Hand on heart, there is no real reason why most of us would want to try this technique – apart from the fact that it is fun and the effects can be amazing!
You need a camera with a B (bulb) setting, a lockable remote release for long exposures and a fully charged battery. A wide-angle lens will allow you to include some foreground and make more of the sky, plus you'll need a tripod
as exposure times are way too long to work hand-held.
Know Your Stars
Some idea of the night sky will help too – that is if you want circles of light moving around a central point, the pole star or Polaris. The pole star is not the brightest in the night sky but it is easy to find. Locate the constellation of the Plough (it is part of Ursa Major) and follow the two end stars to the pole star.
Aim the camera lens at the pole star and everything else will appear to revolve around it.
Moonless and cloudless nights away from big cities are the best times to try this technique. When it is really clear, you will be able to see the Milky Way and the sky will seem to glisten.
Really cold nights can be a problem because you will have frost forming on the lens if you are out there a while – it will be uncomfortable for you too.
Place the camera on your tripod, aiming the lens at the pole star, open the shutter on B and keep it open for... a while. By the way, the stars are obviously a long, long way away but don’t assume setting the lens to infinity will mean the star trails will be sharp. You need to spend a little while getting used to the low light and trying to focus.
A charged battery is important. If the battery dies before the image is written to card all that time you have been waiting will have been wasted. Remember, in cold temperature the battery will run down relatively quickly.
Try setting the lens aperture to f/5.6 and an ISO of around 400 start with exposures at ten minutes and see how it looks. You will probably pick up some light pollution – unless you really are miles from anywhere – and go from there. You could try doubling the exposure or tripling your exposures if the effect is weak. Doubling and tripling a ten minute exposure means 20 and 40 minutes respectively, so this is not a very social form of photography.
Get it right, though, and the effect is worth the effort, so good luck if you want to try this.