This month's usually dependable Perseid meteor shower competes with a nearly full Moon. If you can find a dark viewing location, you might see a bright meteor every few minutes when the shower peaks on the night of August 12–13.
Every three years the Moon displays similar phases at the same dates on the calendar. The rule of thumb is that every phase happens just three days earlier than it did three years ago, on average. You might recall that in 2011 the Perseid meteor shower contended with an almost-full Moon.
Well, it's happening again.
This time the Moon will be two days past full on the peak Perseid night, August 12–13. So it won't be quite as bright as when it's full, but it will illuminate the sky all night, especially from midnight to dawn when the shower's radiant in Perseus is high and the meteors should be most numerous.
But at nightfall the Moon will still be low in the east, and this is when to watch for Earthgrazing Perseids. These are the infrequent, but unusually long and graceful, meteors that you may see when a shower's radiant is low above the horizon.
On their peak night, the Perseids typically produce about 100 meteors per hour when the radiant is near the zenith (directly overhead) and the sky is very dark. The peak rate typically runs for about 12 hours centered on the predicted time, which this year is 0h Universal Time on August 13th (near nightfall on August 12th in North American time zones).
Moonlight will hide faint Perseid meteors, but a nice bright one might show through every few minutes late in the night.
But don't limit yourself to the night of the peak. The shower stays above half of its maximum strength for two days running, and you may see a few Perseids as early the end of July and as late as August 18th, as shown in last year's activity graph at right. The drop-off happens faster than the rise — but the window of Moon-free darkness gets longer on the days following the peak.
Next year's peak Perseid nights will be moonless and ideal.
These meteors occur when bits of debris shed by Comet 109P/Swift-Tuttle slam into Earth's upper atmosphere at a relatively fast 37 miles (60 km) per second. The result is a trail of white-hot plasma (ionized air molecules) along each particle's path, and it's this column of plasma that creates the momentary streaks of light we see in the sky.
Attentive skywatchers realized in the 1830s that the Perseid meteor shower occurs during mid-August each year, though folklore suggests that its annual apparition was known long beforehand. Comet Swift-Tuttle, the shower's source, wasn't discovered until 1862.
Meteor Watching: Fun for All
Meteor watching has become much more of a public thing than it used to be. News media promote the main annual showers in a way that never happened a generation ago. We're proud to have played a part in this; the grand Leonid displays from 1999 to 2002 certainly helped.
But we underestimated how modest a success it takes to get people hooked. Most people know better now that to expect fireworks. Vacationing families with no astronomy experience are often thrilled to see just two or three shooting stars on Perseid night, and many have made it a family tradition.
Remember: you need no equipment — or even knowledge of the constellations — to enjoy the show. Just bring a reclining chair to a dark spot with a wide-open view of the sky, face whatever direction is darkest, lie back, and watch the stars overhead.
Get great tips for watching and studying meteors with Sky & Telescope's ebook, Shooting Stars. It's a free download!
Also: How to photograph a meteor shower (hint: planning, patience, and luck).