The typically reliable Geminid meteor shower will peak this weekend. There will be some windows of opportunity for good viewing, but unfortunately much of the show will be seriously hindered by moonlight.
A bright gibbous moon will have already come over the east-northeast horizon by 10 p.m. on Saturday evening for most locations and will light up the sky through the rest of the overnight hours. This moonlight will overpower many of the shooting stars associated with one of the best annual meteor displays presently visible from Earth.
For those willing to brave the chill, the Geminids are typically a fine winter shower, and usually the most satisfying of all the annual showers, even surpassing the August Perseids.
"If you have not seen a mighty Geminid fireball arcing gracefully across an expanse of sky, then you have not seen a meteor," write astronomers David Levy and Stephen Edberg.
The Geminids stand apart from other meteor showers in another way: The meteors seem to have been spawned not by a comet, but by 3200 Phaeton, an asteroid that crosses the orbit of Earth. Then again, the Geminids may be comet debris after all, for some astronomers consider Phaeton to really be the dead nucleus of a burned-out comet that somehow got trapped into an unusually tight orbit.
Shower under way
Whatever its origins, the Geminids have 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.
Because Geminid meteoroids are several times denser than the cometary dust flakes that supply most meteor showers, and because of the relatively slow speed at which they encounter Earth (22 miles per second, or 35 kilometers per second), Geminid meteors appear to linger a bit longer than most.
The shower has already begun.
Earth moves quickly through this meteor stream, producing a somewhat broad, lopsided activity profile. Hourly rates started to increase steadily this past Wednesday, appearing roughly above a quarter of their peak strength. Were it not for the moon’s interference late Saturday night into Sunday morning, a single observer might average as many as one or two meteor sightings per minute.
After Saturday night, the rates are expected to drop off more sharply, but there is good reason to keep watching for Geminids even after their peak has passed, for those "late" Geminids tend to be especially bright. Renegade late stragglers might be seen for a week or more after the night of maximum activity.
These bright, medium-speed meteors appear to emanate from near the bright star Castor, in the constellation of Gemini the Twins, hence the name "Geminid."
The track of each one does not necessarily begin near Castor, nor 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. In apparent size, that’s less than half the width of the moon. As meteor showers go, this is a rather sharply defined radiant, suggesting the stream is "young" -- perhaps only several thousand years old.
When to watch
Generally speaking, depending on your location, Castor begins to come up above the east-northeast horizon right around the time evening twilight is coming to an end. So the best advice is to be out from then until moonrise when you may catch sight of a few Geminids while the sky is still dark.
Also, as Gemini is beginning to climb the eastern sky just after darkness falls, there is a fair chance of perhaps catching sight of some "Earth-grazing" meteors. Earthgrazers are long, bright shooting stars that streak overhead from a point near to or even just below the horizon. Earthgrazers are distinctive because they follow long paths nearly parallel to our atmosphere.
With all this as a background, the best time to look for Geminids this year will be during early to midevening hours. In fact, Saturday and Sunday will provide us with two "windows" of dark skies spanning the time between the end of evening twilight and the rising of the moon. Generally speaking, there will be nearly three hours of completely dark skies available on Saturday evening, increasing to almost four hours on Sunday. Check the table below for examples at some selected cities.
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 can take the edge off the chill and provide a slight stimulus. It's even better if you can observe with friends. That way, you can cover more sky. Give your eyes time to dark-adapt before starting.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.