It’s that time again. Every August the Earth sails through crumbs dropped by comet Swift-Tuttle. The same way you cross an intersecting street on your way to the store, Earth crosses through the orbital trail of debris left by repeated returns of the comet to the inner solar system. During each visit, solar heating vaporizes a portion of the comet’s dust-embedded ice. Poof goes the ice, but the dust remains and spreads out along Swift-Tuttle’s orbit to become the Perseid meteor shower.
Earth plows through the material, which ranges in size from sand grains to small pebbles. When the bits strike the atmosphere they’re moving at around 37 miles a second (59 km/sec) but come to as sudden skid as they buck up against the air. That raises their temperature to a couple thousand degrees. The particles burn to a fine soot but pass on their kinetic energy (energy of motion) to the air.
Slamming into trillions of atoms, an incoming particle strips air molecules of electrons, those little buzzing bees swarming around an atom’s nucleus. When the electrons rejoin their host molecules, energy is released as the glowing streak of light we call a meteor or shooting star. And it all happens in less time than it takes to read this sentence.
The Perseid meteor shower is one of the best of the year and will peak on Monday night Aug. 12. Under ideal conditions with no moon and the radiant high in the sky you might see up to 80 meteors an hour. I do mean ideal. For most of us, seeing 30-40 per hour is more the norm. But we’ll get some flak from the moon this year. It’s a waxing gibbous situated about 10° (one fist) from Saturn and won’t set until around 4 a.m. Tuesday morning (Aug. 13).
Bright moonlight lightens the sky and wipes away many of the fainter meteors, so we should expect closer to 15-20 per hour. That might be on the conservative side, but I just don’t want to get your hopes up and see you disappointed that it wasn’t raining meteors. My view is that I’m happy to just lollygag on a cheesy plastic lawn chair and look up, ready for what comes whether flood or trickle.
August is such a nice time to be out and skies are generally clear. If they’re not the good news is that the Perseids are already active and will be through mid-month. You won’t see as many as during the Aug. 12-13 peak but you will see some. You can tell a Perseid from a random meteor or members of other minor showers active this time of year by tracing its path backwards in the sky. If it points to the radiant, a spot located just below the W of Cassiopeia in western Perseus over in the northeastern sky you’ve got the genuine item.
The radiant is the direction in the sky from which dribs and drabs of Swift-Tuttle stream our way. Every meteor shower has its own particular radiant. The October Orionids blast out of … Orion.
Here are my viewing suggestions. Go out any night in the next week and you’ll see at least a few Perseids. But if it’s peak action you want, then head out Monday night (Aug. 12) starting about 10 o’clock local time. A reclining chair is the way to go. Bring a banky so you don’t get chilled or wet from dew and face east away from the moon, the better to preserve your night vision. Then relax, inhale and prepare to be surprised by unpredictable flashes of ancient-as-the-solar-system comet dust.
Alternatively, you could get up early Tuesday morning around 3 a.m. and hold vigil in the final hour, hour-and-a-half before the start of dawn. Because the radiant will be at peak altitude and the moon nearly gone you’re bound to see more meteors. You could also get up around 3 a.m. Aug. 10 and 11 when the sky will be moonless for 2 hours and 1 hour respectively before twilight begins. Meteor rates will be lower but you can’t beat a dark sky.
So you can see there are several ways to cut this. I plan to go the easy route and stay up late Monday night, my back to the moon and my eyes to the sky. Can bliss be this simple?