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Updated March 14, 2011
Geminids: December 6-19; Peak December 14, 10h UT
The Geminids are one of the richest meteor showers of the current era, and make for a superb counterpoint to the Leonids as showers of utterly different character. In 2002 observers in Alberta will have a reasonable opportunity to observe the Geminids' peak, in the wee hours of Saturday morning, December 14.
The Geminid meteoroid stream has a unique and fascinating orbit. For one thing, the parent body of the meteoroids is not recognized as a comet at all, rather as the minor planet 3200 Phaethon. It is an Apollo-type asteroid, one whose orbit comes within that of Earth. Indeed, Phaethon goes well inside the orbit of Mercury with a perihelion only 0.14 A.U. (~20 million km) from the Sun. The aphelion of the orbit is in the main asteroid belt between Mars and Jupiter at roughly 2.4 A.U. That periodic comets come from further afield, generally the Kuiper Belt beyond the orbit of Neptune, suggests Phaethon is unlikely to be an extinct (or dormant) comet. It is more likely a small (~5 km diameter) main belt asteroid which had its orbit altered in the manner of a periodic comet, due to a gravitational encounter with a more massive body. It may have once contained some volatile surface materials which burned off in the intense solar radiation of past perihelion passages. No sign of a tail or atmosphere has ever been observed on Phaethon, but there is little doubt it has left a debris trail. The asteroid itself crosses the ecliptic at a safe distance, every few decades making a close pass (<3 million km) which always occurs, like the Geminids, in mid-December.
Because the cigar-shaped, prograde orbit intersects Earth's path in a nearly perpendicular manner from the night side, meteor counts remain fairly consistent throughout the night without the usual strong diurnal bias towards the morning hours. Note that the radiant point is 40 degrees west of that of the Leonids, despite occurring almost a month later when Earth has moved some 25 degrees east in its orbit. The radiant point therefore reaches its highest point due south around 2:30 a.m. local time, as compared to 7 a.m. for that of the Leonids.
The Geminids display a curious asymmetrical character in that the numbers of meteors are higher in the days leading up to the peak, whereas individual meteors seen after the peak are generally seen to be brighter, suggesting fewer but larger particles. This is analogous to a large fall of meteorites in which one end of the fall ellipse will contain the few most massive survivors; due to their increased mass-to-surface-area ratio, larger fragments are less subject to atmospheric resistance and therefore travel further. In the case of meteoroids it suggests that the mass of individual particles may affect their orbital evolution.
Another extremely interesting aspect of the Geminids is that the meteoroid stream appears to be receding from the Sun like a wave rippling gently away from shore, and its outward pass through Earth's orbit is a temporary phenomenon. The shower was unknown before the 1860s, and average numbers of meteors rose from only 15-30 per hour in 1900, to 40-70 per hour in the 1930s, to 80+ per hour in the 1970s. One researcher (Miroslav Plavec) concludes that the point of intersection between the meteoroid stream and the ecliptic has been evolving, from 0.02 A.U. inside Earth's orbit in 1900 to 0.10A.U. outside Earth's orbit in 2100. This suggests that the shower reached its peak performance in the second half of the 20th century, and will have diminished to a non-event by the end of the current century. Better look now while you have the chance!
The writer has experienced the 'Gems' to be plentiful, with observed rates of 90+ per hour under good conditions in 1996. They are relatively faint, colourless, and leave little in the way of persistent trains. Geminids are only about half as fast as Leonids at 35 km/s, largely because their collision angle with Earth is side-on rather than head-on, but in part due to the slower real speed of Geminid meteoroids in their more constrained orbits.
Although the Geminids are generally the friendliest shower for those who like to observe in the evening hours, 2002 nonetheless favours the predawn observer due to the unwelcome presence of a familiar culprit. The Moon will be a 10-day-old, waxing gibbous searchlight on the (Saturday) morning of the peak, some 75% illuminated. It sets just before 3 a.m. for observers around Edmonton. Coincidentally, this is exactly at the predicted peak of this year's shower (Dec. 14 ~10h UT), just when Gemini crests high in the south as seen from Alberta. For the next three hours or so the observer in a dark locale can anticipate spotting one to two meteors per minute on average.
The radiant point is located in northern Gemini, near the famous multiple star Castor. For those still learning to tell the "Twins" apart, a good mnemonic is that the five first magnitude stars that cut in a majestic arc the eastern section of the Winter Hexagon are, from top down, in alphabetical order: Capella, Castor, Pollux, Procyon, Sirius.
Because of the shower's broad and asymmetrical nature, the Geminids are worth watching for at least two nights before and one night after the peak. Moonset times in Edmonton: Dec. 12: 00:37 MST; Dec. 13: 01:47; Dec. 14: 02:57; Dec. 15: 04:08. If you observe several nights apart, see if you can spot the southeastern movement of the radiant point itself.
According to results published by the University of Ghent (http://allserv.rug.ac.be/~hdejongh/astro/meteor/meteor.html ), the Geminids are one of the best showers for the radio observer, with high rates of activity for a day or more around the peak. At +32°, the radiant point slips below Edmonton's horizon for only about four hours each afternoon. Geminids should be detected throughout the rest of the day, although observed rates should exhibit a diurnal rise and fall effect as a function of the radiant's "altitude".
Those equipped with radio observatories can monitor their results for the asymmetry that characterizes the shower, which should theoretically show up in observations. Look for more, smaller spikes early in the shower and relatively fewer, stronger spikes after the nominal peak.
For further information
More on the Geminids is available at the following websites: