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Sunday, July 28, 2013
Many city folks remember heading to the country in their youth and spending a late night enjoying the fireworks of mid-August. They fondly recall lying on a blanket in a grassy field or looking up from the bed of a pickup and counting the streaking meteors as they flew overhead.
Today the same people wonder what happened to the meteors of August. Do they still appear?
The Perseid meteor shower returns year after year — and has been doing so according to ancient Chinese records since at least A.D. 36 — with some years being much better than others. This promises to be a good year.
Annual meteor showers have their origins a million years ago and many billions of miles from Earth. Near the outer reaches of our solar system at the verge of interstellar space, millions of icy mountain-sized bodies drift around the sun, weakly influenced by the gravitational tugs of passing stars. Occasionally one of these mountains, a future cometary nucleus, is nudged toward the sun and begins a million-year orbital free fall.
As the nucleus finally reaches the orbits of the outer large planets, primarily Jupiter, it may be gravitationally pushed again, slightly changing its trajectory. It increases speed, passes the orbits of Mars, Earth, Venus, then Mercury, swoops closely and quickly around the sun, and zips into deep space again. It may not return for hundreds or thousands of years. Perhaps never.
In its wake stretches streams of debris, lined with wispy filaments strewn mostly with grains of water ice. If our planet passes through an orbital stream, skywatchers see a meteor shower.
Every year on either Aug. 12 or 13, Earth crosses the orbital path of Comet Swift-Tuttle — specifically, through one of its associated debris streams. This year meteor and comet scientists predict Earth will pass through a filament field in the late afternoon and early evening hours on Aug. 12.
Swift-Tuttle’s orbital path intersects Earth’s in the celestial direction of the constellation Perseus, causing this meteor shower to emanate from Perseus, and explaining why it is named the Perseids. Perseus doesn’t rise high enough for its major stars or any meteors to be easily seen until 11 p.m., which is fortunate, because this year the sky brightening moon doesn’t set until 11:16. That is the time for meteor watching to commence.
Find a location away from city lights and one where sky glow doesn’t significantly affect viewing the stars in the northeast. You don’t necessarily need to face Perseus to see the best meteors. Many people look more toward either the northwest or southeast because most of the ones spotted in or near Perseus are seen only as short streaks or quick flashes. Long meteors — the most spectacular ones leave sparkling, glowing and sometimes colorful trails called “trains” — are found in regions far from Perseus.
The predicted rate is over 60 per hour, greater than most meteor showers. That number includes many dim, difficult to catch meteors and the ones that pass unseen behind you.
Keep a mental note of their tracks across the sky. Once a handful have been spotted, trace their paths of travel in reverse. They will converge in Perseus, not far from the famous “W” shape of the neighboring constellation Cassiopeia, marking the shower’s radiant.
Binoculars play a useful role, not by being used to watch the fiery streaks, but by enhancing the appearances of their trains of disintegrating icy grains. These can survive for many seconds, resembling slowly dissipating fireworks.
Folks never had views like these when they watched the meteors of August back in the day!
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