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An independent monthly newspaper serving the community since 1988
  Night Sky
 

Enjoy the starry night skies
Be a considerate neighbor. Reduce nighttime glare. Shield your outside lights downward. Let the stars light up the night.

December 2013 night sky

—Charlie Christmann

Geminids

With all eyes watching Comet ISON, the annual Geminids Meteor Shower might be easy to miss. But don’t miss it! This event is the king of winter meteor shows. Only one problem: when this shower peaks on the evening of December 13, the bright, waxing gibbous moon will limit the number of meteors visible. Still, expect to see somewhere around one hundred per hour under dark skies.

The Geminids are unusual because they are not the result of a comet. Most showers are associated with a known or suspected comet. Comets create long tails of dust as they warm near the sun, thus creating the iconic tail flowing away from the sun. It is the comet’s tail that later becomes a meteor shower. As Earth orbits through a dust trail, dust and sand gains create the streaks across the sky as they burn up high in the atmosphere. Occasionally, a small rock enters the atmosphere creating a large, bright fireball. The vast majority of these rocks never get close to the ground.

The Geminids, however, are the result of an asteroid named 3200 Phaethon, discovered in October of 1983 by Simon F. Green and John K. Davies in some images from the Infrared Astronomical Satellite (IRAS). Unlike comets, asteroids normally keep their distance from the sun. 3200 Phaethon dares to approach the sun well inside Mercury’s orbit. At this distance, its surface temperature reaches 1300 degrees Fahrenheit. NASA’s STEREO satellites have witnessed dust ejected from the asteroid’s surface, thus leaving a trail for the Earth to plow into each December and creating this meteor shower.

There is some debate about 3200 Phaethon. Is it really an asteroid nudged into a tight solar orbit, or is it the burned-out remains of an old comet? That question may never be resolved, but it has fueled some arguments about the differences between comets and asteroids. In some ways, 3200 Phaethon is acting more like a comet, ejecting dust into space and briefly showing a faint tail. However, its orbit places its origin in the asteroid belt between Mars and Jupiter. New theories suggest that the high temperatures near the sun may fracture and desiccate the rocks on its surface into a powder. Solar wind and radiation then picks up the resulting dust and launches it into space.

No matter how we define 3200 Phaethon, it can put on a great mid-December show. Look in the sky after dark on the 13th and before sunrise on the 14th. The radiant in the constellation Gemini is well above the horizon after 9:00 p.m.

Ursids

The Geminids are not the only shower in December. The Ursids are a variable shower active from December 17th to the 26th. It can produce around fifty meteors per hour. Occasionally, like in 2000 and from 2006 to 2008, short bursts of nearly one hundred meteors per hour are possible. This shower is the result of comet 8P/Tuttle, which last visited the sun in 2008. Again, the near-full moon will obscure many of these faint, medium-speed meteors.

Venus

No, that is not a UFO, the Christmas Star, or even an aircraft in the evening sky—it is the planet Venus. Venus reaches its brightest this month at a staggering -4.9 magnitude. As a comparison, the sun is a magnitude -26.74, and the moon is -12.7. The brightest star in our sky is Sirius (east if Orion) with a magnitude -1.46. The planet Jupiter is a bit brighter than Sirius at -2.7 this month as it reaches opposition.

ISON Update

Comet ISON rounded the sun on Thanksgiving Day. As of mid-November, there were some indications of the comet fragmenting. If this holds to be true, the December show may be muted. Let’s just keep our fingers crossed that this comet puts on the show as promised.

 
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