Stargazers everywhere are in for a treat on Monday night, as Jupiter and Saturn will appear closer together than they have in centuries, forming a stunningly bright “Great Conjunction.”
“A Conjunction is an astronomical term that we use to describe when two objects are getting close to each other on the sky,” Jackie Faherty, an astrophysicist at the American Museum of Natural History, tells Yahoo Life. “A Great Conjunction seems to define a particularly close Conjunction.”
Given that Monday’s Conjunction involves the two largest planets in our solar system, “great” is a term this event certainly earns, according to Faherty. “On December 21st, if you look up towards the west just after sunset, what you should see is Jupiter and Saturn getting closer to one another than they've been in anyone on this planet's lifetime, at just a tenth of a degree away from each other,” she says.
While Conjunctions occur about every 20 years, Faherty says that Jupiter and Saturn haven’t appeared this close since the year 1623 — but that it would not have been visible from Earth due to an overwhelming glare from the sun’s placement at the time. “You have to go back to the year 1226 in order to get to the next best [Conjunction] which compares to this one that's coming up on December 21st.”
Jupiter has not changed significantly in brightness in the last 800 years, says Faherty, so those who caught a glimpse of the Great Conjunction of 1226 in the Middle Ages would have seen a nearly identical sky to what will be visible in 2020, but with far less context. “We're standing on the shoulders of scientific giants that have taken us to the point where we can take for granted that we understand that Jupiter and Saturn are just having a close moment in their apparent positions on the sky.”
This Great Conjunction of Jupiter and Saturn has been coined the “Christmas Star,” not only because of the festive timing, but because of connections that historical references may have to astronomical events from the past, some experts theorize.
“What happens in astronomy, which I think is really cool, is that we can go back through historical texts and we can locate when somebody has said something in a story or a folklore that we can connect back to an astronomical event,” she says. “A Biblical story that is told around Christmas time is of three wise men being guided by a star.”
Faherty explains that because the atmosphere tends to distort the way things appear, if two objects are near to each other, they can appear blended into one bright spot. “In this case, the star of Bethlehem idea may very well just have been a Great Conjunction of Jupiter and Saturn, which is bright and brilliant, and no one would be missing it.”
It’s important to note that while planets may appear to be close from our viewpoint, they still remain quite far apart in space. “It does not mean that if you were on Jupiter you could just hop leap over to Saturn. They’re still a hundred million miles or so away from each other,” she explains. “Instead they just appear to be close to each other on the sky because of our perspective on Earth as we're going around the sun and their positions going around the sun.”
As far as how to watch the brilliance of the Jupiter-Saturn Conjunction, Faherty says to opt for the darkest sky you can find, where you can see the western hemisphere, and take a look right after the sun sets. “If you have a telescope, you would actually see both [planets] very clearly resolved from each other.” Her other piece of advice, stick around for a while. “Watch for 30 minutes, 45 minutes, an hour. It's actually kind of cool to watch them move through the sky together, what you're watching is the earth rotating.”
To make matters more fascinating, the Great Conjunction coincidentally falls on the same night of the Winter Solstice, and while it may not have a correlation to the astronomical event, it marks the beginning of the season with the longest period of dark skies, perfect for stargazing.
According to Faherty, the next time Jupiter and Saturn will appear this close again will be in the year 2080, and then not again for nearly 400 years in 2417, so you won’t want to miss December 21st’s “Christmas Star.”
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