The two largest planets in the solar system, Jupiter and Saturn, are set to pass each other so closely on 21 December that they will appear to form a "double planet", which is also being referred to as a 'Christmas star' by astronomers.
The last time this phenomenon, known as a planetary conjunction, was clearly visible from Earth was in 1226.
In Finland, the conjunction will be best seen south of Oulu in the early evening, at around 4pm. After the sun sets in southern Finland at 3pm on 21 December, the shortest day of the year and the winter solstice, sky gazers hoping to catch a glimpse of this rare event should find an area with good visibility and look in a south-western direction, according to American space agency NASA.
The planets are set to be at a very low horizon and therefore the view could easily be blocked by buildings and trees, and they will likely appear as one point of light to the naked eye. However, the conjunction will be more easily distinguishable with the use of binoculars or a telescope, including Saturn's rings and Jupiter’s four largest moons: Io, Europa, Ganymede and Callisto.
Witnessing the conjunction will also depend on weather conditions, especially a clear sky.
A similar 'close encounter' occurred about 400 years ago, in 1623, but the planets were too close to the sun on that occasion and therefore very difficult to see from Earth.
Although the planets will appear close, in reality they will be millions of kilometres apart. Jupiter’s average distance from the sun is 779 million kilometres while Saturn is about 1.4 billion kilometres away.