2007 Geminid Meteors
Published on Dec 3, 2007 at 8:46 pm.
2 Comments.
Filed under meteors.
Near the middle of every December we get the Geminid Meteor shower. I wrote about the Geminids last year, but they are one of the year’s best meteor showers, so they are worth writing about again.
This year, the Geminids peak on the morning of December 14, however a few can be seen from December 6 through December 18. The peak activity is expected to reach in excess of 100 meteors per hour. However, that number is for a skilled observer under ideal sky conditions with the radiant of the shower directly overhead. Expect a bit less than that. The meteoroid streams that produce the Geminids are dispersed enough that the meteors don’t all fall at just the day of the peak. You should see about half as many meteors per hour the day before or the day after the peak. This is good, because I am going to be doing a public star party the night of December 15, and there should be a meteor every few minutes or so that night. The Moon is rather favorable for the meteor shower this year. It is a waxing crescent moon that will set a little before midnight. About the best time to watch the meteor shower is between midnight and local dawn, anyway, so that is fine. The Geminids tend to favor the northern hemisphere, so activity will start to pick up a few hours before midnight (but should be much more active after midnight). Southern hemisphere observers will see very few, if any meteors, until a little after midnight. Then, because Gemini is a northern constellation, the number of meteors seen will be somewhat reduced, compared with northern hemisphere observers.
The Geminids are so named because they appear to radiate away from the constellation Gemini. The accompanying chart that I created using Stellarium software shows meteors radiating from Gemini. The two prominent stars of Gemini, Castor and Pollux, are very near the meteor shower’s radiant. Now, often people new to star gazing will wonder how they are supposed to find the constellation that is the radiant of a meteor shower. Normally, I tell them that it doesn’t matter. All you need to do is just to look UP. Meteors will appear pretty much anywhere in the sky. If they are part of the shower, though, then they will appear to be shooting away from Gemini no matter where they are seen in the sky. Around midnight, Gemini is high overhead for those of us in the northern hemisphere. All you need to know is the approximate location, unless you happen to be a serious meteor observer and you are trying to differentiate between the Geminids and stray sporadic meteors that occur every night. But, Gemini is actually pretty easy to find. First, you find Orion. Castor and Pollux are the pair of stars to the northeast of Orion in the sky. But, this year, there is an even easier way to find Gemini. Mars happens to be in Gemini this year! Mars moves around, about a constellation per month, but right now it is in the vicinity of Gemini, and Mars is bright and reddish orange, so it is pretty easy to spot northeast of Orion.
 The meteoroids that make up the Geminid meteor shower strike Earth at about 35 km/s. That is fast, of course, not so fast for a meteor shower. That means that the Geminids ofter produce long bright streaks across the sky. They really are a pretty meteor shower. The meteorids themselves are tiny: about the size of a sand grain or a rice grain. You don’t really see them. When they strike the atmosphere, they compress the air in front of them. That heats up and glows white hot. It is that ionized air that you really see when you see a meteor. The meteoroids rarely survive passage through the atmosphere. They “burn up,” though that is not really the correct terminology, since they do not burn in the chemical sense of the word.
 Most meteor showers are believed to be associated with comets. As the comets swing by the Sun, they shed material into orbits similar to that of the comet. The meteoroid streams then orbit the Sun gradually spreading out. If the comet passes rather near Earth, then the meteoroid stream may eventually intersect Earth’s orbit. Whenever Earth passes through the meteoroid stream, then the meteoroids strike the atmosphere, creating meteors. That is the accepted origin for meteor showers. For many meteor showers, we know the comet associated with the shower. That is particularly true for most of the major meteor showers. The Geminids, though, seem to be an exception. No known parent comet for the Geminids was found. However, in 1983, astronomers Simon Green and John Davies, looking at data from IRAS (the Infrared Astronomical Satellite), found a small object moving in an orbit almost the same as that of the Geminid meteoroid stream. This object was eventually named 3200 Phaethon. It appeared to be an asteroid. But, asteroids are not normally expected to be the progenitors of meteor showers. As I pointed out, though, in an earlier posting, the “asteroid” Phaethon may actually be an extinct comet. Comets are composed of volatile materials such as ices and frozen gasses as well as silicates and other rocky components. After enough years, most of the volatiles may be driven away by the heat of the Sun, leaving what looks like an asteroid behind. Read my earlier posting for more information on that.
 This year, interestingly, Phaethon is passing fairly near Earth. On December 10, Phaethon will be only 18 million kilometers from Earth. That is a near miss in cosmic terms. In fact, that distance is only about 47 times farther from Earth than our moon. Phaethon passes close enough to Earth’s orbit that it is on the list of Potentially Hazardous Asteroids being monitored. It is not calculated to impact Earth at any time in the forseable future, but these things still need to be monitored. As I have said in previous postings, asteroids and comets can have their orbits shifted every now and then. So, an asteroid that safely misses Earth can have its orbit shifted into one that is a very real threat.
 At any rate, this year looks to be another good one for viewing the Geminids. If you get a chance, then go out and look for them. Remember, the best time will be from about midnight until dawn. The best morning will be December 14, but a day or two on either side of that will still give some meteors.
 -Astroprof
Stargazer on December 11, 2007 at 1:07 pm: 1
Well said! Getting out of bed past midnight and driving to a dark field is a lot of work, and your article is quite informative enough that I can with confidence embark on my meteor- shower- viewing expedition based on its information. I know exactly when to be where to view the peak shower activity. It is a celestial event I hope not to miss.
Temple on December 13, 2007 at 10:23 pm: 2
My sons (ages 9 & 11) and I watched the sky for a little while before 10pm tonight and we were impressed with the amount of meteors we saw. I can’t wait to go out after midnight and watch some more! This is thier first meteor shower, but they are hooked-it won’t be their last!