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Of all the annual meteor showers, arguably the best one of all is the Geminids. Also known as the "Winter Fireworks", the Geminids are active from December 4th to December 17th, with peak activity occurring on December 13th. The peak zenithal hourly rate (ZHR) or the number of meteors seen under perfect conditions with the radiant overhead, is around 120 per hour. The only other comparable annual meteor shower to attain such reliable highs is the August Perseids. Like the Geminids, the Perseids are superb but with a lower ZHR of around 80.

Since the new Moon occurs on the same day as the peak, moonlight will not interfere and the 2012 Geminids are set for a spectacular show.

A Geminid meteor streaks through Ursa Major in 1998 (Yukihiro Kida)

Parent asteroid

The Geminids are unusual in that the source object is an asteroid, 3200 Phaethon. Together with the Quadrantids, they are the only major meteor showers not originating from a comet. Phaethon has an unusual orbit that brings it closer to the Sun than any other named asteroid, although there are several unnamed asteroids that do approach closer. At perihelion, the point of closest approach to the Sun, Phaethon is only 0.14 AU distant with the orbit being more like that of a comet than an asteroid. At perihelion it approaches the Sun to within less than half that distance of the innermost planet Mercury, while at aphelion (the point of furthest distance from the Sun) it is 2.4 AU distant and further from the Sun than Mars. It is perhaps a little strange that no cometary activity has ever been connected to Phaethon but nevertheless, the Earth passes through the debris field, resulting in the Geminids meteor shower.

Phaethon was the first asteroid to be discovered using images from a spacecraft. While investigating data from the Infrared Astronomical Satellite (IRAS), Simon Green and John Davies discovered Phaethon on October 11, 1983. Its diameter is only 5.1 kilometres (3.2 miles).

Radiant

The Geminids radiant or the point in the sky where the meteors appear to emerge from is very easy to find. At the time of peak activity, it is located very close to Castor (α Gem). Castor is a multiple star system that has a combined apparent magnitude of 1.6, making it one of the brightest stars in the sky. Even brighter at magnitude 1.1 and located only a few degrees southeast of Castor is Pollux (β Gem), the brightest star in Gemini.

Geminids Radiant and Star Chart

Geminids Radiant and Star Chart - pdf format

What to expect

The Geminids are slow moving meteors and often bright. With a declination of +33 degrees, the radiant appears very high in the sky after midnight - even close to overhead - from mid-latitude northern latitudes. At the moment, the shower activity appears to be intensifying each year and with no interfering Moon, northern hemisphere observers should see a spectacular and dramatic meteor show. The best two nights to look are on December 12/13 and 13/14. If you can brave the cold and stay up all-night, peak activity will occur and persist long enough to produce an impressive display of meteor activity. Observers from southern hemisphere don't have it quite so good due to the lower altitude of the radiant, but the temperatures are more pleasant and the shower is still excellent.

As with all meteor showers it is best not to look directly at the radiant itself; the meteors can appear many degrees away from it and even in a completely different area of the sky. To be certain you have seen a Geminid, trace back the meteor trail and it should go all the way back to the radiant.

Of course if it is cold, wrap up warm. A good observing tip when looking for meteors is to scan the general region of the sky around the radiant, ideally when lying down on a deck chair or something similar.

Geminids 2012 Data Table

Meteor shower nameGeminids
Radiant ConstellationGemini
Dates4th December -> 17th December
Peak Date13th December
RA (J2000)7hr 28m
DEC (J2000)+33d
Speed (km/s)35
ZHR 120
Parent3200 Phaethon (asteroid)