The Leonid meteor shower

Most years, the Leonid meteor shower tends to be rather humdrum. At most, one can expect to see maybe 10 to 20 meteors per hour zip fleetingly across the sky in a dark location out in the country. But every 33 years, this shower suddenly becomes a storm of 'shooting stars.' The last major meteor storm generated by the Leonids occurred in 1966 when observers in the central United States saw over 5,000 meteors per hour.

Why does a meteor storm happen only occasionally? The answer doesn't have as much to do with where meteors are in space as much as it has to do with where comets are. Comets are the primary sources of meteoroids.

Comets are like snow-encrusted cars barreling down a highway in winter, leaving in their wake an icy, blustery trail. When a comet comes out of the deep freeze of space and visits the inner solar system, its surface is warmed by the Sun. Part of its icy crust suddenly turns to vapor, which solar radiation blows off the comet's surface, leaving a trail of debris through which Earth passes at a specific time of the year. This 'blowback' material contains ice, dust, and small gravelly material -stuff that later burns up in our atmosphere during a meteor shower.

The Leonids in particular are composed of debris from periodic Comet Tempel-Tuttle. This comet has a period of 33.18 years. As the

comet approaches the solar system, it brings with it a concentrated trail of debris, which Earth passes through. When this happens, we see a greater display of meteors than usual.

Look for the Leonids on two mornings around November 17. If there is a half Moon or less in the sky, the extra light shouldn't be too much of a problem. If the Moon is full, however, you'll probably only see the brightest of the Leonids. Leo rises in the east around 1:30 a.m., local time, just as Earth is meeting these meteors head on. The greatest number, however, will be seen when the radiant (near the Lion's 'sickle') is halfway up in the eastern sky. That occurs around 4 a.m.

In a typical year, you can expect to see a half-dozen or more meteors per hour from a dark-sky sight. Then again, if we pass through a 'knot' of debris, as occasionally happens with the Leonids, you may see a brief 'flurry' of meteors.

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