Geminids: The meteors are — for those willing to brave the chill of a December night — a fine winter shower, and usually the most satisfying of all the annual showers.
November's Leonid meteor shower may have passed us by, but that doesn't mean there aren't any good meteor displays to look forward to as the end of 2010 draws near. In fact, one of the best is just around the corner, scheduled to reach its peak Monday: the Geminid meteor shower.
The Geminids get their name from the constellation of Gemini, the Twins, where the display of so-called "shooting stars" appears to originate from the night sky. During the overnight hours of Dec. 13 and Dec. 14, the night of this shower's peak, the meteors should appear to emanate from a spot in the sky near the bright star Castor in Gemini.
This sky map shows where to look for the Geminid meteor shower when it peaks on Dec. 13 and Dec. 14.
December's meteor gems
The Geminid meteors are — for those willing to brave the chill of a December night — a fine winter shower, and usually the most satisfying of all the annual showers. They can even surpass the famous Perseid meteors of August at their peak.
Studies of past displays show that this shower has a reputation for being rich in slow, bright, graceful meteors and fireballs as well as faint meteors, with relatively fewer objects of medium brightness. Many appear yellowish in hue. Some even seem to form jagged or divided paths.
According to the late meteor specialist Neil Bone, at 2 grams per cubic centimeter (0.07 pounds per cubic inch) 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 typically encounter Earth — about 22 miles (35 km) per second — about half the speed of a Leonid meteor, and you have a recipe for meteors that linger a bit longer in view than most.
The Earth moves quickly through this meteor stream. The rate of meteors increases steadily for two or three days before maximum, reaching roughly above a quarter of its peak strength, then drops off sharply, lasting for only about a day afterward.
Those late Geminids, however, tend to be especially bright. A few renegade forerunners and late stragglers might be seen for a week or more before and after the peak night. One interesting finding made recently from video analysis by the International Meteor Organization was that Geminids have been detected as early as Nov. 30 — totally unexpected from past visual observations.
Some lunar Interference
The Geminids usually perform splendidly every year, although as was the case for last month's Leonids, the moon is going to pose a bit of a problem this time. In fact, the moon will reach first quarter phase on Dec. 13, the same night as the Geminid peak, shining brightly in the dim constellation of Pisces, the Fishes. That means that many of the fainter Geminid streaks will likely be washed out by the bright moonlight.
But unlike the Leonids, where the moon was brightly illuminating the sky most of the night, in the case of the Geminids the moon will be setting around 12:30 a.m. (your local time) early on Tuesday, Dec. 14. That means that the sky will be dark and moonless for the balance of the night, making for perfect viewing conditions for the shower.
In addition, according to Margaret Campbell-Brown and Peter Brown in the 2010 Observer's Handbook of the Royal Astronomical Society of Canada, peak activity is projected to occur at or near 6 a.m. EST on Dec. 14.
Under normal conditions on the night of maximum activity, with ideal dark-sky conditions, at least 60 to 120 Geminid meteors can be expected to burst across the sky every hour on average (light pollution greatly cuts these numbers). So, in 2010, along with a lack of significant moonlight, North Americans are projected to be the best situated to catch the very crest of the shower, when the hourly rates conceivably could exceed 120!
British meteor astronomer Alastair McBeath also points out that a new detailed analysis confirms that Geminid near-peak activity is very persistent with hourly rates of around 80 to 130 often seen for almost a day around the predicted time of maximum, corresponding this year from about 1900 UT on Dec. 13 to 1600 UT on Dec. 14. So from virtually anywhere on Earth, an excellent Geminid show can be anticipated.
A productive Geminid watch can actually begin as early as 10 p.m. local time, because the shower's radiant (where the meteors appear to originate from) is already fairly high in the eastern sky by then. Even with that annoyingly bright moon still high in the western sky, it will be worth watching for some early "Gems."
But keep this in mind: At this time of year, meteor watching can be a long, cold business. You wait and you wait for meteors to appear. When they don't appear right away, and if you're cold and uncomfortable, you're not going to be looking for meteors for very long. Therefore, make sure you're warm and comfortable.
Warm cocoa or coffee brought along in a thermos can take the edge off the chill, as well as provide a slight stimulus. It's even better if you can observe with friends. That way, you can keep each other awake, as well as cover more sky. Give your eyes time to adapt to the dark before starting.
Debris from a dead comet?
The Geminids will be especially noticeable right after the moon sets, as their radiant point will be passing nearly overhead. The higher a shower's radiant, the more meteors it produces all over the sky.
The track of each meteor does not necessarily begin near Castor, or even in the constellation Gemini, but it always turns out that the path of a Geminid extended backward along the direction of flight passes through a tiny region of sky about 0.2 degrees in diameter (an effect of perspective). In apparent size, that's less than half the width of the moon.
As such, this is a rather sharply defined radiant as meteor showers go, suggesting the stream is "young" — perhaps only several thousand years old.
Geminids stand apart from the other meteor showers in that they seem to have been spawned not by a comet, but by 3200 Phaeton, an Earth-crossing asteroid. Then again, the Geminids may be comet debris after all, for some astronomers consider Phaeton to be the dead nucleus of a burned-out comet that somehow got trapped into an unusually tight orbit.