Moon Muscles in on December Geminids

This week brings us what usually is considered to be the most satisfying of all the annual meteor displays, even surpassing the famous Perseids of August: the December Geminid Meteor Shower. But this year, before making any elaborate plans to view this year's Geminid display, prospective sky watchers should be aware that they will be facing a major obstacle to observe this year's Geminid performance, namely the moon.

Unfortunately, as luck would have it, 2008 will see the moon will turn full on Dec. 12 and as such will seriously hamper — if not all but prevent — observation of the peak of the Geminids, which is predicted to occur for the night of Dec. 13-14. Bright moonlight will flood the sky through much of that night, and will certainly play havoc with any serious attempts to observe these meteors.

The Geminids are already around, having been active only in a very weak and scattered form from earlier this month. But a noticeable upswing in Geminid activity has occurred since then, leading up to their peak night this weekend. Historically, this shower has a reputation for being rich both in slow, bright, meteors as well as rather faint meteors, with relatively few of medium brightness. Many Geminids appear yellowish in hue.

Great balls of fire

But for those who are grasping at straws in light of the poor conditions for Geminid viewing this year, it should be noted that every once in a while, a dazzling Geminid fireball will blaze forth, bright enough to be quite spectacular and more than capable to attract attention even in bright moonlight. In their book, "Observe Meteors," published by the Astronomical League, astronomers David Levy and Stephen Edberg note that, "If you have not yet seen a mighty Geminid fireball arcing gracefully across an expanse of sky, then you have not seen a meteor."

With this as a background, perhaps the best times to look this year will be during the evening hours on those nights after the night of full moon. That's when the constellation Gemini (from where the meteors get their name) will be rising above the east-northeast horizon.

Catch an earthgrazer on Sunday

In fact, early on Sunday evening, December 14, there will be a "window of opportunity" to look for Geminids in a dark, moonless sky. Between the times that evening twilight ends and the time that the bright waning gibbous moon rises, there will be about an hour of dark sky available.

For most locations, moonrise is around 7 p.m. To find the time of moonrise for your specific location, go to: http://aa.usno.navy.mil/data/docs/RS_OneDay.php

During the moonless interval, there possibly may be an opportunity to catch a glimpse of an unusually beautiful type of meteor called an "Earthgrazer." Earthgrazers are long, bright shooting stars that streak overhead from just below the horizon. They often display colorful halos and long-lasting trails. Earthgrazers are so distinctive because they follow a path nearly parallel to our atmosphere, analogous to a rock skimming across the top of a pond. The Geminid radiant — near to the bright star, Castor — will be near the horizon as our planet is passing through the Geminid stream.

Much better in 2009

In the absence of moonlight a single observer might see upwards to 120 meteors per hour on the peak night, a number that sadly can not be hoped to be approached in 2008. Looking ahead to 2009, the Geminids will reach their peak just two days before a new moon, meaning viewing conditions will be nearly perfect. So it appears that this year, Geminid fans will be uttering the same lament that the old Dodger fans in Brooklyn used to:

"Just wait till next year!"

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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.