Friday, August 31, 2012
An easy and fun way to get started in astronomical observation is meteor watching.
Meteors appear as fast little streaks of light you see in the night sky. They are often called falling or shooting stars, although this name is misleading since they are not stars at all.
The solar system is filled with rocky pieces of space debris called meteoroids, most of which range in size from grains of sand to small pebbles.
These pieces of space debris can enter the Earth’s atmosphere at very high speeds, seven to 46 miles per second. Most are then vaporized by the heat of the air friction, forming the white-hot streaks we see.
If the pieces are large enough to survive the atmosphere and not be completely vaporized, they will fall to the Earth’s surface. These rocky fragments are called meteorites.
However, not all meteoroids that enter the Earth’s atmosphere create meteors. Most of them are micrometeoroids, space debris particles only about the size of dust grains. Because these particles are very small, when they hit the Earth’s atmosphere the air resistance slows them so quickly that instead of burning up, they fall gently to the surface of the Earth. Although we cannot see them, it is estimated that 5,000 to 10,000 tons of this material falls to Earth each day.
The word Meteor is derived from the Greek word meteoros, which means “high in the air.” Meteors become visible at very high altitudes — 50 to 70 miles — with the faster moving particles visible at the greater heights. They lose their brightness when they are still 10 to 12 miles above the ground.
Meteors most often appear white, but they can be blue, green, yellow, orange or red. If they appear brighter than the planets, they are called fireballs; if they explode at the end of their flight, they are called bolides.
During any night, at a dark site there are on average two to seven meteors per hour. Because these meteors come from random directions and appear anywhere in the sky, they are called sporadic meteors.
There are some nights when you have a better chance of seeing meteors. As the Earth orbits the sun, it passes through the orbits of comets. These areas are filled with debris shed from the comets and therefore produce displays of meteors at a higher rate than normal — a dozen or more meteors per hour. These events are called meteor showers and occur at about the same time every year. In dense regions of the debris, the rates can be as high as several hundred (a meteor outburst) to more than 1,000 per hour (a meteor storm).
In meteor showers, the meteors are on parallel paths; as a result they all appear to come from the same point in the sky, which is known as the radiant. The shower takes the name of the constellation where the radiant is located; for example, the Perseid shower has a radiant in the constellation Perseus.
There will be several major meteor showers between now and the end of the year. The first one, the Draconids, occurs Oct. 7-8, with an average maximum rate of 10-20 per hour. Next come the Orionids, Oct. 20-21, with a rate of about 25 per hour; then the Leonids, Nov. 17-18, 10-15 per hour; and finally, the Geminids, Dec. 13-14, 60-120 per hour.
Because of the direction of the Earth’s rotation and orbit, the best time to look for meteors is after midnight, with the very best time being one to two hours before dawn. This is because at that time you are facing the direction of the Earth’s motion and hitting the particles head-on, as when driving more bugs hit your car’s windshield than the rear window. So get a reclining lawn chair and a cup of hot chocolate, find a dark site and give meteor watching a try.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.