Alignments between the planets, known as a 'conjunction', is "rather rare," Rice University astronomer Patrick Hartigan explained in a statement, "but this conjunction is exceptionally rare because of how close the planets will appear to one another."
The last time these gas giants appeared so closely, at a visible separation of only 0.1 degrees, was in the Middle Ages: at predawn on March 4, 1226.
Of course, the appearance of the 'Christmas Star' or 'Star of Bethlehem'—so named because the closeness of the planets creates a shining point of light—is a phenomenon only observed from Earth. In reality, Jupiter and Saturn remain millions of miles apart.
Where to see the 'Christmas Star'
Saturn and Jupiter have been moving steadily closer to each other since summer 2020.
Taking the time to look for these planets over the coming nights is worth it. "You can watch [the planets] move which is super cool, because you're actually seeing planets in orbit" Hartigan told USA Today, and watching for the pair coming together before solstice night will make identifying them that bit easier on the 21st.