Perseid Meteors
Published on Aug 11, 2009 at 10:57 am.
3 Comments.
Filed under meteors, observing, skywatching.
Every year, in mid-August, the Earth passes near the orbit of Comet 109P/Swift-Tuttle. The comet, itself, is seldom seen. It was last easily viewed by amateur astronomers in 1992 and 1993. With an orbital period of about 133 years, the comet will next be in the inner Solar System in the year 2126, when it will pass fairly close to Earth (about 15 million miles). But, this comet has been following this same orbit for a very long time. It has made many passes through the Solar System, and each time that it comes near the Sun it sheds a little material. Much of the material shed is in the form of gases. However, some of the material is dust grains or tiny meteoroids. These small bodies then move in orbits similar to that of the parent comet. The orbits are not identical, but similar. Since the orbits are not exactly the same, then over time they become quite separated from the parent comet. Eventually, a whole swarm of this debris is spread out following a path not too dissimilar to the comet’s orbit. Each August, Earth passes through the swarm. When that happens, the debris strikes Earth’s atmosphere and burns up, creating a meteor as seen from an observer on the surface of the planet. These meteors appear to shoot away from the constellation Perseus, so we call them the Perseid Meteors.
The Perseid meteor shower is typically one of the best of the year. It is also very reliable. Some other meteor showers tend to be more active, but not every year or not to every observer. The Perseids are a very old shower. The meteors have had a chance to distribute themselves all around the sky, so the Perseids tend to always put on a good show. Since the Perseids get their name because the meteors appear to radiate away from Perseus, the Perseids put on the best show for observers in the Northern Hemisphere (Perseus is a quite northerly constellation). For observers south of about 30° S, don’t expect to see much. This year, Earth passes through the middle of the debris swarm in the early morning hours of August 12, 2009. That is when you see the most meteors. However, since the Perseid Meteors are such an old meteor shower, the debris field is so spread out that activity starts to pick up a couple weeks before and lasts until a couple weeks after the peak. In fact, you get really good meteor shower activity for a couple days, or so, before and after the peak.
Because the radiant for the Perseids is so far north, the Northern Hemisphere observers get a bonus: the meteors are often seen a bit earlier in the evening. In general, meteor showers tend to be best on the front side of the Earth (much as you get more bugs on the front side of a car as you are driving). From the accompanying diagram, you can see that the “front” side of Earth is from about midnight until about noon.
Midnight and noon, in this context, refer to local midnight and noon, not necessarily midnight and noon according to the clock. Local midnight, for observers here in north central Texas, is about 1:30am, Central Daylight Time. So, from then until dawn would normally be the best time to observe meteor showers. However, since the radiant is so far north, and since the meteors are coming much faster than Earth’s orbital velocity, you’ll start picking up activity as soon as the radiant rises. For us, that will be about 9 or 10 pm (clock time). These meteors skim along and just clip the Earth’s atmosphere. Often you see meteors with long trails when that happens. Typically, though, activity still picks up substantially after 1:30 am and lasts until dawn. That is when you will get the highest level of meteor activity.
What I’ve said is typical for generic Perseid meteors. Remember, this is an old shower, and the debris is pretty much spread out all over the orbit. But, every time the comet comes by, it sheds material, and that fresh material makes a tighter bunch of debris for a very long time until it spreads out. It is very common when watching meteor showers to see activity wax and wane as Earth passes through various densities of debris swarms. This year, Earth is expected to pass through one of the denser parts of the meteoroid debris swarm from about 08:00 to 09:00 UT (that is about 3 to 4 am on August 12 for us in Texas). Unfortunately, the Moon will be very high and bright at that time, making it tough to see the meteors. (Actually, the weather forecast for my location is for clouds all night, so we might not get to see much of anything). The Moon is a waning gibbous right now (a couple days shy of Last Quarter), and it will rise a bit after 11 pm (CDT) for us tonight. So, even without the clouds, this is not a great year to be watching the Perseids. Next year, the Moon will be nearly new, so 2010 will be a very good Perseid observing year.
If you get clear skies at your location, though, then go ahead and go out tonight and look for the Perseid meteors. If you go out before local midnight, then you will see one every few minutes or so. At the peak, you might see one every minute or two (you’d see more if the sky were not so bright from the Moon), and maybe even more if you observe from a dark site.
-Astroprof
Aglolink on August 11, 2009 at 10:05 pm: 1
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best time to see perseids | sodini video on August 13, 2009 at 3:25 am: 2
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