At first glance, the annual Geminid meteor shower appears to be on a collision course with a bright Moon. But a closer look reveals a happy window of dark skies.
The Geminids are coming! While the meteor shower is the year's richest it also happens to occur under the glare of the waxing gibbous Moon. Normally, this would spell doom for viewing any but the brightest meteors, but thanks to winter's long nights, a choice observing window opens up in the small hours after moonset.
The shower peaks on Monday night–Tuesday morning December 13–14 with up to 120 meteors per hour visible from a perfectly dark location. Observing from a semi-rural sky I typically see about half that number. Most arrive singly, but sometimes two or three zip by in succession followed by several minutes of inactivity. These quiet spells only increase the suspense and seductively stretch a one-hour meteor-watching session into one lasting twice as long.
During the evening portion of the shower, a 79-percent-illuminated Moon will shine from Pisces and scrub out the fainter members. But the Geminids possess a secret weapon — fireballs! The shower ranks second only after the Perseids for bright meteors, so expect to see a modest amount of activity during the evening hours despite moonlight.
Geminids stream from a radiant or point in the sky located just a couple degrees southwest of the bright star Castor in Gemini, the Twins. As the Earth wings around the Sun at some 107,000 kilometers per hour, it encounters the trail of debris spalled from the asteroid 3200 Phaethon, the Geminids' parent. Like a car plowing through a snowstorm at night, we barrel straight into the material, which creates the illusion of meteors streaming from a point in the sky.
In reality, the fragments strike our atmosphere on approximately parallel paths. The combined speeds of the Earth and shower material add up to an average incoming Geminid speed of around 127,000 kilometers per hour, fast enough for friction with the air to heat and vaporize the particle, creating a luminous trail several kilometers long of ionized metals (including iron and magnesium) and atmospheric molecules. Heating and ionization combined create the familiar sight most people know as a "shooting star."
Moon or no, you can start watching the shower as soon as twilight's end on December 13th, when the radiant has barely risen in the northeastern sky. That's when Geminid earthgrazers will be on the prowl — meteoroids that strike the atmosphere at a very low trajectory and produce long-lasting, often bright trails. I've only seen a few of these over the years, but each has been memorable.
By 9 p.m. local time, when the radiant has risen to around 30°, you can begin observing in earnest. Just remember that lunar glare will reduce the meteor count. For the best view, observe with the Moon at your back to preserve your night vision. This year, the International Meteor Organization is asking for help in determining the effect of bright moonlight on meteor activity. You simply need to observe the shower for at least one hour and estimate your limiting magnitude every 30 minutes by counting the number of stars visible within certain areas of the sky. You'll find charts for these areas here, with Field #7 on Chart 1 easiest to use. Complete the free registration form and then submit your report here.
The best show begins around moonset (approximately 3 a.m. local time on December 14th). For the next few hours before the start of dawn, Gemini towers in the southwestern sky, with the Geminids at full force. As often happens in our hobby, the best observing hours coincide with the coldest temperatures of the night, making this year's shower a bit more challenging than usual. Dress warmly. Use chemical handwarmers inside your boots and gloves and keep a hot beverage at the ready. A lounge-style chair you can settle into comfortably is a must. No one says that amateur astronomy necessitates a spartan life style. Creature comforts make us better observers.
As with most meteor showers, the great majority of Geminids are small, ranging from dust- to pea-size, but occasionally larger pebbles shed by the asteroid produce brilliant fireballs, which are meteors that equal or exceed the brightness of Venus (magnitude –4).
The Geminid stream stands apart from other showers because it originates from 3200 Phaethon, a 5.8-kilometer asteroid with a cometary orbit that occasionally sprouts a dust tail. Most meteor showers instead stem from comets. Phaethon swings closer to the Sun than any other major asteroid, leading astronomers to hypothesize that intense solar heating causes its surface to crack and release dust and rocks that create and feed the Geminids. Earth cuts across Phaethon's orbit during the first three weeks of December, and we blast through the core on December 13–14, the reason for the rapid increase in activity on those dates.
As far as the best direction to face to view the shower, keep the radiant off to one side of your vision. That way you'll bag a mix of both short-trailed meteors and the "long-streakers" that flare 90° to either side of the radiant. For the Geminids I like to face southeast during the evening hours and south or northwest before dawn. But don't fret — Phaethon's fragments will cross the sky in all directions. And you can identify any meteor as a Geminid by following its trail backwards. If it points to Castor, you've got a keeper.