If we're lucky, then in just a few days time we could witness one of the most spectacular sights in the night sky for a generation or more. Astronomers hope that on 3 December a comet will appear on the eastern horizon - Comet Ison. For the whole month of December, millions of people across the northern hemisphere should be able to see its tail, which is several millions of kilometres long, stretching across the dawn sky. Ison has come from the Oort cloud, a belt of comets on the very edge of the Solar System, where it has been for the last 4.6 billion years. http://www.bbc.co.uk/news/science-environment-25052236
