What are the Geminids?
The Geminids meteor shower occurs in mid-December each year and is one of the most reliable and most spectacular meteor showers visible from the southern hemisphere. The meteors are caused by small particles of rock and dust from the asteroid-comet (3200) Phaethon entering the Earth’s atmosphere. As they vaporise high overhead the particles leave long, bright and sometimes colourful trails across the sky.
As Phaethon orbits the Sun it passes relatively close to the Sun, its surface bakes and crumbles in the sunlight and forms a cloud of dusty particles surrounding the asteroid-comet. These particles are larger than the typical dust from most comets which makes the Geminid meteors particularly bright and long-lasting. The meteors we see each December departed Phaethon long ago and formed a swarm of particles along Phaethon’s orbit. When Earth passes through this swarm each December, like a car driving through rain we see the meteors appear to radiate from the constellation of Gemini.
How can I see the Geminids?
All meteor showers have a broad window covering many days when at least an occasional meteor will be seen, but also a peak period lasting less than a day, and sometimes less than an hour, when maybe hundreds of meteors are seen. The Geminids are visible from about December 4 to 20 and their peak activity lasts several hours.
For the best chance of seeing the Geminids in Australia in 2020 plan ahead for the night of Sunday-Monday, December 13-14. Early on Monday morning, from about midnight to 4am, will be the best viewing time.
No telescope or binoculars are required. Your eyes are the best instruments for viewing a meteor shower.
Find a dark, country site with a clear view of as much of the sky as you can get – the whole sky if possible. If you are stuck in a city or a large town find the darkest (and safest) place you can – a large park, a coastal headland, a suburb on the outskirts. Take a blanket, something warm to wear and maybe something warm to drink – you will be awake for a while yet and even in summer it gets surprisingly cold lying still watching the sky.
Get yourself into position from about 11pm on Sunday, get comfortable, lie back and watch the sky overhead and to the north. After 15-minutes your eyes will have adapted to the darkness. Now, every minute or two (more often if we are lucky) you should see a bright meteor fly silently overhead. Some will leave short trails, some will be faint. But, if everything goes to plan, some meteors will cross the whole sky from north to south, shining brightly and even showing a hint of colour! Once you see the first one you will be hooked for the night.
From a light-polluted city location only the brightest meteors will be visible. From a dark country site the viewing is good all night because the Moon is below the horizon. Even when it rises at about 5am it is just a thin waning crescent (New Moon falls on early Tuesday morning) and will cast little light – but by then early twilight will begin to interfere.
If Sunday night is clouded out try again on Monday night, although the shower will be just past its best.
Update: While a lot of the Australian east coast was clouded out, or raining heavily, the cloud forecast sites noted in the comments below showed some clear areas west of Sydney. I managed to view the meteors from Mt Canobolas (near Orange). It was windy and cool and more light-polluted from Orange than I expected, but I saw about one meteor every couple of minutes or so from about midnight to 3am. Most were bright, some arced across the whole sky, but I didn’t notice any colour.