The 2015 Geminid Meteor Shower

Late autumn offers us an astronomical event that nearly anyone could observe if the conditions are right. The Geminid meteors make their annual appearance on the night of December 13. If the skies are clear, and you are willing to brave the cool temperatures for a few minutes, you should be able to see anywhere from 30 to 50 or more little streaks of light coming out of the direction of the constellation Gemini the Twins. Gemini, named for the brothers who accompanied Jason in search of the Golden Fleece, is easily visible in the east northeast by 9:00 p.m. Under perfect conditions the numbers (based on past observing) could be up to 120 per hour. But due to our location so near a bright city, we rarely have the perfect conditions needed to see them at their best.

I am sometimes asked how we can predict the appearance of relatively small rocks (many only as big as a grain of sand) entering and burning up in earth’s atmosphere. The answer is in their regularity. We see them every year. Of course they aren’t the same exact rocks, but since they appear when the earth returns to the same place in its orbit, it tells us there is a large cloud of them crossing earth’s path at this location. They don’t just stay here in wait for the earth’s arrival. The cloud is in motion too, and its path intersects the earth at this time and place. The ones that get directly in our way strike the atmosphere at high speed, heat the air around them so it glows brightly for a short time (usually only a fraction of a second). These quickly moving points of light looked like fast moving stars, giving us the misleading term “shooting stars”.

GeminidsWhere does this cloud come from? By studying the direction of the shower particles as they enter the atmosphere, we can determine the motion of the parent cloud. This reveals something interesting about the source of the Geminid shower. While most such clouds are found to follow the path of a known comet, the Geminids instead trail the orbit of an earth-crossing asteroid known as Phaethon. It turns out that it is one of several asteroids of a class called Apollo asteroids, characterized by paths that take them in close to the sun. In fact Phaethon gets closer to the sun than any other in its class. This leads many astronomers to believe that it is actually a dead comet, one that has lost most or all of its frozen gasses.

Although it crosses earth’s path along with its dust cloud, we needn’t worry. It’s barely 3 miles across and will not get within 2 million miles of us until a fairly close pass on December 14, 2093. Until then we can look for pieces of this asteroid to streak across our sky for several hours on the evening of December 13 and continuing through the early morning hours of the next day.

The best place to view them is away from the city lights, but even city dwellers might see a few of the brightest ones. Dress appropriately for the temperature, settle into a lawn chair with a warm drink and look up. There’s no need to watch any particular part of the sky, but you should commit at least half an hour for the best chance to see some of the show, which will last through the night and into the morning. This year the Moon is just past New and will not interfere a bit, making this a great chance to see one of the two biggest meteor showers of the year.

For more information about meteors, meteorites and other wonders of the universe come to the Sharpe Planetarium set to reopen late next month.

David Maness, Sharpe Planetarium Supervisor
Pink Palace Museum

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