Meteors provide celestial fireworks

It's a sight many of us have experienced. The sun has long-since disappeared below the western horizon as clear skies reveal the first stars of the evening. With an upward glance, you pause for a moment to notice the deepening shades of color in the sky as encroaching darkness signals the end of another day. Suddenly, your reverie is broken by a streak of light that darts across the sky, perhaps lasting only a second or two. In all likelihood, you have just witnessed a shooting star, more properly referred to as a meteor.

OK, fine. But what is a meteor, exactly?

Meteors are caused by the entry of small pieces of rock, dust, ice or metal from nearby space into Earth's atmosphere at extremely high speeds. Most of this material is surprisingly small, ranging in size from grains of sand to pea-size particles. As these bodies enter the atmosphere, they are heated to very high temperatures by friction with the air.

The tremendous energy that is generated is transferred to the surrounding atoms and molecules in the upper atmosphere, thus producing the bright trails of light we see in the sky. Traveling at speeds up to 44 miles per second (that's nearly 160,000 miles per hour!), meteors begin to glow at altitudes of 40 to 75 miles.

It's time to introduce some terminology. Technically, particles of debris that are still floating around in space before they enter Earth's atmosphere are called meteoroids. The streaks of light we see in the sky are meteors. Objects large enough to survive the fiery journey through the air and land on the ground (and aren't completely vaporized by the impact), are known as meteorites.

If there is no interfering light from the moon or a nearby city, an observer can usually see four to seven meteors per hour (fewer in the evening, more in the early morning). These numbers can jump dramatically, however, when Earth encounters a greater number of meteoroids. When this happens, it is referred to as a meteor shower.

Meteor showers occur when Earth plows through streams of particles left behind by periodic comets. As comets enter the inner solar system, heat from the sun warms the frozen nucleus, causing the release of gases and debris that become distributed along the comet's orbit. It is this material that sometimes forms the tail of a comet.

On rare occasions, Earth encounters a particularly dense cloud of cometary fragments that will produce more than 1,000 meteors in an hour. Such an event is called a meteor storm.

One of the greatest meteor displays in recent history was caused by the 1833 Leonid storm. At its peak, it was estimated that more than 200,000 meteors per hour filled the night sky; that's 55 every second! In 1966, the count was in excess of 10,000 in an hour. Then, in 1999 and again in 2001, the rate was about 3,000 per hour. These particular meteors occur when Earth moves through the debris field of comet Temple-Tuttle, in mid November.

Inconveniently for most of us, meteor counts are normally highest just before dawn. This is because during this time observers are looking toward the direction of Earth's motion through space. An analogy would be like riding in a car when it's raining. More raindrops can be seen hitting the front-facing windshield than the back window because of the car's forward motion. Searching for meteors shortly after dusk is like looking out the back window of the car.

Of the seven primary meteor showers that occur every year, the reliable and prolific Perseid shower is probably the best known. Occurring during the second week of August, it gets its name from the fact that the streaks of light appear to emanate from the constellation of Perseus. At its peak, this shower can produce 60 to 100 meteors in an hour. More people have seen meteors from this shower than any other, in large part because the show can be enjoyed in the relative comfort of warm summer nights.

But there is another display that produces at least as many flashes of light as do the Perseids. These meteors radiate from the constellation of Gemini. The good news is the Geminid shower can generate high meteor counts (anywhere from 65 to 100 per hour, on average), and activity levels are greater earlier in the evening than for most other showers. Now the bad news - it peaks around December 13 or 14. And during the winter months, when it's clear, it's usually cold!

The 2009 edition of this one is just around the corner, and moonlight will not interfere. So, for those who are up for the challenge, put on your Nanook-of-the-North outfit, grab a few blankets and a thermos of hot drink, settle back on a chaise, and enjoy the show.