October’s Orionid Meteors
Published on Oct 12, 2006 at 1:11 am.
2 Comments.
Filed under meteors.
October of every year brings meteors. Actually there are several meteor showers in October, but the biggest one is the Orionid meteor shower. The Orionids aren’t the biggest, nor the best meteor showers of the year, but they are predictable and quite reliable. Another nice thing about the Orionids is that they are associated with one of the most famous of comets: Halley’s Comet.
Meteor showers, as such, were first recognized following the spectacular showing of the Leonid meteors of 1833. Prior to that time, astronomers had never really paid much attention to meteors. Even the name “meteor” simply means “atmospheric,” because astronomers used to think that meteors were simply atmospheric phenmoina, similar to lightning. The spectacular shower of 1933, though, finally got astronomers interested in meteors. Soon, it became apparent through the work of astronomers such as Denison Olmsted that meteors are caused by extraterrestrial objects hitting and burning up in the Earth’s atmosphere.Â
The Orionid meteors were apparently first recognized as a distinct meteor shower in about 1839 by Edward Herrick, though detailed observations weren’t made for nearly 25 years. Like other meteor showers, the Orionids occur when the Earth passes through a swarm of meteoroids sharing similar orbits. On any given night, you can see a handful of meteors per hour from just random meteoroids hitting Earth’s atmosphere. However, when the Earth passes through a swarm of meteoroids, you see more than average, and from the perspective of an observer the meteors of the swarm appear to come from a point in the sky, as seen in my drawing above. Now, that drawing, with meteors all over the sky, isn’t normally what you see! Normally, you see a meteor only every now and then, and so you can imagine that drawing as an hour long time exposure of a meteor shower.  So, despite what common opinion might be, a meteor shower is not normally the sky being lit up with meteors (that is called a meteor storm, and such events are exceedingly rare). Instead, you get a meteor every few minutes. As I said earlier, the meteors of a shower appear to radiate from a point in the sky. That point is called the radiant of the meteor shower. Meteor showers are named for the constellation in which the radiant is located, a practice that dates back to a suggestion by the famous astronomer Giovanni Schiaparelli. For example, the Leonid Meteors of November appear to shoot away from Leo, the Perseids of August appear to radiate from Perseus, and December’s Geminids appear to radiate from Gemini. It is thus not hard to figure that the Orionids radiate from Orion.
They are also a fairly broad shower, too. Unlike some meteor showers, most of whose activity is in the course of a night, the Orionids are active through most of the second half of October. Now, that doesn’t mean that any night from mid-October onwards is equally good. The Orionids peak in the early morning hours of October 21. But they are nearly as active the night before or the night after. Also, they are fairly decent for two or three days before or after October 21. And anytime from about now until early November, you will see a few extra meteors on most nights shooting away from the constellation Orion, but more than a week from the peak and you see only a very few Orionid meteors. Also, as I said earlier, the Orionids are not the most active shower of the year, but they are always pretty reliable. You can expect about 20 meteors per hour (paying attention, looking up, in dark skies, near the peak). My own observations of the Orionids over the years seem to indicate that there a lot of moderately dim meteors with the Orionids (dim enough to really kill off a lot of the fun when observing from bright city skies) with an occasional really bright meteor (once or twice per hour).  The orientation of the meteoroid stream seems to slightly favor Southern Hemisphere observers, who often see a few more Orionids per hour than those of us in the Northern Hemisphere. I have also noticed a sort of “clumping” of the Orionids. The technical term for this in meteor circles is filamentary structure, but clumpiness gets the idea across. While the average may be 20 per hour, you will often go for long periods of time with very few meteors, and then get a handful in just a few minutes. Actually, it turns out that this is typical behavior for many meteor showers. Also, like most meteor showers, you don’t see much of anything until between local midnight and dawn (more on that later).
Early on, I mentioned that the Orionids are associated with Halley’s comet. Let me explain. As comets pass through the Solar System, they move in very elliptical orbits. When they get close to the Sun, some of the gasses that make up the comet begin to sublimate. Comets are made of a mixture of frozen gasses, ice, and dust and rock. The dust is thrown out by the escaping gas. Now, if the comet is a periodic comet, that is if its orbit takes only years, decades, or a few centuries to complete, then each time it nears the Sun the comet sheds some of this dust. These dust particles then orbit the Sun in orbits very similar to that of the comet. The orbits are not exactly the same, and so the dust particles gradually separate from the comet. Eventually, if the comet’s orbit itself passes near enough to Earth, then the swarm of the dust particles’ orbits may begin to intersect Earth’s orbit. If so, they you get a meteor shower. Halley’s Comet has been returning for a very long time, so it has shed a lot of these particles. Their orbits have dispersed all along the approximate path of the comet, which crosses near Earth’s orbit in a couple of places, thus producing two meteor showers. In the early part of the 20th Century, Charles Olivier noticed that the Orionids have an orbit very similar to the Eta Aquarid Meteors that happen in May. The Eta Aquarids had been shown to be associated with Halley’s Comet as early as 1876, when Alexander Herschel demonstrated a mathematical correlation between the orbits of the Eta Aquarid meteorids and Halley’s Comet. But, the connection between the Orionids and the Eta Aquarids (and hence Halley’s Comet) wasn’t really shown until 1983 when McIntosh and Hajduk published calculations showing how the two were related.
Also, it turns out that the radiant of the Orionids has some interesting properties. First of all, the filamentary nature of the shower results in there being two or more very closely spaced radiants. Furthermore, the position of the radiant actually drifts a bit over the time that the shower is active. In the picture below, the oval is the approximate point of the radiant. Near the peak, it is near the center of the oval, and a week earlier it is near the right hand side. A week later, it is near the left hand side.Â
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So, now that you know to go look for the Orionids, how and when should you observe them? Well, as I said earlier, the peak is the morning of October 21, so stay up late the night of October 20, or else get up several hours before dawn on October 21. That is the best time. The Earth is running into the meteors, so you see more when on the front side of the Earth — that is from local midnight until noon. For most of us, that means the best observing is from about 1am until just before dawn. You can see them a couple nights before or after. This year, the Moon is nearly New, so it won’t be a problem at all like it was last year. The meteors will appear to radiate from the northern part of Orion, as shown in the top picture. However, you don’t want to look right at the radiant. The longer and prettier meteor trails are typically some 45 degrees from the radiant. So, just look up. Lay in a lawn chair, or something, and look up. It works better if you have a buddy to talk to to help keep you awake. And then, just relax. Personally, I find meteor observing very relaxing. You just lay down and look up. There isn’t much thinking or activity involved. You can take notes when you see one, but otherwise, you just watch and wait. Given that this is October, you probably want to dress accordingly. There is little telling what the temperature will be, but it is always coolest at that time of the day, so you may as well be prepared for cold weather.
If you do observe the Orionids, I’d love to hear what you see!
-Astroprof
(Images generated using Starry Night Pro, and then modified by me.)
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A Ler…-- Rastos de Luz on October 12, 2006 at 5:47 am: 1
[…] “October’s Orionid meteors“, uma referência ás Oriónidas, no Astroprof’s Page; […]
Peter Naake on October 22, 2007 at 9:30 am: 2
My son and I went to a local park at about 6:00 a.m., here in Louisville Kentucky on Sunday morning (Oct 21st). We saw several bright shooting stars even though this is a highly light-polluted area. They were green in color and most lasted only across the Orion constellation. He would watch for a few minutes while I played with the dog, and then we would switch. This morning, it was too cloudy.