2007 Perseid Meteors
Published on Aug 9, 2007 at 12:55 pm.
2 Comments.
Filed under meteors, skywatching.
Every August the Earth passes through a swarm of material shed by comet 109P/Swift-Tuttle. This debris, mostly very small things about the size of a rice grain or sand grain, strikes the Earth’s atmosphere at 60 km/s. At that speed, they compress and ionize the air, creating a glowing spot streaking across the sky that we call a meteor. This phenominon is also sometimes, very inaccurately, called a “shooting star” or a “falling star” in layman’s terms. I really don’t like those term sbecause all too often people who know nothing about astronomy actually think that it is a star that is moving or falling from the sky.
There are actually a large number of swarms of small particles (meteoroids) moving around the Sun. Whenever the Earth passes through one of these swarms, then we can see a number of meteors shooting away from the same area of the sky. This is a perspective thing. You get the same effect looking at long straight railroad tracks: they appear to radiate from a single point on the horizon. Or, when you see the Sun rising or setting behind clouds, or even simply hidden by a cloud, then you can see great streaks across the sky called crepuscular rays. This gives the appearance that the meteors are shooting away from a particular spot in the sky that we call the radiant of the meteor shower. Often we name the meteor shower by the apparent location of the radiant in the sky. In the case of the meteor shower coming up this weekend, the meteors appear to shoot away from the constellation Perseus, so we call them the Perseid Meteors. I wrote about them a year ago, and you may want to read that posting, too.
Comet 109/P Swift Tuttle orbits the Sun once every 133 years. The last time it came by was in 1992. So, if the Perseid Meteors are material shed by the comet, why are we seeing them now, and basically every August? Well, the comet has been doing this for quite some time. The material is shed each time by, but as it is pushed off of the comet and is then pushed around by solar light pressure, it acquires a slightly different orbit than the parent comet. This means that some of the material can wind up ahead of the comet and some of it behind the comet. The swarm spreads out along a path roughly along the path of the parent comet. That is good for meteor observers, because it means that we can see meteors in years that the comet is nowhere near Earth. It is also good, because the spread of the meteoroid swarm means that you don’t need perfect alignment of the orbit of the comet with the Earth to get the meteor shower. In other words, Earth just passing pretty near the path of the comet means that we’ll be getting some meteors. In fact, the Perseid Meteor Shower’s meteoroid swarm is so spread out that the Perseids last for several weeks. The Peak, though, is normally about August 12 (give or take a day). But, a day early or late you will still see nearly half as many meteors as you would on the day of the peak itself, and you’ll see about one third of the peak number two days early, and there will still be a handful per hour for a week before and after the peak. That means that you won’t miss the entire show if it is cloudy on the day of the peak activity!
Also, since the meteoroids come from material shed by the comet as it dives near the Sun, the shower, like most meteor showers, tends to be “clumpy.” In other words, you may go for half an hour or so and hardly see any meteors, and then you’ll see two or three a minute for a while, and then it will calm down a bit, and then pick up activity a bit, and then calm down, and so forth. Also, there are sometimes sub-swarms of meteoroids that make a second, or even third, meteor activity peak other than the main one. This peak is sometimes seen as a different radiant (some of the meteors appearing to radiate from a slightly different spot in the sky than the main shower’s radiant) or as a peak at a different time, sometimes a day or two earlier or later than the main peak. Since the meteoroids themselves are too small for us to detect until they run into the atmosphere, guessing the activity is even less accurate than forecasting the weather (and here in North Texas, that is normally a pure guessing game!). The variability in activity for the Perseids is typified in how many are seen each year. The count rate increased in the 1990s, as expected with the passage of the comet, and some years recorded nearly 400 per hour. Activity has now dropped to typically about 80 to 100 per hour. But, in 1920 there was a sudden spurt of about 200 per hour. There were similar outbursts reported in 1931 and 1945. Of course the other extreme happens, too. In 1912, meteor observers reported only about 12 Perseids per hour at its peak, and in 1911, there were only 4 per hour. (Note: I got much of this data from the International Meteor Organization.)
Now, the meteor counts that I give above are perhaps not what you, as an untrained observer, would expect to see. These are corrected numbers called the Zenithal Hourly Rate (ZHR). This is what may be seen when the radiant is near the zenith and the meteor shower is being observed from ideal clear dark skies. You should expect to see lower numbers. Even so, the Perseids are an active enough meteor shower that anytime this weekend you ought to be able to see several meteors per hour, if you observe at the right time of night.
So, when is the best time to observe the meteors? Obviously, the closer that you are observing to the peak, the more that you’ll expect to see. This year, the peak activity occurs about 05:00 UT August 13, 2007. That is about midnight here in Texas (with Daylight Saving Time in effect) on the night of August 12. That is Sunday night. So, I am helping host a star party at Dinosaur Valley State Park southwest of Fort Worth on Saturday night, August 11, 2007. There will be nearly as many meteors then, and it is a Saturday night, which is far more convenient for most people.
So, now you know the dates to go look. What about the time? The best time to observe the meteors is when the Earth’s motion through space helps to sweep them up (like bugs on the front of a car). That occurs from local midnight until local noon. Here in North Texas, with Daylight Saving Time in effect, local midnight is about 1:30am. So, you would expect the best show from 1:30am until the dawn twilight begins. But, Perseus is a far northern constellation. So for those of us in the Northern Hemisphere, we get a bonus. Once Perseus gets above the horizon, then you’ll start to see some meteors. With the bug and car analogy, this would be like very fast flying bugs crossing the road as you are driving by and hitting the side of the car. As fast at the meteoroids go, this is a very real possibility. So, activity would be expected to actually begin sometime between 10 and 11 pm at night, getting better as the night progresses, and then getting even better after 1:30am. Once nice thing about these meteors that come in before the ideal time is that they often leave longer trails. Also, they sometimes leave persistent trains: glowing trails in the sky left by the passage of the meteor. That can happen with any of the meteors, of course. The Perseids are pretty good about leaving such persistent trains (not all meteor showers do that to the same extent). Sadly, with the Perseid radiant being so far north, my friends in the Southern Hemisphere don’t get as good a view of the Perseids as we do here in the north.
So, how do you observe the meteors? That’s easy. You just go outside and look. You don’t need to use telescopes or binoculars or any other viewing aid (except glasses or contacts, if your eyes are as bad as mine!). You just go look. The best thing to do is to get comfortable, such as laying on a lawn chair or a blanket looking up. You do not have to look at the radiant. In fact, the meteors near the radiant will be coming almost straight at you, so they will appear as much shorter trails in the sky. About the best place to look is about 45 degrees away from the radiant. The other key is to get away from lights. The more light around, the sky brighter the sky will be, and the harder it will be to see meteors. So, you need to be somewhere dark. That is why I am doing my star party in a state park. It is darker than here in the city. But, the Perseids are fairly bright, so if you want to observe from the city, then go for it. You will just have to realize that you will see far fewer meteors when fighting the city lights, but you will still see some, and it should still be worth your time. This year, the meteor shower is falling very near New Moon, so there will not be much in the way of moonlight to interfere with observing meteors. Now, how do you find the radiant?
Near the top of this posting is a finder chart that I made using Starry Night Pro software. It shows the sky at about 4am local time. Perseus is well up in the NE sky at that time. Down and to the right of Perseus, you’ll see Mars in the NNE, and up and to the left of Mars will be the Pleiades (a small cluster of stars that looks like a tiny dipper). Meteors that appear to radiate from Perseus will undoubtedly be Perseids. However, on any given night of the year, you’ll see a handful of random meteors per hour just shooting across the sky. There will be some of these sporadic meteors during the Perseid shower. Also, there are a few minor meteor showers this time of year, too, so there will be several meteors per hour that appear to radiate from other spots in the sky. But, unless we just happen to have one of those rare years with a hole in the meteoroid swarm (like 1911 or 1912), then the Perseids will be active enough that there will be plenty of meteors shooting away from Perseus.
So, if you get a chance, go out and observe the Perseid Meteors this weekend.
-Astroprof
Mike Plaiss on August 13, 2007 at 9:38 am: 1
Speaking of “falling stars”, your post reminded me of a recent column by a Chicago Tribune reporter who is one of those people to whom you write in questions (she is like “The Answer Lady”). She recently posted a brief list of some of the craziest questions she has ever received. One of those came from a woman who had just returned from a trip out west and, while out there, had seen several “falling stars”. Her question went something like this: “Having seen all these falling stars, the following question occurred to me - have any stars ever fallen out of a familiar constellation?”
William on August 13, 2007 at 4:20 pm: 2
Outstanding blog. Nice links, too, I should have lots to read here. Thanks.