We have our very own Christmas star to look forward to: it is time to get ready for a spectacular conjunction next week between the giant planets Jupiter and Saturn. On 21 December, visible in the south-western evening sky, the two planets will line up so precisely that they may become indistinguishable from each other. The meeting of these great planets happens only once every 20 years. It is known as a great conjunction, and what is making this one so particularly memorable is that the planets will appear to be closer to each other than at any time since 16 July 1623. The planets are already close together in the sky, having been inching towards each other since the autumn. It will be worth watching them night after night this week to see them make their final approach. The chart shows the view looking south-west from London on 17 December at 17.00 GMT. On this evening, they will be joined by a beautiful waxing crescent moon. It will be a delightful sliver, with just 11% of its surface illuminated.