The Moonlit 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 as was the case with last month's Leonid Meteors, prospective sky watchers should be aware that once again they will be facing a major obstacle in their attempt to observe this year's Geminid performance, namely, the Moon.

Unfortunately, as luck would have it, 2005 will see the Moon will turn full on December 15 and as such will seriously hamper, if not all but prevent observation of the peak of the Geminids, 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 since around December 7.  But a noticeable upswing in Geminid activity is expected to begin during this weekend, leading up to their peak night next week. 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. 

And every once in a while, a 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 predawn hours several mornings before the night of full Moon.  That's when the constellation Gemini (from where the meteors get their name) will be standing high in the northwest sky. 

In fact, three "windows" of dark skies will be available between moonset and the first light of dawn on the mornings of Dec. 11, 12 and 13.  Generally speaking, there will be about 2-½ hours of completely dark skies available on the morning of the 11th.  This shrinks to about 1-½ hours on the 12th, and to less than ½ hour by the morning of the 13th.  Check the table below for examples at some selected cities. 

All times are a.m. and are local standard times.  "MS" is the time of moonset.  "Dawn" is the time when morning (astronomical) twilight begins.  "Win" is the available window of dark sky composed of the number of minutes between the time of moonset and the start of twilight.  Example: "When will the sky be dark and moonless for Geminid viewing on the morning of December 12 from Houston?"  Answer: there will be a 92-minute period of dark skies beginning at moonset (4:10 a.m.) and continuing until dawn breaks (5:42 a.m.).

For Seattle and Helena, dawn breaks before the Moon sets on Dec. 13, so no dark sky window is available.

Perhaps about a dozen or more of the forerunners of the main Geminid display might appear to steak by within an hour's watch on these mornings.

In the absence of moonlight a single observer might see up to 120 meteors per hour on the peak night, a number that sadly can not be hoped to be approached in 2005.  In fact, 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!"

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.