One of the delights of dark skies is to see a sudden brief shining streak across the sky. They're commonly known as shooting stars and many ancient cultures thought they were souls ascending to heaven.
“Shooting stars” are correctly known as meteors, bits of space debris burning up in the Earth's atmosphere. The word meteor comes from a Greek word referring to atmospheric phenomena. Greek philosopher Aristotle (384 BC – 322 BC) thought they were related to the atmosphere and weather. This is how meteorology came to be the study of weather rather than meteors.
Meteoroids are bits of space debris that orbit the Sun. Most of them are small, though their sizes range from tiny particles to great boulders. Some meteoroids are material thrown into space by collisions involving other Solar System bodies. Others are leftovers from the formation of the Solar System. Still others, as we'll see, come from comets.
When a meteoroid enters the Earth's atmosphere, we call it a meteor. Friction with the air heats it to 3000 degrees Fahrenheit (1650 degrees Celsius), vaporizing it and leaving behind glowing gases and particles. This is what creates the meteor trail, which does look like a bright star shooting across the sky. It usually lasts for no more than a second or so.
Most incoming debris simply burns up in the atmosphere, but some pieces are large enough to reach the ground. These remnants are called meteorites. Since very little extraterrestrial material has been collected by astronauts or space probes, meteorites are of great interest to astronomers.
Yet sometimes there is a not only a noticeable increase in meteor activity, but of meteors originating from the same region of the sky. This is a meteor shower and the apparent point of origin is called the radiant. The meteoroids that create these showers come from comets.
When a comet comes near the Sun, it warms up. The warming comet ejects gases and a lot of debris, which it leaves as a trail along its orbit. When our orbit takes us through this debris, a meteor shower occurs. They recur on the same dates every year.
One of the best known meteor showers is the Perseids. A meteor shower is called after the location of its radiant and the Perseids seem to come from the constellation Perseus, named for the Greek hero who saved Princess Andromeda from the sea monster.
The Perseids occur in mid-August and records of heightened August meteor activity go back over two thousand years, with the earliest accounts by Chinese astronomers.
The Perseids were the first meteor shower to be connected to a specific comet. Late nineteenth century Italian astronomer G.V. Schiaparelli made calculations relating them to comet Swift-Tuttle. Since then the same has been done for the other main meteor showers.
Earth can encounter debris from Swift-Tuttle from mid-July on through the third week in August, with the peak activity occurring somewhere in the period August 9-14. Sometimes the Perseids are known as the "tears of St Lawrence" since his feast day, the 10th of August, is within this peak period.
People in the northern hemisphere get the best view of the Perseids because in the southern hemisphere the radiant is below the horizon. Nonetheless southern hemisphere observers can see a worthwhile number of meteors coming up from the horizon.
As the Earth turns, the radiant gets higher in the sky. The higher the radiant the more meteors you'll see, so watching after midnight is worthwhile. The very best time is an hour or two before dawn.
Although it's called a shower, don't expect meteors cascading like raindrops. At best, under clear, dark skies and with an unobstructed horizon, you might see two or three a minute, but there will also be gaps of several minutes in which nothing seems to be happening. The brightest of them will outshine the brightest stars. As well as the meteors streaking away from Perseus, you may see some random meteors going in a completely different direction. These are not part of the shower.
Also there is a good chance of seeing meteors before and after the predicted peak, so it's worth looking on more than one evening if you can, although the numbers will be lower.
In choosing a viewing place, the darker the better. Be sure to tape some red cellophane over your flashlight so that you can see without ruining your eyes' dark adaptation. A reclining lawn chair or camping mattress is useful to avoid getting a stiff neck while trying to get a good view of the sky. Even though it's August, remember to take some warm clothing.
By the way, you don't need binoculars or a telescope. You get a wider view of the sky without them. Let your eyes relax – your dark-adapted eyes are very sensitive to motion.
Give yourself plenty of time. Be patient and with any luck, you'll see one of the sky's loveliest light shows.
(1) Sharpton, Virgil L. "Meteor." World Book Online Reference Center. 2005. World Book, Inc. http://www.nasa.gov/worldbook/meteor_worldbook.html
(2) Kronk, Gary W."What Is a Meteor Shower?" http://meteorshowersonline.com/ [accessed 2010-08-01]