Keep your eyes to the sky the next few nights. Earth has entered the Geminid meteor stream, and sparks are gonna fly! The annual Geminid meteor shower is the richest of the year, even beating out the Perseids of August, and it peaks Thursday night into Friday morning Dec. 13-14. One of the great things about this shower is that you don’t necessarily have to rise at 2 a.m. to see it. The radiant — the point in the sky from which the meteors will appear to stream — lies in the constellation Gemini near the star Castor. And Gemini’s high enough in the eastern sky by 10 p.m. to see a good show before bedtime.
You can even start earlier at 7 to 8 p.m. to look for Geminid Earthgrazers — slow meteors that streak up from near the horizon and flare a long time before finally fading out. Earth is just beginning to turn into the shower, so incoming meteors scrape the top of the atmosphere where the air is thinner, causing them to burn more slowly. As the radiant climbs higher, meteors strike the atmosphere more directly and cut shorter, quicker paths across the sky. While uncommon, Earthgrazers are amazing to watch because they can last for many seconds.
You’ll read elsewhere of 120 Geminids visible per hour at shower peak, but this is how many you’d see from the darkest sky with the radiant directly overhead. Few of us live in such a meteor utopia. Expect something closer to 60 per hour from reasonably dark skies. Based on my own observations under good conditions with no moon in the sky, I typically see around 30 to 40 per hour. At least the moon won’t cause any trouble this time around. It will be a waxing crescent off to the southwest and set around 10:30 p.m.
I usually spend an hour with the Geminids in the evening then go to bed and get up again around 2-3 a.m. (Friday morning) to catch the peak of the shower, when the radiant stands almost overhead. The higher the radiant, the more meteors you’ll see because fewer get cut off by the horizon. The sky is also darker the higher up you gaze, making it easier to see fainter meteors. Every shower has a lot more fainter meteors than bright ones.
Pick your time and plan to spend at least an hour watching. This will allow enough time for your eyes to get used to the darkness and spot at least a few Geminids. I should follow my own advice but rarely do. I might plan to be out an hour but inevitably stay longer for “just one more.” If you’re out in the evening, keep the moon at your back to protect your night vision and cozy up in a reclining chair, so you don’t get a crick in your neck from standing up. I use a cheap, fold-out lawn chair and drape myself in a sleeping bag. Dress warmly and wear insulated boots — you probably won’t be moving around a lot.
Geminids can appear anywhere in the sky, but they all fly out of Gemini. That means you can face any direction to watch the shower. I like south or east because Orion’s there, one of my favorite constellations and many other bright stars of the season. Once settled in, keep an informal count of how many meteors you see. Most appear singly, but sometimes two will pop into view simultaneously (very exciting!). Busy spells with one meteor after another can be followed by no activity at all for minutes at a time. Use those times to appreciate the beauty of the stars or try to spot comet 46P/Wirtanen, which will share the sky with the Geminids.
Want to photograph the shower? Check out this “How To” for everything you’ll need to know.
If clouds are in the forecast for Thursday, go out Wednesday and Friday nights. Numbers will be lower, but the shower will still be active. Each meteor you see is a piece of dust or small rock sloughed off the asteroid 3200 Phaethon, sometimes referred to as a “rock comet.” Unlike normal comets, which are composed mostly of dust-impregnated ice, Phaethon appears to be made of rock and rarely shows the kind of activity — tail and fuzzy coma — typical of comets.
Every 1.4 years, the 3-mile-wide asteroid passes extremely close to the sun, just 13 million miles (21 million km). Astronomers think the intense heat (1,500° F) stresses and ruptures Phaethon’s surface, releasing dust and debris that trails along its orbit. Every December, Earth crosses that orbit and pieces of Phaethon slam into our atmosphere at 79,000 mph (127 km/hour). The friction excites the surrounding air to glow, and that’s what makes a meteor.
So if you want to enjoy a great show hosted by a crumbling asteroid, dress up in your finest winter gear, stride purposely out the door and look up.