The Geminid meteor shower

¡SkyCaramba! Weekly astronomy blog for the week ending December 15, 2012

This could be a great year to watch the Geminid meteor shower. There can be up to 50 Geminid meteors per hour. It’s a regular and dependable shower. And the moon will be new on the predicted peak dates of December 13 and 14, so there won’t be any full moon light to obscure the dimmer meteors.

A meteor is a small bit of rock or other material that burns up in the earth’s atmosphere. As our planet moves in orbit, our atmosphere sweeps up debris left over from astronomical events of long ago. When there are lot of meteors over a few hours or days all seeming to originate from the same point in the sky, we identify it as a meteor shower and name it for the constellation the meteors originate from.

Most meteor showers happen as Earth passes through dust trails left by comets. Earth and most comets have regular enough orbits for the encounters to happen at about the same time every year. We know to look for Orionids in November because that’s when Earth meets dust from Halley’s Comet. Likewise, when our atmosphere strikes leftovers from a Comet Tempel-Tuttle passage in November, we see Leonids.

The Geminid meteor shower, however, has been linked to an asteroid. Named 3200 Phaethon, this rock discovered in 1983 is an unusual asteroid. Most asteroids have orbits that look like planetary orbits. Their orbits are elliptical, but they look more like circles than ovals. Comet orbits are highly elliptical, coming very close to the sun and going far away from it over years or decades. 3200 Phaethon gets closer to the sun than Mercury and almost as far away as the asteroid belt between Mars and Jupiter. That’s the kind of orbit we expect comets to have!

But there’s no dust coming off 3200 Phaethon forming a glowing head or tail. It’s just a rock. Astronomers think this rock is an example of what’s left when a comet runs out of looser material it can shed in the solar wind. And 3200 Phaethon apparently had lots of material to lose at one time. When Earth passes near regions where 3200 Phaethon has been, we see Geminid meteors. But we don’t know of any comet that has been seen where 3200 Phaethon has been.

The nights of the 12th to 13th and 13th to 14th are when Earth is passing through the Geminid dust trail. Since it’s not a narrow trail, you could still see some Geminids a few mornings before and a few mornings after. The best way to know if a meteor you see is a Geminid is to pay attention to where it came from. If you can extend a line back to the constellation Gemini, you’ve probably seen a Geminid. It’s possible to see Geminids without being able to see Gemini. You’ll see those in the early evening before Gemini rises. However, this is the time of year when the constellation is up nearly all night. So if you can see the constellation and you see a meteor, you can probably tell whether the meteor is a Geminid.

If you want to watch the Geminids easily, do it while lying down. A reclining lawn chair or a blanket on the ground can save a lot of neck strain from all the looking up you’ll do. Make sure you’re watching from a place where you have dark skies. A few bright lights can ruin the show more than the moon. If you live in a brightly lit city, you’ll need to head out of town. Don’t bother bringing a telescope for the meteor shower. You can watch other wonders up there with the telescope, but you’ll never be able to aim it at a meteor in time. Dress warm.