Winter solstice this year there will be a a great planetary conjunction between Jupiter and Saturn on December 21st, 2020. The most recent great conjunction was in spring 2000, and the next one will be in 2040. In 2239 there will be a triple conjunction, where Jupiter appears to pass Saturn three times.

Saturn and Jupiter will appear in conjunction at their closest since 1623, and their closest visible from Earth since the Middle Ages (1226). It occurs because Jupiter takes 11.86 years to orbit the Sun and Saturn takes 29.4 years, which means they will sometimes appear to pass each other from Earth's point of view. Some wonder if the star of Bethlehem was a similar celestial event.

Johannes Kepler calculated these conjunctions back to the first decade BC. His calculations showed that Jesus was born a few years earlier and that "four years must be added to the Epoch of Christianity [Anno Domini] now in use" - Read more: Kepler and the Star of Bethlehem.

Winter solstice takes place on Monday, December 21st, starting just after midnight in Hawaii, 2am on the West coast of North America, 5am on the East coast, and so on. The great conjunction will become visible from December 20th to 24th, but will be at it's climax on solstice. Whether or not you believe it's some sort of end-of-the-world prophecy, it'll be a cool astronomical event!