66 August 2014 sky & telescope
Breaking the Meteor Rules–1,000 km 1,000 km200 km100 kmActual meteor zone of the atmosphere, to scale
meteors500 milesEarth’s atmosphere is quite thin compared to Earth’s diameter, as this true-scale diagram shows. This enhances the diff erence between
grazing and straight-down meteors. Most meteors shine in the upper blue zone: from about 120 to 80 kilometers (75 to 50 miles) high.Below: The historic “Earth skimming” fi reball of August 10, 1972,
caught peoples’ attention in broad daylight from Utah to Alberta.
At Jackson Lake, Wyoming, James M. Baker grabbed his camera
and shot pictures of it passing behind Teton Mountain. Esti-
mated to be a few meters in diameter, it continued on right out
of the upper atmosphere and back into space.hour was still fairly early.
“In the next 20 minutes we saw 20 more like this,” he
recalls. “We could not avoid seeing them!” The experience
was all the more remarkable because the radiant was only
about 25° to 30° up, so in theory, they were seeing only
40% to 50% as many meteors as they would have seen
had the radiant been overhead.
Earthgrazers (extreme cases are called “skippers”)
are memorable enough to make up for their rarity. The
meteors in a shower are tiny pieces of low-density comet
material — dust clumps rather than rocks or iron —
arriving at 30 to 70 km/second (70,000 to 160,000 mph).
When they dive into Earth’s atmosphere at a steep angle,
they burn out fast. But when they enter at a low angle,
they can survive long and travel far. This is especially true
if the meteor is inherently big and bright.
An earthgrazer may seem to move slowly, but this
can easily be an illusion due to its long duration. It will
indeed appear slow if it’s near your horizon and therefore
is very far away, especially if you’re looking toward oraway from the radiant and the motion is foreshortened
along your line of sight.A New Way to Watch Meteors
Watching for earthgrazers means violating the standard
advice. Wait until the radiant is high? That’s for watchers
who hope to see the greatest numbers, or for those seek-
ing to make a statistically signifi cant meteor count. But
early evening is, on average, the most likely time to see
meteors coming in at a shallow slant.
This year Earth should pass through the thickest part
of the Perseid stream for 12 hours or more centered on
0 h August 13th Universal Time. This covers nightfall onS&T:LEAH TISCIONE