Parsing the Perseids

Last week was the peak of the Perseids meteor shower, one of the best and most reliable sky shows of the year. I received approximately eleventy billion tweets about it, and it seems that most folks saw several dozen meteors per hour. I have old friends visiting, and we camped out on the driveway for an hour and saw a couple of dozen ourselves. Many were very bright, leaving visible glowing trains (what astronomers call the long, streak in the sky left after the meteoroid itself burns up; it's m...