The Geminids: An Exception to the Rule

Geminid meteor
A bright meteor from the 2004 Geminid meteor shower was captured with a tripod-mounted digital camera and a 16-mm lens. It’s a 1-minute exposure at f/2.8 with an ISO setting of 800.
Courtesy Alan Dyer.
Comets unleash streams of dusty debris as they approach the Sun. The Earth, speeding around its orbit, crosses some of these streams. And we see stunning meteor showers.

But that’s not always the case. The Geminid meteor shower, peaking on the night of December 13–14, is a rare exception to this rule. Broken fragments from a pseudo asteroid-comet with a mysterious composition create the Geminids, which are growing in intensity every year.

While the Leonids and Perseids have been documented for centuries, the Geminids were first observed only 150 years ago, spawning scientific query into its origin. In October 1983, after searching survey data from the Infrared Astronomical Satellite, Simon Green and John Davies found an asteroid-like object, later named 3200 Phaethon. Dynamicist Fred Whipple then noted the body’s orbit matches that of the Geminids. It was the first time an asteroid had been linked to a meteor shower.

Geminid meteor
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.
But is it an asteroid or is it a comet? Described as a dormant comet coated with a thick layer of dust, Phaethon has an extremely elliptical, 1.4-year-long orbit. Meteors created as Phaethon advances toward the Sun are much denser than those usually created by comets. Yet they are less dense than typical asteroid compositions. Furthermore, Phaethon doesn’t have a characteristic comet tail and its spectra indicate a rocky surface. While Phaethon’s true nature is up for debate, the annual meteor showers it triggers certainly aren’t.

The Geminid meteors, due to peak in 2004 on the night of December 13–14, may be the best annual shower in a typical year, even surpassing the Perseids of August. This year there’s no glare from moonlight to worry about, because the Moon is a two-day-old crescent setting early in the evening sky. As many as 75 slow, graceful Geminids might be seen per hour under ideal conditions. They tend to be bright and appear yellow.

Rates increase steadily for several days before maximum, then drop off quickly. The meteors that do appear after maximum, however, tend to be especially bright.

A productive Geminid watch can begin as early as 10 p.m. local time, because the shower’s radiant (near Castor) is already fairly high in the sky by then, peaking around 2 a.m. The higher a shower’s radiant, the more meteors it produces all over the sky. Bundle up warmly!

Observers can use our interactive sky chart to see the appearance of the sky at 2:00 a.m. during the peak morning of the Geminids. The chart is set at 40° north latitude for central North America. On the chart, the meteor shower name and symbol is visible in both windows. Click on the "change" button to alter either the date and time or viewing location displayed by the chart.

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