The Geminids meteor shower
Key facts
- Peak
- around 13–14 December
- Active
- about 4–20 December
- Rate at peak (ZHR)
- about 150
- Speed
- 35 km/s
- Radiant
- Gemini (near the bright star Castor)
- Parent body
- Asteroid 3200 Phaethon
Dates shift slightly from year to year. This year's peak is in the events calendar, and the meteor-shower planner shows how high the radiant climbs from where you live, night by night.
The Geminids are, in most years, the richest meteor shower of all. They are reliable, plentiful and bright, and because the radiant is up for most of the night they suit evening watchers as well as early risers. Their only drawback is that they arrive in the cold of mid-December.
A young shower. Compared with ancient showers like the Lyrids, the Geminids are newcomers. They were first noticed in 1862, when observers in England and the United States independently reported meteors coming from Gemini in mid-December. At first the shower was weak, but it has grown steadily stronger. Today its ZHR is about 150, higher than the Perseids. The change is caused by the gravity of the planets gradually shifting the stream so that the Earth now passes through a denser part of it.
A rock comet. For a long time no one knew where the Geminids came from. Then in 1983 two astronomers, Simon Green and John Davies, found a small object in data from IRAS, an infrared space telescope. The American astronomer Fred Whipple soon pointed out that its orbit matched the Geminid stream. The object was named 3200 Phaethon, after the son of the Sun god in Greek myth, because it passes extremely close to the Sun, closer than the planet Mercury.
Phaethon is about 5 to 6 km across and looks like an asteroid, not a comet. So how does it make a meteor shower? At its closest approach, its surface becomes hot enough to bake and crack rock, and it has been seen to brighten briefly near the Sun, though whether it still sheds significant dust is debated. Some scientists call it a rock comet. It may be the remains of a larger body that broke apart, or a comet that lost its ice long ago. A Japanese mission called DESTINY+ is planned to fly past it and study it close up.
What to expect. Geminid meteoroids appear to be denser and tougher than typical comet dust, so they survive deeper into the atmosphere. They enter at a moderate 35 km/s, so they last a little longer than Perseids or Leonids. Many are bright and white, and some show yellow, green or blue colours. Under a dark sky at the peak an observer might see 60 to 100 or more an hour, with occasional fireballs.
Viewing from the UK. The UK is excellent for the Geminids. Gemini rises in the north-east in the early evening and is high in the south by about 2 a.m., so you can see meteors from mid-evening right through to dawn. The highest rates usually come after midnight, when the radiant is highest, but evening viewing is worthwhile too and suits children. Look for the two bright stars Castor and Pollux, which mark the heads of the Twins. December nights are long but very cold, so dress warmly.
Viewing worldwide. The Geminids are best from the northern hemisphere, and the radiant, about 32 degrees north of the celestial equator, is also well placed from the tropics, especially around local midnight to 2 a.m. From mid-southern latitudes such as Australia and South Africa it is still visible, but lower, with reduced rates.
Peak dates vary slightly from year to year, and a bright Moon will cut the count, so check the lunar phase.
Viewing tips
- Start watching from mid-evening, because the radiant is up from soon after dark.
- For the best rates, watch around 1 to 3 a.m., when Gemini is high in the south.
- Look for the bright stars Castor and Pollux to find the radiant.
- Dress for freezing temperatures and bring a hot drink.
- Watch for colours: many Geminids are white, with some yellow, green or blue.
Its parent is thought to be Phaethon, an asteroid that may be a burned-out comet. See the other comets behind meteor showers.