EVENT:  The annual Leonid meteor shower
DATES:  Early Saturday and Sunday mornings, November 17 and 18.
BEST OBSERVATION TIMES:  Midnight to 5 am, with a peak predicted for 3 to 5 am on Saturday the 17th.
DIRECTION:  Overhead and toward the east and southeast.
EQUIPMENT:  Warm clothing.


Way, way out in the cold reaches of the Solar System, way beyond the orbits of Pluto and Neptune, thousands of gritty snowballs drift along in lazy orbits.  They range in size from dozens of feet to dozens of miles.  Occasionally one of these snowballs will swing close enough to be nudged by the gravity of a neighbor, dropping into a long orbit that brings it past the sun.

The heat of the sun will boil off an outer layer of ice and the imbedded dust and grit to produce a cloud thousands or millions of miles across and, behold, we have a comet swinging around the sun shedding gas, dust, grit, and dirt like a jalopy burning oil.  This cloud of cast-off debris orbits the sun, following more less in the comet's path.

A meteor shower occurs when Earth's orbit carries it close to the orbit of a comet where it runs through a cloud of that boiled-off debris.  Astronomical radar studies — pulses sent from arrays of huge radio telescopes — and decades of visual observations have succeeded in mapping the approximate locations of individual clouds of comet debris so the chances of a particularly weak or strong shower can be estimated.  This is still an inexact science, of course.

Virtually all meteor showers seem to be fed by debris from comets although a few have been linked to asteroids.  The Orionid meteors come from the dust and grit that Halley's Comet has shed — and will shed again on its next loop around the sun.  The Leonid meteors come from the sheddings of a comet named Tempel-Tuttle, discovered in the nineteenth century.

This year's Leonid meteor shower is expected to be good but not great.  The key word here is "expected":  despite marked improvements in the art of predicting meteors, the forecasts are still inexact and subject to surprises.  The coming shower could be disappointing.  It could also be great.
   "Disappointing" is easy to understand.  For a reminder of what "great" means, here are some photographs of previous Leonid showers:

<http://apod.nasa.gov/apod/ap101212.html>

<http://apod.nasa.gov/apod/ap061118.html>

<http://apod.nasa.gov/apod/ap021030.html>

Because of orbital geometry and the inability of astronomers to understand basic marketing principles, most meteor showers are best observed in the hours before dawn and the Leonids are no exception.  Anyone seeing a clear sky in the wee hours can enjoy, in addition to the meteors, a brilliant Jupiter above the Pleiades star cluster in the west, mighty Orion coasting toward bedtime in the southwest, and the planet Venus rising in the east just to the left of the blue-white star Spica.  The Big Dipper will be high overhead and below it, Leo the Lion, the constellation which gives this meteor shower its name.

Telescopes and binoculars are not generally useful for watching fast-moving meteors, but they do a marvelous job of bringing out detail in the Pleiades, the Orion Nebula, and in Jupiter and also in Saturn, which will rise below Venus in a sky brightening with the coming dawn.

An illustration of the eastern sky with the Leonid radiant is at <http://meteorshowersonline.com/leonids.html>.


     Keep looking up!
     - Bob Hamlin
     <rhamlinatdartmouth.edu<http://rhamlinatdartmouth.edu/>>




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