The 2004 Perseid Meteor Shower
The Perseids may be the best annual meteor shower. For my home state of Oregon, this shower certainly offers the best odds for clear skies around the peak night. Other regions complain of hazy skies, though, and in recent years forest fires have been a problem in the Western US. For several years, US West Coast observers have not seen the Perseids at their best due to moonlight and/or bad timing of maximum activity. 2004 should be better. The Moon is a waning crescent, but shouldn't be too much of a problem. The nominal peak time will be near the beginning of morning twilight on August 12. Although we might miss part of the maximum in twilight, it's almost certain that activity will be high during the morning darkness when the Perseid radiant is high in the sky. All in all, conditions are similar to 1996. In that year, observing from White River Canyon near Mt. Hood, I saw good rates all night, and 96 Perseids in my last hour of observing. That memory has me licking my chops for this year!
The first Perseids are noted in mid-July each year, but are very few and far between. In late July, rates increase to 2-3/hour and add to the high level of meteor activity that can be seen before dawn. Rates climb slowly through the first 1.5 weeks of August, until the majority of meteors seen are Perseids. There is typically a steep increase in activity on August 11, with a maximum on August 12 or 13. During this leap year, the maximum is predicted for Thursday morning, August 12 at around 4:00am PDT (7:00 EDT). That's for the traditional maximum. Esko Lyytinen, one of several people who had so much success in predicting the recent Leonid outbursts, says that there is also a good possibility for enhanced Perseid activity on August 11, due to the Earth's crossing of particles ejected from the Perseids' parent comet in 1862. This activity would occur during US daylight hours (roughly 2pm PDT), would be short-lived (maybe only 15 minutes!), and would favor observers in parts of Europe and Asia. This predicted time is around 2100 UT on August 11; favored observers could see rats of several hundred Perseids/hour, but outburst rates are extremely difficult to predict. After the peak, activity falls off rather quickly, although rates are still fairly impressive for a couple of days.
To see the highest rates, you'll want to be watching near the peak. Although you can start observing as soon as the sky gets dark (around 10:00pm local time for midnorthern latitudes), the Perseid radiant is low in the sky at that time. On Wednesday evening, August 11, initial rates will probably be around 20-30 per hour from a dark site. Rates will increase throughout the night as the peak approaches and the radiant rises. From 1:00am on Thursday morning until the sky starts to brighten with twilight (around 4:30am), the Perseids should really be hopping. A perceptive observer in a dark Central Oregon sky (hopefully clear and relatively smoke-free!) might expect to see 80-100 Perseids per hour during this key time. The Perseids won't be the only meteors you'll see; expect 15-25 "others" each hour during the morning. Most of these will be random sporadics, but several minor showers are active, including a radiant complex in Aquarius and the slow Kappa Cygnids near the head of Draco. The Northern Apex is a diffuse radiant near the feet of Perseus; unwary observers may confuse these meteors with Perseids. Light pollution will cut rates dramatically, so get out to a decent dark site.
While the timing of the traditional maximum may favor the US West Coast and the eastern Pacific (the sky will be bright with sunlight on the East Coast), the rest of the US should not despair. The same 1:00am-4:30am local time period on Thursday morning should be good for just about everyone in the western hemisphere. The farther east you are, the further from the peak you are, but the Perseids produce high rates for many hours. In fact, rates following the maximum on Thursday night/Friday morning may still be 30-50/hour. This falling branch of the activity will actually slightly favor more easterly observers.
For casual observing, it is not necessary to distinguish between Perseids and other meteors. If one is doing a formal count or pursuing a program such as the Astronomical League's Meteor Club, Perseids must be identified. The Perseid radiant is near Eta Persei, often drawn as the pointy head of the constellation. When a meteor appears anywhere in the sky, mentally prolong its path backward. If the backward extension of the path passes within a few degrees of Eta Persei, the meteor should be counted as a Perseid. A shoestring or other cord held up to the sky can aid in alignment. Perseids, except those seen near the radiant, tend to be fast-moving meteors, a consequence of their entry into the atmosphere at nearly 60 km/s. The brighter ones often leave glowing trains in their wakes. Perseids are fairly bright on average, but run the gamut from an occasional fireball (brighter than Venus) to very dim meteors that are easy to miss.
Since Perseids can appear anywhere in the sky, it is not necessary to look right at the radiant. When trying to identify Perseids, however, it is best to have the radiant somewhere within your field of view. Your field of view should also be centered high enough that it is not obstructed by the horizon. Keep any glaring light sources (such as local streetlights or the rising Moon) out of your view. Comfort is extremely important; you want to be in a position where you can keep your eyes on the stars for several hours (take an occasional break, though!) A reclining lounge chair or sleeping bag with pillows may work for you.
I plan to watch the 2004 Perseids from the dark skies of the Oregon Star Party. Wherever you're enjoying this annual astronomical tradition, good luck and have fun!
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The Perseid radiant appears to drift eastward against the starry background from night to night; around the shower’s maximum it is near the star Eta Persei. The distinctive W-shaped figure of Cassiopeia is a good signpost.
Adapted from the International Meteor Organization’s Meteor Shower Calendar.
