HERE COME THE LEONIDS
Dr. Terry Goforth
Associate Professor of Physics, SWOSU
November 2000

Well, Halloween is over. Thanksgiving has yet to come. It must be time for the Leonid meteor showers again.

In 1999, experts cautiously predicted a meteor storm (when meteor activity is far above normal) for Europe and Northern Africa. The peak of the storm was predicted to occur on November 17 between 7 p.m. and 10 p.m. CST. Activity rates were forecast to be around 500 to 1000 meteors per hour. Experts and observers in Europe and the Middle East weren’t disappointed. Activity peaked at 8:05 p.m. with maximum rates of 1800 meteors per hour recorded in Spain. Unfortunately for North American observers, the storm was over before we were in position to see it.

The 2000 shower will be dampened by the third-quarter moon which will make it difficult to see all but the brightest meteors. However, experts are predicting two, possibly three, mild outbursts–nothing like last year’s storm, but a nice show.  For North American observers, they suggest a possible burst of activity on Friday morning (Thursday night) around 1:50 a.m. CST with activity rates around 215 meteors per hour, and a second more likely burst on Saturday morning (Friday night) around 1:51 a.m. CST with activity rates around 100 or 700 (depending on who’s predicting) meteors per hour. Of course, sporadic activity can occur any time on November 17 or 18 as we pass through the broad orbit of the comet.

A meteor is observed when a small bit of space debris (typically the size of a pebble or smaller) enters the Earth’s atmosphere at very high speed, usually in excess of 25,000 mph. The pebble gets very hot and glows brightly as it vaporizes in the atmosphere. The resulting streak of light is often called a shooting star. Random meteors fall all the time, but when the Earth moves through a comet’s orbit, the rate at which meteors can be seen will increase, and the result is a meteor shower. Comets, best described as dirty snowballs, are a loose pack of frozen water, methane, and ammonia (collectively referred to as ices) with small pieces of rocky material mixed in. Far from the sun where it is very cold this frozen snowball is stable, but when a comet comes into the warmer regions “close” to the sun, the ices in the comet will sublime (convert directly from a frozen state to a gaseous state, like dry ice) and some of the rocky material will be freed from the snowball. No longer a part of the comet, these particles will continue to move around the sun in the same orbit as the comet. Thus, a comet’s orbit is littered with small debris that has been released during previous trips near the sun.

Meteors can be observed at any time in a dark sky, but the best chances for seeing meteors occur after midnight. As the Earth moves around the sun, those regions on the earth where the local time is after midnight and before noon are on the leading edge of the Earth. Regions where the local time is after noon and before midnight are on the trailing edge. (See diagram.) Now think about driving a car in the summer. An occasional bug hits your windshield, but if you drive into a swarm of bugs, several hit the windshield in a very short period of time. For the car, most of the bugs spatter the windshield while very few hit the rear window. Similarly, as the Earth moves through this swarm of cometary debris, most of the particles dive into the Earth’s atmosphere on the leading edge of the Earth, while very few enter the atmosphere from the trailing edge.

More meteors strike the leading edge of the Earth than strike the trailing edge.

Each year in mid November, the Earth passes through the orbit of Comet Tempel-Tuttle producing the Leonid Meteor Shower. This meteor shower usually results in about 10-20 meteors per hour, or about one every 3-6 minutes. However, every 33 years Comet Tempel-Tuttle makes a pass around the sun, leaving additional debris behind it. For the next 2-4 years, this debris and swarms of debris from previous passes trail the comet, giving Earth a chance at significantly increased meteor activity. Tempel-Tuttle’s last pass around the sun occurred in 1998, so we are likely to experience increased activity this year and possibly in 2001 and 2002.

Meteors associated with a meteor shower always appear to radiate from a single point in the sky. Again, think about driving your car, this time through a snowstorm. The snowflakes surround you, passing by you on all sides, but they all seem to be coming from a point directly in front of the car. This is what we see during a meteor shower. All the meteors seem to be coming from a point in front of the Earth (this point is called the radiant), but they shoot through the atmosphere in all parts of the sky. Meteor showers are named for the constellation nearest the radiant. The upcoming meteor shower is known as the Leonid Meteor Shower because the meteors appear to radiate from the constellation Leo, but they’ll be visible in all parts of the sky. In fact, this year the moon will be right in Leo, making it almost impossible to see many meteors in that direction. The best advice is to sit back (a recliner is best) so you can take in as much of the sky as possible, looking away from the moon. If you can’t get out exactly at the predicted time for the peaks, anytime between midnight and sunrise is usually best (that’s when we’re riding on Earth’s “windshield.”)

Meteor observing tips from the experts:

Of course, cloudy skies can put an end to any observing plans. But if the weather interferes this year, take heart. Some experts are already predicting a storm with 1500 meteors per hour visible from North America in 2001.

Get out and enjoy!

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1999 Leonids
2001 Leonids
2002 Leonids

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Last update:  November 15, 2002