On just about any evening under a dark sky, you?re likely to see a meteor or two streak across the starscape. They catch you by surprise, serving as a reminder that space is not static ? things are moving out there, and fast.
What Are Meteors?
Often called "shooting stars," meteors are really particles from outer space ? fragments of comets and asteroids ? that burn up from friction as they enter the Earth?s atmosphere. While still in space, the particles are called meteoroids. A very bright meteor is about the size of a grape; typical meteors are more the size of tiny pebbles or grains of sand. A very large meteor may break up as it penetrates the atmosphere, throwing off sparks. Occasionally, a meteor may even make noise as it shoots through the air, though this is rare. The largest meteors do not completely incinerate and actually land on Earth as rocks, called meteorites.
A fireball is a very bright meteor loosely defined as being brighter than the planet Venus, whose maximum magnitude is -4.7.
Ordinarily, you can see about one meteor per hour at a dark-sky location if you watch the sky continuously. These are sporadic (random) meteors, not associated with specific meteor showers. You will see more meteors after midnight, since you are then on the "forward" side of the Earth as it moves along in its orbit.
Meteors En Masse ? Meteor Showers
A meteor shower happens when the Earth enters a swarm of meteoroids, usually debris from a comet that trails along in the comet?s orbit. At certain times of the year, the Earth?s orbit intersects that of a comet, and we get pelted with the debris.
Then, the meteor rate rises to 10 or 20 per hour, and possibly even 100 or more per hour at the maximum of some showers! The meteors in a shower all appear to come from the same general area in the sky. More precisely, if you trace the path of each meteor backward, you?ll find that they converge near one point in the sky. That point is called the radiant, and it is different for different showers. Each shower is named for the constellation in which its radiant is located. For example, the Perseid shower in August has its radiant in the constellation Perseus. It is often hard to tell whether a meteor belongs to a particular shower or not, since even during a shower, some meteors appear far from the radiant or are moving in slightly different directions.
Because they occur at particular positions in the Earth?s orbit around the Sun, meteor showers recur on the same date every year. Most showers last two or three days; some are longer, and some are very brief.
Some meteoroid swarms have orbital periodicities of their own, so we don?t encounter them every year. For example, the Leonids were spectacular in 1933, 1966 and 2001. In the "off" years, few or no Leonid meteors are seen.
Below is a list of major showers; there are many minor ones producing smaller numbers of meteors. One major shower, the Quadrantids, is named for an obsolete constellation corresponding to part of Bootes.
Table of Annual Meteor Showers
|Date (every year)||Name of Shower||Remarks|
|Jan. 1-5||Quadrantid||Brief maximum, Jan. 3 or 4. Radiant in Bootes.|
|April 21-23||Lyrid||Brief and variable|
|May 1-8||Eta Aquarid||Radiant in Aquarius|
|July 15 - Aug. 15||Delta Aquarid||Maximum July 27/28|
|July 25 - Aug. 18||Perseid||"Old reliable" ? strong and regular. Maximum Aug. 12.|
|Oct. 16-26||Orionid||Maximum Oct. 21|
|Nov. 15-19||Leonid||Spectacular in 1933, 1966, 2001. Check astronomy magazines for predicted maximum.|
|Dec. 11-17||Geminid||Usually a fine shower|
Meteor watching is like fishing. You cast your sights upward, sit back, wait a while, and see if you get any bites. You?ll increase your chances by going to a dark-sky location, away from suburban light pollution. Most meteors aren?t very bright, so you?ll see more of them if you go to a dark country site. Your chances of logging meteors are also improved when the Moon is below the horizon, or during the new Moon phase, as moonlight can wash out all but the brightest fireballs.
Observing meteors is a naked-eye activity. You don?t need, or even want, a telescope or binoculars to view meteors, as these will only restrict your field of view. You want to be able to visually canvas as much of the sky as possible, because meteors can flash anywhere. For scientific work you?ll need a star chart (for plotting meteor paths) and a ruler (to hold up against the sky to note exactly where a meteor went). More important is a lawn chair and, if appropriate, blankets to keep you comfortable during your vigil.
You will notice that meteors not only differ in brightness, but also in speed and length of travel. Some appear to move relatively slowly, glowing for a second or two while others streak quickly and are gone in a fraction of a second. Some of the brightest meteors leave vapor trails in the sky that can linger for several seconds after the meteor has disappeared.
While looking for meteors you?ll see plenty of airplanes and satellites, too. How do you distinguish them from meteors? For one, meteors move much faster. Another dead giveaway for an airplane is a blinking light. Binoculars will help you identify airplanes, which normally have more than one light. A satellite looks like a slowly moving star. An iridium satellite occasionally reflects sunlight so that it looks like a bright, slow meteor; however, the movement is much slower and it fades out rather than burning up suddenly.
Occasionally, you?ll see a tiny, distant meteor in binoculars or a low-power telescope. These are called telescopic meteors and are so rare that you probably shouldn?t spend time looking for them ? but take note when you happen to see one while viewing other things.
Photograph a Falling Star
Photographing meteors is relatively easy ? if you encounter a meteor at the right place and right time! The technique is to use a DSLR or camera with a manual exposure setting using anormal or wide-angle lens (preferably f/2.8 or faster) and take a long exposure of the stars. If a bright meteor comes through the field, the camera will probably catch it.
The camera can stand on a fixed tripod (recording the stars as trails due to the Earth?s motion) or on a clock-driven telescope (to get pinpoint images of the stars). Light pollution and your local sky qulaity conditions will limit how long the exposure can be before the image quality degrades. Experiment with different exposures and ISO settings to determine the best results for your set-up and location.
You?ll likely catch a lot of airplane and satellite trails in your pictures, too. You can distinguish them from meteor trails by their uniform thickness (or the regular-spaced, dots from an airplane?s blinking light). A meteor trail is usually not uniformly thick, but rather almost always exhibits a bright "head" where the meteor burned up.