3 a.m., and now plenty of meteors also streak downward from Perseus. There’s a lot more to see, but is it still best to face northeast?

Not at all. In that direction, you only see the ones coming straight at you. Those exhibit short streaks. If instead you face 90° from the radiant, like high in the south, you watch meteors that are zooming sideways to you. They look more dramatic because they fly across more of the sky.

Meteors are visible around 60 miles (100 kilometers) up, then disintegrate and vanish as the apple-seed-sized particles have slowed so much that they no longer heat the surrounding air to incandescence. Here’s a cool fact using those same numbers: The incoming Perseid meteoroids are 60 to 100 miles (100 to 160 km) apart! Obviously, even this rich shower isn’t exactly buckshot. Yet another reminder why it’s called “space.”


The Perseids are all debris from Comet 109P/Swift-Tuttle. Discovered during the Civil War, this comet has a 133-year orbit that brought it back around here in 1992. But who were those guys Swift and Tuttle? Were they a team like Laurel and Hardy?

Actually, Lewis Swift and Horace Tuttle were well-known American astronomers who each discovered lots of comets. Tuttle, 17 years younger than Swift, was the guy who found the parent comet of the sometimes-super Leonid shower. He also discovered the Ursid meteor shower comet. During the Lincoln presidency, these men were the real deal, the equivalent of today’s Australian superstars Robert McNaught and Terry Lovejoy. They enjoyed long professional careers. In 1862, soon after the pair independently found the comet that would forever link their names, Swift-Tuttle grew as bright as Polaris.

No such luck during its 1992 return. Just like the Halley bust in 1986, this is not the right lifetime for earthlings to enjoy Swift-Tuttle. It had put on a darned good show in the second century, as chronicled by Chinese astronomers. And it will again attain naked-eye brilliance next time around, in August 2126, when it passes half as close as Venus ever gets.

Comet Swift-Tuttle has a small but nonzero chance of hitting us in the far future. With its 16-mile-wide (26km) nucleus and super-fast 37-mile-per-second (60 km/s) speed, it would be far more devastating than the impact that ended the dinosaurs. Swift-Tuttle is the largest object in the known universe that makes repeated close approaches to Earth. It’s the most hazardous celestial body we’re aware of. Yet, thanks to their fragile composition, individual Perseid particles never make it to the ground. Of the 50,000 known meteorites, exactly zero are Swift-Tuttle fragments.

Jupiter can take credit for this comet’s reliable clockwork period. They are linked into an 11:1 resonance. Swift-Tuttle performs one orbit around the Sun in the exact time Jupiter makes 11 circuits.

To maximally see all that juicy cometary debris, look between midnight and dawn August 12/13, when rural observers should average a meteor a minute. One night earlier offers some cloud insurance. While the sheer numbers may be 20 percent fewer during that “bonus night,” the compensation on August 11/12 is a slight tendency for more bright ones and also a higher percentage with glowing trains.

So visit those friends in the country. Douse the house lights. Drag out a few lawn chairs. Share some cool Perseid factoids. And while enjoying astronomy’s most famous shower, maybe salute the bearded Lewis Swift and the mustachioed Horace Tuttle.

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