Posted on 12/12/2001 1:47:31 AM PST by 2Trievers
Above: A midwinter night's meteor. Lance Oldham caught this dazzling Geminid skirting the Big Dipper's Bowl in 1988 while watching the shower from Red Rock Canyon in California. This year's shower peaks on the largely moonless night of December 13–14. Copyright 1989 Lance Oldham.
Meteors, meteors, meteors! Having looked forward to both a rare Giacobinid storm on October 8th and the Leonids in November, it might seem hard to get excited about December's same-as-ever Geminid shower, which is due to peak on the night of December 13–14. Besides, for the Northern Hemisphere observers favored by the Geminids, mid-December is a mighty cold time for lying on one's back at night staring up at the sky.
But experienced meteor watchers know better; indeed, many of them favor the Geminids over August's better-known Perseids as the year's most satisfying shower. And for good reason. The shower's shooting stars radiate from a point (at 7h30m, +33°, near Castor) that is nearly as far from the Sun as one can get in mid-December. As a result, the radiant rises to a respectable elevation only a few hours after sundown and remains high for the rest of the night, crossing the zenith around 2 a.m. (for observers at midnorthern latitudes, that is). This means you can see the shower at a reasonably convenient hour – any time after about 10 p.m. The activity peak lasts for about a day, enabling all longitudes to share in the action, and it is generous: in recent years Geminid meteors have come at maximum rates of 110 to 130 hourly as seen by a single observer under ideal conditions. (Light pollution greatly cuts the numbers.)
December 1998 favors Geminid viewers with a waning crescent Moon that rises only in the morning's wee hours – well-deserved compensation for last year's full-Moon floodlighting. As suggested by the diagram below, North Americans and Hawaiians should be best situated to catch the very crest of the shower, when zenithal hourly rates should exceed 100 (assuming that the shower behaves as it did in 1996, its most recent Moon-free year). By contrast, observers in Asia and Europe will have to content themselves with the still-respectable flanks of the shower's broad, lopsided profile. The Geminids should also be active at lesser levels for several nights before and after the peak.
Above: In recent years the Geminid meteor shower has displayed a broad peak that rises slowly to a maximum rate of nearly two per minute before dropping more rapidly. It wasn't always so, however; in the 1940s the profile rose rapidly, then declined slowly. The difference attests to the changing profile of the debris stream that Earth crosses each December 13th or 14th. The change is due to the stream's slow drift with respect to Earth's orbit. A close look at the Geminids for 1996 should help observers predict how they might fare this year. This graph incorporates 19,604 Geminids that 119 observers counted and reported to the International Meteor Organization. Vertical bars show the range of statistical uncertainty for each point.
The Geminids stand apart from most meteor showers by dint of their origin: they seem to have been spawned by 3200 Phaeton, an Earth-crossing asteroid in an unusually elongated orbit that gets as close as 21 million kilometers (0.14 astronomical unit) to the Sun. By contrast, most other meteor showers are attributed to comets (that is, when their parent bodies can be identified). Then again, the Geminids may be comet debris after all: some astronomers consider Phaeton to be the dead nucleus of a burned-out comet that somehow got trapped into an unusually tight orbit (S&T: December 1990, page 587, and October 1985, page 317). Astronomers have yet to convincingly explain Phaeton's orbital and physical properties. But when they do, careful amateur observations of the Geminid meteor shower may play a role (the methods for making a scientifically useful meteor count are given on page 90 of the August 1997 Sky & Telescope and in our How to Observe Meteors page).
Decades of careful visual counts have enabled researchers to map out the complex structure of the Geminid debris stream, while two-station photography of individual meteors has shed light on the particles' trajectories and densities. According to meteor specialist Neil Bone, at 2 grams per cubic centimeter on average, Geminid meteoroids are several times denser than the cometary dust flakes that supply most meteor showers. Add this to the relatively slow speed with which Geminids encounter Earth (35 kilometers per second), and you have the recipe for meteors that linger a bit longer in view than most.
Joshua Roth is technicl editor of Sky & Telescope and an avid backyard observer.
I went out on my front lawn at 4:00 AM for the last one, and was rewarded with seeing more than one per minute. That's in suburban Atlanta, with loads of light pollution.
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