The Geminids or "Winter Fireworks" are the richest of the annual meteor showers. This year they peak on the evening of December 13th/14th when up to 120 meteors per hour can be seen. Of all other annual showers only the August Perseids comes close to attaining such highs. In addition this year's event has excellent prospects; on the main evening the waxing crescent Moon will have long set before peak activity kicks in.
The Geminids are unusual in that the source 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 - Phaethon is only 0.14 AU distant from the Sun and much closer than innermost planet Mercury.
On the other side at aphelion (the point of furthest distance from the Sun) Phaethon moves out to 2.4 AU and therefore beyond the orbit of Mars. It's perhaps a little strange that no cometary activity has ever been associated with Phaethon but nevertheless the Earth passes through the dust 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).
The Geminids radiant or the point in the sky where the meteors appear to originate is very easy to find. At the time of peak activity it's located very close to Castor (α Gem) a multiple star system that has a combined apparent magnitude of +1.6 and 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.
What to expect
The Geminids are active from December 4th to 17th. They are slow moving meteors that are 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. The best time to starting watching is before midnight on the evening of December 13/14th although Geminid "shooting stars" can be seen as soon as it's dark enough. Observers from the Southern Hemisphere don't have it quite so good due to the lower altitude of the radiant.
As with all meteor showers it's best not to look directly at the radiant itself as 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.
Geminids Data Table 2015
|Meteor shower name||Geminids|
|Meteor shower abbreviation||GEM|
|Activity||December 4th -> December 17th|
|Peak Date||December 13th / 14th|
|RA (J2000)||7hr 28m|
|Parent body||3200 Phaethon (asteroid)|
|Notes||Together with the Quadrantids, they are the only major meteor showers not originating from a comet.|
Asteroid 3200 Phaethon (at epoch April 18, 2013)
|Discoverer||Simon Green / John Davies / IRAS|
|Discovery date||October 11th, 1983|
|Semi-major axis (AU)||1.27128|
|Orbital period (years)||1.43345|
|Longitude of ascending node (degrees)||265.267|
|Perihelion||March 15th, 2015|
|Notes||Phaethon approaches the Sun closer than any other named asteroid|