'Gem' of a Meteor Shower Underway

If you missed out on last month's Leonid Meteor Shower, don't fret. What potentially will be the best meteor display of the year is just around the corner, scheduled to reach its peak during the overnight hours of Dec. 13-14. The Geminid Meteor Shower, in fact, is already underway.

If the Geminids occurred during a warmer month, they would be as familiar to most people as the famous August Perseids, which people often notice by chance when they're out camping or otherwise enjoying a warm summer night. The Geminids, on the other hand, come at a time when much of the Northern Hemisphere is experiencing bone-chilling cold.

But if you are willing to bundle up, this coming Sunday night into early Monday morning will be when the Geminids are predicted to be at their peak. Depending on dark your location is, and how much of the sky you can see, meteors may streak into view that night at an average rate of one or two per minute.

A rich display expected

The Geminids are ? for those willing to brave the chill of a December night ? a very fine winter shower, and usually the most satisfying of all the annual showers, even surpassing the Perseids.

Studies of past displays show that this shower has a reputation for being rich both 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 appear to form jagged or divided paths.??

These 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 or even in the constellation Gemini, but it always turns out that the path of a Geminid extended backward passes through a tiny region of sky about 0.2-degree 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 most meteor showers go; suggesting the stream is "young" ? perhaps only several thousand years old.

Excellent viewing conditions

This year's Geminids will not be hindered at all by moonlight. On Monday morning, the moon ? a narrow sliver of a crescent, just two days before its new phase, will come up over the east-southeast horizon soon after 5:30 a.m. for most locations and will pose very little, if any interference for meteor watchers.

Best of all, Earth is predicted to be passing through the densest part of the Geminid swarm at around 5:00 GMT on Dec. 14. That means that much of Europe and North America will be in excellent position to catch the absolute peak of this display, which should persist for more than several hours.

But don't get hung up on that exact timing. In fact, regardless of where you live, the very best time to watch for the Geminids will come at around 2 a.m. local time on Monday morning. At that hour, the constellation Gemini will appear almost directly overhead.

Late Sunday night on through daybreak Monday morning, a single observer blessed with a dark and unobstructed view of the sky might average as many as 60 to 120 meteors per hour ? one to two meteor sightings per minute. People in cities and suburbs will see far fewer.

Watch for Earthgrazers

Generally speaking, depending on your location, the star Castor begins to come up above the east-northeast horizon right around the time evening twilight is coming to an end. 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 early on Sunday evening.

Earthgrazers are long, bright "shooting stars" that streak overhead from a point near to even just below the horizon. Such meteors are so distinctive because they follow very long paths nearly parallel to our atmosphere. As Gemini climbs higher into the sky, however, the meteors these very long paths will become much shorter.

The Geminids begin to appear noticeably more numerous in the hours after 10 p.m. local time, because the shower's radiant is already fairly high in the eastern sky by then. As was already noted, the best views come at around 2 a.m., when their radiant point will be passing very nearly overhead. The higher a shower's radiant, the more meteors it produces all over the sky.

Because Geminid meteoroids are several times denser than the cometary dust flakes that supply most meteor showers, and because of their relatively slow speed with which they encounter Earth ? 22 miles per second (35 km per second) ? Geminid meteors appear to linger a bit longer in view than most. As compared to an Orionid or Leonid meteor that can whiz across your line of sight in less than a second, a Geminid meteor moves only about half as fast.

Their movement reminds me of field mice scurrying from one part of the sky to another.

Get out now

The Earth moves quickly through this meteor stream, producing a somewhat broad, lopsided activity profile. Hourly rates begin to slowly increase on Dec. 10, appearing roughly above one-quarter peak strength.

So you might want to go out any night between now and Sunday for a practice session. Don't expect a great display, but it's a good opportunity to scout a location and see a few shooting stars. Again, the best time would be 2 a.m. your time.

After Sunday night, the shower's intensity should drop off sharply: rates on Monday night/Tuesday morning will have diminished to about 30 to 60 per hour. Yet, there is good reason to keep watching for Geminids even after their peak has passed, for those "late" Geminids tend to be especially bright. And renegade late stragglers might be seen for a week or more after the night of maximum activity.

Prepare properly

Make sure you're warm and comfortable. Meteor watching in December can be a long, cold business. You wait and you wait for meteors to appear. If 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!

The late Henry Neely, who for many years served as a popular lecturer at New York's Hayden Planetarium, once had this to say about watching for the Geminids: "Take the advice of a man whose teeth have chattered on many a winter's night ? wrap up much more warmly than you think is necessary!" Hot cocoa, tea or coffee can take the edge off the chill, as well as provide a slight stimulus.

It's even better if you can observe with a friend. That way, you can keep each other awake, as well as cover more sky. Give your eyes at least 15 minutes to dark-adapt before getting serious about scanning the sky. Telescopes and binoculars are of no use. Bring a blanket or lounge chairs so you can lie back, stare up and scan the sky for long stretches of time.

As you enjoy the display, here's something to ponder: 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 really be the dead nucleus of a burned-out comet that somehow got trapped into an unusually tight orbit.

• How Meteor Showers Work, Plus Viewing Tips
• Leonid Meteor Shower Galleries 1, 2, 3, 4
• Meteor Shower Photography

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.