The moon giveth, and the moon taketh away. The very thing that will make us go gaga this weekend — a close, bright supermoon — will also take a bite out of the Leonid meteor shower. Leonids appear to fly out of the constellation of Leo the Lion, hence their name. The shower occurs every mid-November when the Earth passes through the trail of debris left by comet 55P/Tempel-Tuttle.
Normally during shower peak, which happens Thursday morning Nov. 17 before dawn, we’d see about 15 meteors an hour. But this year, the bright, waning gibbous moon will shower the sky with light from its high perch in the constellation Gemini the Twins. The bright sky will reduce the number of meteors you’ll see in half.
If you’re game for a look or just happen to be out and wonder why there are more meteors than usual visible, the best time to watch is starting around 4 a.m. local time, about an hour and a half before the start of dawn. Leo’s well up in the southeastern sky at that hour. Grab a chair, face east or south and enjoy the show. The Leonids are among the swiftest of meteors, crashing into our planet’s airy envelope at over 160,000 mph (72 km/sec), the reason the shower’s known for bright fireballs (meteors equal to or brighter than Venus) and persistent trains.
Trains are the streak-like trails that can glow for many seconds after the bright meteor flash has disappeared. What you’re seeing is a tube of ionized or electrified air created when the incoming comet crumb’s kinetic energy (energy of motion) is transformed into the jiggling of air molecules. As the molecules return to their previous “rest state” they release energy as light.
While you watch the Leonids, keep in mind that most of the material that makes for meteors is very tiny, on the order of sand grains. Brilliant ones, called fireballs, come from larger pieces of comet about 10mm across — about the size of a shirt button — and weigh half a gram. To date, there have been no known meteorites recovered from comets, which are made of crumbly, friable stuff that quickly burns up in the atmosphere. Asteroids, made of more-tightly compacted rock, are responsible for the meteorites that are recovered after some especially bright fireball sightings.
The Leonids are famous for their “meteor storms” that happen about every 33 years when Earth passes through a denser filament of debris dribbled out by Temple-Tuttle. The last great Leonid show occurred in 2001-2002 when up to 3,000 meteors per hour furrowed the sky from certain locations. The next outbursts aren’t expected to begin until about 2028.