The sky — astronomy

The Geminids: The December Meteor Shower

The Geminids are a meteor shower that peaks around December 13–14, when their radiant in the constellation Gemini is high in the sky. They come from debris left by the asteroid 3200 Phaethon and are among the most abundant showers of the year. They are best watched from a dark site, around and after midnight.

What the Geminids are

The Geminids are an annual meteor shower that lights up December nights. As Earth travels around the Sun, it crosses streams of small particles left along the orbits of other bodies. These grains hit the atmosphere at high speed and burn up, producing the streaks of light we call meteors. For the general picture, see our article on meteor showers.

The Geminids have a reputation for being rich and reliable. Under a dark sky, with the radiant high, an observer can count over a hundred meteors per hour. The International Meteor Organization (IMO), which gathers observations from amateur skywatchers worldwide, lists them among the strongest showers of the year, alongside the August Perseids.

A shower from an asteroid: 3200 Phaethon

What makes the Geminids special is their source. Most meteor showers come from comets (read more about comets), but the Geminids are linked to an asteroid named 3200 Phaethon. It was discovered in 1983 by the infrared space telescope IRAS. Phaethon is a few kilometres across and circles the Sun in roughly a year and a half.

Its orbit is strongly elongated and carries it closer to the Sun than the orbit of Mercury. That is one reason scientists find it so interesting. Near its closest approach Phaethon shows weak, comet-like activity, which is why it is sometimes called a “rock comet”. Exactly how it sheds material is still being studied, so it is honest to say the picture is not fully settled.

The particles left along its orbit form a stream. When Earth crosses it in December, we see the Geminids. The material is denser and more rock-like than typical comet dust, which helps explain why the meteors are often bright and relatively slow. They enter the atmosphere at about 35 kilometres per second, noticeably slower than the Perseids.

The radiant in Gemini

Meteors from one shower seem to fan out from a single point in the sky, called the radiant. For the Geminids it lies in the constellation Gemini, not far from the bright star Castor. The shower takes its name from Gemini. This is an effect of perspective: the particles travel almost in parallel, and we see them spreading from one point, like lines converging in the distance.

You do not need to stare at the radiant. Meteors can appear anywhere in the sky, and the longest trails often show up some distance from it. A wide view of the sky works best. The radiant rises in the evening and climbs higher through the night, reaching near its highest point around two hours after local midnight. That makes the Geminids one of the few major showers worth watching from late evening. If you are curious about the cultural side of this constellation, see Gemini, keeping in mind that astronomy and astrology are different things.

When is the peak?

The Geminids are active from roughly early to mid December. Their maximum is traditionally around December 13–14. The exact hour of the peak can shift slightly from year to year, and the phase of the Moon decides how many faint meteors you can see. A bright Moon washes out the dimmer ones, while a New Moon or a thin crescent gives excellent conditions. Before planning your night, check the Moon phase and a current forecast from the IMO or NASA.

The good news is that the Geminid peak is fairly broad. Even if you miss the very best hour, the nights before and after can still bring plenty of meteors.

Watching on a cold night

December nights are long, which helps, but the cold is real. A few practical tips:

• A dark place. Get away from street lights. Light pollution sharply cuts the number of meteors you can see.
• Warm layers. Watching means staying still, so bring a hat, gloves, a flask of something hot and a blanket or sleeping bag.
• A comfortable position. A reclining chair or a mat lets you lie back and look up without straining your neck.
• Let your eyes adapt. It takes about 20–30 minutes in the dark to see more. Avoid your phone screen, or use a red light.
• No binoculars or telescope. They narrow the field of view. Your own eyes are the best instrument for meteors.
• Patience. Meteors arrive in bursts. There may be minutes with none, followed by several at once.

For more ideas, see how to watch the sky.

Why it is worth it

The Geminids remind us that the sky is alive. A small rocky body swinging close to the Sun leaves a trail that, long afterwards, becomes a show of light above our heads. It is science you can feel with your own eyes on a cold December night.

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