The event started after sunset on Dec. 21, 2020, the beginning of the winter solstice. Though this event happens roughly once every 20 years, astronomers have to go back to the Middle Ages to find an event where the two gas giants were in such close alignment, Rice University astronomer Patrick Hartigan said in a statement.
"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," said Rice Hartigan said. "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."
Between Dec. 16 and Dec. 25, the two planets will be separated by less than a full moon, Hartigan added.
"On the evening of closest approach, on Dec. 21, they will look like a double planet, separated by only fifth the diameter of the full moon," Hartigan explained. "For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening."
The nativity star may also have been a rare celestial alignment, with Jupiter, Venus and a star known as Regulus all aligning around 2 B.C., Justin Mason, a director of Old Dominion University's Pretlow Planetarium, told The Virginian-Pilot.
The celestial event that has been dubbed the Christmas Star, or Star of Bethlehem, can be observed anywhere on Earth, but Hartigan noted the farther north someone is, "the less time they have to catch a glimpse."
Franck Marchis, senior planetary astronomer at the SETI Institute and Chief Scientific Officer at Unistellar, told Fox News this is a celestial event "you don't want to miss."
Stargazers should try to see the event this month or risk waiting a long time for the next occurrence. The two planets won't be this close to each other again until March 15, 2080, and sometime after the year 2400, Hartigan said.
A version of this story was originally published on Dec. 21. It has been updated to include additional information.