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Astronomy From Your Back Yard - 12/08 to 12/14 2010

By Dave Grosvold

  
If you can get out and observe in the two hours before the coming of dawn this week, Venus blazes in the southeast, even after sunrise. Venus is still at its highest luminosity, at magnitude 4.8 and nearly as high as it gets. Can you watch Venus past sunrise with your naked eye? Look for much fainter Spica to Venus' upper right (about 20°,) and for Saturn about 20° above Spica.

In the evenings this week, Mercury remains in view low about the southwest horizon during twilight. Look about 40 minutes after sunset. Mercury fades from magnitude 0.4 to +0.1 this week.

  
On Wednesday Dec 8th, by 7 or 8 PM CST, bright Capella in Auriga is high in the east-northeast. To its right in the east is the Pleiades (M45), or Seven Sisters, and below it is ruddy Aldebaran in Taurus. Below Capella lie Castor and Pollux, the heads of the twins of Gemini. It will be important to know the location of these twins early next week.

As soon as it gets dark on Thursday evening, look for the waxing crescent Moon in the southwest. To the right of the Moon about 7°, is tiny Alpha Capricorni (α Cap.) If you have sharp vision you can barely see that this is a very close double star. You can resolve it easily in binoculars.

Jupiter still shines brightly this week at magnitude -2.5. Just after dusk on Friday evening look far to the lower right of Jupiter for the waxing crescent Moon. Look below Jupiter to the lower left of the Moon for Fomalhaut (FOAM-a-lout), in Piscis Austrinus, sometimes called “the Autumn Star.”

By Saturday evening, the nearly first-quarter Moon this evening forms a roughly equilateral triangle with bright Jupiter to its upper left and Fomalhaut to its lower left. The Moon reaches First Quarter at 7:59 AM CST, on Monday December 13th.

  
Monday, December 13th near midnight marks the start of the peak of the Geminid meteor shower for this year, and for most observers, it is the highlight of the week. The peak of the shower continues through the night into the morning hours of Tuesday, December 14th. The name “Geminids” refers to the fact that the radiant for this meteor shower appears to come from the heart of the Twins of Gemini. When meteors are traced back to their apparent origin, the paths they followed all converge at a point on the sky known as the radiant.

From now until December 13th, hourly rates increase until a peak of 50-80 meteors per hour is attained. The rate then tapers off until the last Geminids on December 18th, when the rates fall to one every hour or so. Meteor showers occur all the time, but most are very weak. So there may be stray meteors that are not part of the Geminids during this time as well. When you see a meteor, how do you know if it's a Geminid? Trace the path of the meteor back to its apparent origin. If that origin ends up being in Gemini or close to it, then it's probably a Geminid.

The best way to observe the Geminid Meteor Shower is lie back on a chaise lounge with a clear view of the sky toward Gemini. This time of year, be sure to dress warmly, get a thermos of coffee or hot chocolate, and maybe even throw on a blanket or two. As you lie there, look high up and all around the sky, and don't focus in just one place.

  
Meteors will appear all over the sky, but the majority will appear to have come from the direction of Gemini, so we face the chaise lounge that way. But we scan from overhead down to about 30° up the sky. We also scan off to the right, and then to the left, looking at the big picture rather than focusing in any one area. Don't discount the sky opposite Gemini, either. Meteors can be seen all over the sky. See if you can count the number of meteors you see in an hour's time, and note the time of night.

On a historical note, the Geminid meteor shower appeared suddenly in the latter part of the 1800's. R. P. Greg of Manchester in the UK first noted several meteors that had a radiant in Gemini in December of 1862. Several other observers independently discovered the same activity the same year from the United States.

Early observations in the late nineteenth century reported the hourly rates at about 14 per hour. As the century waned, the hourly rate seemed to increase to about 23 per hour, with many more bright meteors. These rates have continued to increase throughout the 20th century, averaging about 50 per hour mid-century with the peak rate reaching an average of 80 per hour in the 1980s. The peak hourly rate continues to remain at 80 per hour today. Scientists now think the Geminids are tied to the passage of an asteroid (or minor planet,) 3200 Phaethon, in our solar system across Earth orbit every few years. The Earth passes through the dust and debris left in its wake, creating tiny meteors in our upper atmosphere as we pass.
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