Jupiter and Saturn will appear very close together in the night sky on 21st December in what astronomers refer to as a rare ‘Great Conjunction’. They are not literally close together, they will be millions of kilometres apart but as viewed from Earth they will appear to be separated by less than a fifth the diameter of the full moon as it appears in the sky. This is the closest they have been in conjunction, just 0.1 degrees of arc, since the seventeenth century (the year 1623), in a rare ‘Great Conjunction’. In that year, Wilhelm Schickard invented his Calculating Clock, a mechanical precursor of the pocket calculator.
When that last similar conjunction occurred the two planets were close to each other in the sky but also appeared close to the Sun so would’ve been difficult to observe. Prior to that, an observable conjunction occurred in 1226 long before the invention of the telescope in the year that Saint Francis of Assisi died, apparently.
Just to clarify, as planets orbit the Sun, they occasionally appear to be close to each other in the sky, it’s an optical illusion. On 21st December, Jupiter and Saturn will be almost 800 million kilometres apart in the solar system.
To see the conjunction, look low in the south-west after sunset. As the sky darkens, first Jupiter and then Saturn will become visible. Both planets are bright – in the case of Jupiter brighter than all the stars – so will be obvious in a clear sky. By 17h00 GMT both planets will be less than ten degrees above the horizon for UK observers, so it is important to find a line of sight without tall buildings or trees that will block the view.
They will be visible to the naked eye, but with a small telescope you will be able to see both planets in the same view and Jupiter’s cloud belts will be apparent as will Saturn’s rings. The peak of the conjunction is the 21st, but they will appear to move apart from each other only slowly in the days that follow. There is lots of musing as to whether such a conjunction in history was the origin of the myth of the Star of Bethlehem.