NBC News reports that on Dec. 21—which is the longest night of the year in the northern hemisphere—the two giant planets will appear the closest that they have been in centuries. Some astronomers refer to the celestial event as the Christmas Star due to a “belief that the biblical tale of the Star of Bethlehem could have been a planetary conjunction,” the outlet writes. Various reports have also said it will look like a "double planet," with the two bodies appearing a mere 0.1 degrees apart.
About two thousand years ago, Venus and Jupiter were nearest—and the last time Saturn and Jupiter were this close to each other was in 1623, though stargazing conditions back then meant that people on Earth couldn’t see it. The last time such a conjunction could be seen by the naked eye was almost 800 years ago, in 1226.
From Dec. 15 to Dec. 18, stargazers can observe Jupiter and Saturn moving toward each other by looking at the waxing crescent moon in the western sky 45 minutes after sunset, according to NASA's Night Sky Network.
“Keep in mind that while the two gas giants may appear close, in reality they are hundreds of millions of miles apart,” NASA writes.” This will still be quite a striking sight, but you will need to look fast as both planets will set shortly after sunset.”
On Dec. 21, the moon will sit high in the sky, and Jupiter and Saturn will sit closer to the horizon in the western sky so that they appear to be one big star. It’s possible that stargazers can see the planets and some of their moons at the same time.