This is the spot in the sky from where the Geminid meteors originate, known to astronomers as the radiant. Drawing Nick Lomb
Every year at this time the Earth runs into a stream of dust circling the Sun. As it hits the stream the dust particles burn up 100 km or so above us. As they do, each dust particle leave behind a streak of light – a meteor. These particular meteors all appear to originate from a point in the constellation of Gemini and hence they are known as the Geminids. The meteors can appear anywhere in the sky, but if you trace them back, you will see that they originate from Gemini.
In 2010 the Geminid Meteor Shower is expected to peak in the afternoon or evening of Tuesday 14 December in Australian time. Of course, in Australia they are not visible at that time as the sky is not dark and the constellation of Gemini has not risen above the horizon. For Australia the best time to view is in the early morning before dawn, say between 2:00 and 4:00 am summer time. Fortunately, the Moon sets about 1 am AEST in Sydney, a little later from Melbourne, and will not disturb the viewing by brightening the sky. As indicated in the diagram above look towards the north, but note that the meteors can appear anywhere in the sky.
It is best to observe from as dark a sky as possible. If you cannot leave the city, you can still try to find a suitable location. For Sydneysiders it could be looking from Bondi Beach. Of course, the sky needs to be clear as you would not expect to glimpse the meteors though cloud.
How many meteors can you expect to see? At the peak of the shower, from a dark site, with Gemini high in the sky almost two meteors a minute are expected. By the time we in Australia have an opportunity to view the shower the rate is likely to have dropped to at least a half. Plus Gemini is low in the sky and that also cuts the rate. So even from a dark site the best that can be expected is a meteor every two or three minutes. However, the Geminids are known for bright meteors, especially after the peak, so it is still worth watching.
The path of the asteroid 3200 Phaeton, the parent body of the Geminids around the Sun. From JPL Small-Body Database Browser
The origin of the Geminids is a mystery. All other meteor showers are due to comets, but the parent body of the Geminids is a 5 km wide asteroid or space rock called 3200 Phaeton. Why is it losing dust? One possibility is that it does so each time it approaches the Sun, as at that time is only 21 million km from it, well within the path of the innermost planet Mercury. However, at its close approach in 2009, Phaeton was closely monitored by spacecraft and little dust seemed to be emitted. The case is not yet closed!
Let Sydney Observatory know if you see the Geminids. Good observing!