From The Compass, Vol. 3, No. 1, September, 1998.
Stars falling from the sky? Well, not really. But there are two meteor showers this fall that have the possibility of being meteor storms.
Almost everyone has looked up at the sky and seen a falling star. Of course you are not actually seeing a star that's falling. What you are seeing is a meteor. On a clear dark night, away from the lights of the city, anyone can see about a half-dozen meteors every hour.
The falling stars or meteors that you see are just bits of rock that burn up when they run into Earth's atmosphere. Most of them are no larger than a grain of sand and they burn up in a fraction of a second. If you happen to see a bright meteor that lasts for more than a fraction of a second, you've probably seen something that's pebble-sized. A meteor that is fist-sized or larger will not only put on a spectacular sky show but will most likely survive the trip to the ground.
For a few nights a couple of dozen times each year you can go out and see anywhere from a dozen to a few dozen meteors per hour. This is a meteor shower. To understand why meteor showers occur, we have to understand a little bit about comets. The easiest way to describe a comet, especially when it is in the frigid depths of the solar system, billions of miles away from the sun, is to think of a dirty snowball; that is a dirty snowball that's 10 miles across. This dirty snowball is the nucleus of the comet.
As this comet nucleus approaches the sun, the sun's heat warms its surface, and it begins to boil. As the ice boils off, dust particles, are released. This gas and dust is pushed away from the comet's nucleus by sunlight and the solar wind to form a tail. Although over the course of time the dust particles move away from the comet's nucleus, they continue to orbit the sun in roughly the same orbit as the comet from which they came. When Earth passes through one of these particle streams, the dust particles run into our atmosphere and we have a meteor shower.
As you might expect, these streams of dust are densest closest to the comet. When Earth and the comet are in close proximity to each other a large number of particles run into Earth's atmosphere and we have an intense meteor shower which is referred to as a meteor storm.
The comet that is responsible for the Giacobinid meteor shower which occurs on October 8th/9th is Giacobini-Zinner. Most years no Giacobinid meteors are seen. This indicates that the particles haven't had a chance to spread around the comets orbit yet. However this year Earth passes just inside of Comet Giacobini-Zinner's orbit 50 days before the comet. Previously when Earth has passed inside the orbit just after the comet, meteor viewers have been treated to spectacular showers. When Earth passes just outside the comet's orbit, regardless of whether it has done so just before or just after the comet has reached that point, the showers have been disappointing. What will happen this year when Earth passes inside the orbit, but ahead of the comet, no one really knows. There could be a meteor storm with hundreds of meteor per hour or there could be nothing. The only way to make sure you don't miss anything is to watch for the shower. Go out on October 8th from the time it gets dark at 8:00 p.m. until moonrise at 9:30 p.m. The meteors will appear to come from the northern part of the sky.
If you miss out on the Giacobinids don't despair, just wait for the Leonids in November. The comet responsible for the Leonid shower, Temple-Tuttle, crossed Earth's orbit in March, 1998 so astronomers expect this shower to be very active. Unfortunately for those of us in North America, the peak of the shower is expected to occur during the afternoon of November 17th. This means that the prime meteor viewing area will be Japan and western Asia where hundreds or thousands of meteors per hour may be seen. In 1966, the last time the comet was in the vicinity of Earth, nearly 800 meteors were seen from Arizona in only 20 minutes.
For those of us in the central United States, our best chance to see Leonid meteors will occur on the evening of the 17th and morning of the 18th. Although we won't see the hundreds or thousands that western Asia may see, rates of 30 to 40 per hour are predicted. As with many astronomical predictions, there is a degree of uncertainty, so if the peak is a few hours later than predicted, we may see more.
The best time to look will be after midnight on the morning of the 18th. (The shower will continue to be visible for the next couple of mornings but with a decreasing number of meteors.) The meteors will appear to come from the direction of the constellation Leo which rises in the northeast at about 1:00 a.m. As with all meteor observing, dark skies are essential, so make sure you get away from the lights of the city and that your eastern horizon is dark. Other essential meteor observing equipment includes a reclining lawn chair or blanket, bug spray, and some patience. The best meteor watching technique is to just lay back and watch the sky.
If you would like additional information about events taking place in the sky, call the Lake Afton Public Observatory program and sky information line at WSU-STAR (978-7827). Good luck meteor hunting!