The Meteoric Gales of November

© 1998 by Robert C. Moler

On the evening of November 16-17 1996 my bride and I were outside the observatory of the Shreveport Astronomical Society gazing up through partly cloudy skies at the Leonid meteor shower. The display was about as active as the Perseids. But the cold and having to be at work the next day we decided to end our viewing session at 2 a.m. However by 4 a.m. the shower picked up dramatically. Over the western US all hell... well actually, all heaven broke loose, with reported guestimated counts of hundreds a minute to thousands an hour. I've regretted that decision to this day.

Actually the big display was supposed to have occurred in 1965, but it didn't materialize. The Leonids are a lackluster shower most years however every 33 or so years the Leonids are spectacular. Such was the shower of 1933. The reason is that the Leonids like all meteor showers are associated with a comet.

Comets are known polluters, shedding dust, gas and bits of rock every time they pass near the sun. The gasses and dust get blown away by the solar wind of charged particles and pressure of sunlight. The larger bits then slowly migrate away from the comet along the orbit ahead of and behind the comet. So if a comet's orbit crosses the earth's orbit we have a chance to see a meteor shower in the day of the year the earth itself crosses that spot. Comet orbits are reasonably fixed in space unless the comet comes close to a planet. Still even the most stable comet's orbit will change a bit with time. And each particle is independently influenced by, primarily the suns' gravitation and also the minute pull of all the other planets. With time the debris spreads out both farther along the orbit from the comet, but also away from the comet's orbit. The Perseid meteor shower peaks on August 12th, but its meteors can be seen three weeks before and a week after that date.

Since the orbits of the meteoroids are parallel to each other the seem to come from the same direction in the sky. The Leonids are so named because the come from the head of the constellation Leo the lion, inside the left edge of the asterism of the Sickle. The point in the sky where the meteors seem to diverge from due to perspective is called the radiant. For the Leonids it's located around 10 hours right ascension and +22 degrees declination.

The comet in the case of the Leonids was independently discovered in 1866 by Ernst Tempel and Horace Tuttle. The comet had a short period as comets go of just over 33 years. Also the comet's orbit is opposite the direction of the planets; clockwise when seen from the north. That means the comet, if it should ever hit the earth would hit head on at 71 kilometers per second (km/s). That's more than twice earth's velocity orbiting the sun. The Space Shuttle orbits the earth at a mere 8 km/s. Luckily the probability of the comet nucleus striking the earth is remote. However the cometary debris does hit the earth every year as the Leonid meteor shower. In addition there seems to be a huge amount of debris following a year or behind the comet. And when we encounter that we get to witness a meteor storm.

The nights of November 16-17 both 1998 and 1999 are prime candidates for the meteor storm because Comet Tempel-Tuttle passed by earlier this year. If this is like 1965 when the comet last passed through, the meteor storm occurred during the 1966 shower rather than the 1965 date.

So what are the prospects for 1998 and 1999? This November 16-17 the moon will be out of the way, a thin crescent two days before new rising around 5:55 a.m. for Traverse City. For 1999 the first quarter moon will set at 12:38 a.m. and be out of the way for the prime viewing hours. The radiant will be above the horizon starting about 11 p.m. and will be rising the rest of the night reaching its highest in the south shortly before 7 a.m. However morning twilight starts about 6 a.m. The weather in Western Michigan is quite cloudy in November, so many amateur astronomers are heading to clearer parts of the country and the world for this event. As for me, I'll make sure I'm dressed warmly.

(This is a reprint, with some changes, of an article by the author for the Inside Orbit, a publication of the Grand Rapids Astronomical Association.)

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Updated: 12/30/99