Radiant point for December’s Geminid meteor shower

Where do you look to see December’s famous Geminid meteor shower? In an open sky, because these meteors fly in many different directions and in front of numerous age-old constellations. However, if you trace the paths of the Geminid meteors backward, they all appear to radiate from the constellation Gemini the Twins. Do you need to find Gemini to watch the shower? No, but it’s fun to spot the radiant point in the night sky.

The Geminid shower comes on strong around this time every year. In 2012, the best nights will be December 12, 13 and 14. In a moon-free sky, you might see as many as 50 meteors per hour in the wee hours after midnight – maybe more. Fortunately, in 2012, the new moon guarantees dark skies for the peak nights. No matter where you live worldwide, the greatest number of Geminid meteors usually fall after midnight. But you can watch this shower in late evening as well. In 2012, the morning of December 14 will probably be the peak of the Geminid shower. But look on the nights around then as well.

Hard to say how many meteors you’ll see in an inky black sky, but we’ll be eager to hear your reports. If the forecast holds – which is a big IF – Europe and far western Asia will probably have the best view of this year’s Geminid meteor shower. But you never know for sure which part of the world will win the prize.

Why do meteor showers have radiant points? No matter where you are on the globe, expect the most Geminids to streak the night from about 1 a.m. to 3 a.m. local time. That’s when the Geminids’ radiant point will be highest in the sky. What is a radiant point? Remember, the meteors are arriving from outer space. When we see them, it’s as though we’re standing in the middle of railroad tracks, gazing down the tracks and watching them converge in the distance. Meteor shower radiant points are kin to this illusion. They’re really arriving on parallel paths. But, to us on Earth’s surface, their paths appear to converge in the distance.

The Geminid shower radiates from the constellation Gemini the Twins, as shown on the chart in this post. That’s how this shower got its name. The meteors come from a point near the star Castor in Gemini, one of the brightest stars in the sky. If you watch this shower – and trace the paths of the meteors backwards on the sky’s dome – you’ll find that the meteor paths seem to converge at a point in the sky near this star.

To see Castor, look fairly low in the east-northeast sky around 9 p.m. This star is noticeable for being bright and near another star of almost equal brightness – its brother star in Gemini – called Pollux. The stars Castor and Pollux, and the Geminid meteor shower radiant, swing upward through the night and climb pretty much overhead by around 2 a.m. That’s what’s important about a meteor shower’s radiant point: the higher the radiant rises into in your sky, the more meteors you’re likely to see.

That means you can expect to see the most Geminids around 2 a.m., when Castor will be highest in the sky, and the meteors will be raining down from overhead.

The moderately fast Geminid meteors tend to be bright. The exact number you can see varies with a number of factors, but 40 to 90 per hour at peak is a reasonable estimate in years when the moon is out of the way. With no moon to intrude on this year’s production, some Geminids should be bright enough to offer a little extra holiday lighting on these cold December nights. Since the peak of a meteor shower isn’t all that easy to predict with absolute precision, meteor watching enthusiasts will be watching the Geminids for the several nights, centered upon the expected peak night on December 13/14.

By the way, meteors in annual showers – like the Geminids – stem from debris left behind by comets as they orbit the sun. A meteor shower’s radiant point represents the direction in the sky at which Earth’s orbit intersects the orbit of that (usually) long-lost comet.

The parent object of the Geminids is very special. It looks more like an asteroid than a comet, and it has an asteroid name: 3200 Phaethon. Perhaps the link between comets and asteroids is closer than we think! Plus, in 2012, astronomers are contemplating a new name for objects like 3200 Phaethon. They’re calling them rock comets. Read more about the mysterious origins of the Geminid meteor shower and rock comets here.


Bottom line: You don’t need to find the radiant point for a meteor shower to see the meteors. But it’s fun to locate the radiant in the sky. This post tells you how to find the radiant point for the December Geminid meteor shower. Watch the shower on December 12, 13 or 14. Just lie back comfortably and let your gaze wander to all parts of the sky.

- EARTHSKY