The Perseid Meteors
Published on Aug 11, 2006 at 12:22 am.
1 Comment.
Filed under meteors, skywatching.
Saturday, August 12, is the peak of the Perseid meteor shower. The Perseids always put on a good show, but this year is likely to be pretty washed out by a very large, bright moon. In fact, this hasn’t been a great year for meteor showers. It seems that the Moon has been getting in the way a lot this year. Well, that means that the Moon will be less of a problem next year.
Meteors are commonly called “shooting stars” because most appear as tiny dots, like stars, streaking across the sky. But, they are most definitely not stars! Meteors are bits of interplanetary debris hitting the Earth’s atmosphere and burning up. As they streak through the atmosphere, friction slows and heats them, but the friction also strips electrons off of the air molecules. This is what you see when you look up and see the meteor.
For a long time, astronomers didn’t think of meteors as having anything to do with astronomy. After all, it was clear that they weren’t stars. It was also clear that they had something to do with the atmosphere. So, most scientists just thought that they were some sort of high altitude atmospheric phenomenon, like some sort of lightning. In fact, the word meteor means atmospheric. That is why meteorologists study the atmosphere, not meteorites! It wasn’t until the Leonid meteor storm in 1833 that astronomers began to realize that meteors were actually extraterrestrial in origin. (The Leonids occur in November, and I’ll do a blog entry on them and the history of meteor showers then.)
On any given night, you can see meteors. These are normally just random bits of material flying around the Solar System. But, once in a while, there comes a night in which you see a lot more meteors than normal, and they seem to come from a particular point in the sky. This is a meteor shower. Meteor showers come from material shed by some celestial body orbiting the Sun, usually a comet. This material orbits as a swarm of particles. Before they enter the atmosphere, these things are called meteoroids. If the meteor survives passage through the atmosphere to strike the surface of the Earth, we call it a meteorite. Since the meteoroids from a meteor shower are moving in a swarm, when they hit Earth’s atmosphere, they will appear to radiate from a point in space. This is called the radiant of the meteor shower. Meteor showers are named for their radiant. The meteors this weekend will appear to come away from the constellation Perseus, so we call them the Perseid meteors. In December, there is a meteor shower that appears to come from Gemini, so they are the Geminids. The Leonids of November appear to come from Leo, and so it goes with other showers over the course of the year.
So where did this particular swarm come from? The Perseids come from Comet Swift-Tuttle, which last passed by in 1992. As one would expect, the Perseids were pretty spectacular through the 90’s. They have been a bit less wonderful lately, but they are still one of the best meteor showers of the year, and they are very predictable. Comet Swift-Tuttle was not very impressive when it came by in 1992 since it was far from Earth when it was closest to the Sun. With an orbital period of just over 133 years, it will next come back in 2126. The comet should be spectacular then. It will actually be passing very close to Earth in 2126. For a while, in fact, there was talk that it might actually collide with Earth. That was a very low probability from the start, but the media really picked up on the possibility. More careful studies of the comet’s orbit show that it will definitely pass the Earth at a very safe distance when it next comes by us.
While the Perseids may produce nearly 100 meteors per hour from about midnight until dawn near its peak activity, don’t expect to see that many meteors. As I said, the Moon will interfere. With so much moonlight, you’ll only see the brightest meteors. So, expect only a few per hour if you go look. But, if you do look, where should you look, and when? Basically, look up. The meteors will appear to radiate away from Perseus, which will be in the NE sky in the mornings. However, they can be seen almost anywhere in the sky. They are actually easier to see if you look away from the radiant by 45 to 90 degrees. So, just look straight up. As for when, that would be before dawn. As the Earth orbits the Sun, it will be running into the meteors, so the side of the Earth towards the front will get hit the most. The front side of the Earth will be from local midnight until local noon. This isn’t exacly clock midnight and noon. Here in Texas, local midnight is about 1:30am due to an offset as a result of our longitude and an hour delay for Daylight Saving Time. My students always whine when I tell them that meteor showers are best (or sometimes only) seen a few hours before dawn. Sorry, but I don’t have control over the timing!
If you go observe the meteors, just be patient. As I said, you’ll be missing the dim ones, and that means that you’ll just see the really bright ones and there may be several minutes between bright meteors.
-Astroprof
Astroprof’s Page » 2007 Perseid Meteors on August 9, 2007 at 12:55 pm: 1
[…] 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. […]