If you’ve never seen a meteor this is the week to see one. Many times a year the Earth passes through the debris trail of various comets. Each passage produces a short-lived meteor shower though most aren’t much to see. A few times a year the Earth encounters a major shower which produces enough meteors to be noticeable by anyone. This week marks one of those major showers as the Perseids meteor shower will be best seen on the morning of August 13.
The Perseids were produced as dust was ejected from the nucleus of Comet Swift-Tuttle, a Halley-type comet which orbits the Sun roughly once every 135 years. The comet was discovered independently by 2 American astronomers, Lewis A. Swift and Horace P. Tuttle, in 1862. Ancient records suggest the comet was actually first seen in 68 BC. In addition to 68 BC and 1862, the comet has been observed on 3 other occasions, most recently in 1992. During its next return in 2126, the comet will pass within 0.15 AU of Earth and become a brilliant naked eye object.
Though the Perseids will peak on August 12/13 they have been active and producing meteors for weeks now. Video observations compiled for the AKM/IMO Video Meteor Network by Sirko Molau (of which my cameras are members) find that the Perseids are active between July 13 and August 26. During most of this time the Earth was encountering dust released over the past ~15,000+ years. Most of this ancient dust never crosses the Earth’s orbit so Perseid rates remain low (at best 5-10 meteors per hour). This week we encounter dust released more recently, within the past 5,000 years. Since the comet’s orbit over the past few thousands of years has intersected the Earth’s, we will see a large increase in meteor rates this week.
The peak of this year’s Perseids is predicted to take place around August 13 at 3h20m UT (11:20 pm EDT, 8:20 pm PDT, 8:20 pm Tucson time) which is not a great time for us in Tucson. Still, the maximum is broad and can last for many hours. The last 2 years have also seen a number of spikes in activity on the night or 2 around the predicted peak. What this means is that high Perseids numbers can happen at any time even if tens of hours before or after the predicted peak. This year will see passage near a few older trails which may enhance the number of meteors by 5-10 per hour. Predictions by Jeremie Vaubaillon and Mikhail Maslov show encounters with dust trails produced in 441, 1479 and 1862.
So how can you observe the Perseids? The key is to get outside Thursday night/Friday morning. The reason these meteors are called the Perseids is because they appear to radiate (move out from) a point (called the radiant) in the constellation of Perseus. For most of the night Perseus is located low in the northeastern sky so most meteors should come from that direction. Late in the night the radiant will be located just north of overhead. Note, you don’t have to look directly at the radiant. Meteors will be visible all over the sky and it is usually best to look about 45-90 degrees away from the radiant.
Though the radiant will be visible for nearly the entire night it will be too low in the sky to produce high numbers of meteors until after midnight. The later in the night you observe, the greater the number of meteors. By 3 to 4 am, the radiant is well placed nearly overhead. The Stellarium star chart below shows the sky over Tucson at 4 am on Friday morning with the Perseid radiant (yellow star) nearly overhead.
How many meteors can you see? This depends on a lot of variables. The peak rate for the Perseids is usually between 60 and 100 meteors per hour. But… that rate (called the zenithal hourly rate, or ZHR) is only valid from a very dark site (where stars down to magnitude 6.5 can be seen), within a few hours of maximum, and with the radiant directly overhead. In reality, very few of us will be watching the meteors under such perfect conditions. Instead, most of us will be observing a few hours from maximum with the radiant lower in the sky and under a light polluted sky.
Over the past 3 years, the Perseids have produced ZHRs of between 50 and 200 meteors per hour on the night of maximum (and sometimes even the night before) and it should be safe to expect ZHRs of 50 to 120 during this year’s shower. If you are observing from a dark site where the Milky Way is bright and easy to see, you should be able to detect 35-85 meteors per hour. Under moderate light pollution (like that found in most of Tucson), apparent rates will be lower with 15 to 40 being seen per hour. Even if you observe from the middle of heavily light polluted cities you will still see a handful of meteors per hour. Again, rates will be lower the earlier you observe during the night since the radiant will be lower in the sky.
To follow the latest observations and ZHR measurements, visit the International Meteor Organization’s Perseids 2010 ZHR Live page.
The best way to observe meteors is to find a spot with a clear view of the sky and no (or few) bright lights shining in your eyes. This means getting away from the glare of pesky streetlights and security lights. A lawn chair is the most comfortable way to relax and see the sky. For most of us the temperature will be warm but if you are further north or at altitude, dress warm. You may be surprised with how cold the early morning hours can be. Also be prepared for bugs. Even though the early morning in Tucson is nice and warm (maybe even too warm) I’ll be wearing longs pants and a long sleeve shirt to try to minimize mosquito bites.