The two largest planets in the solar system, Jupiter and Saturn, have fascinated astronomers for hundreds of years.
But the two gas giants will do something this month not seen since the Middle Ages — they will look like a double planet. The rare occurrence will happen after sunset on December 21, the start of the winter solstice. Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to one another.
You’d have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.
Back in 1614, German astronomer Johannes Kepler suggested that a conjunction of Jupiter and Saturn may be what was referred to as the “Star of Bethlehem” in the Nativity story, while others have suggested that the “three wise men” could have been a triple conjunction of Jupiter, Saturn and Venus. But it’s unknown if the “Christmas Star” was a real astronomical event, like a planetary conjunction or a comet.