A once-in-a-lifetime meeting in the sky

If you look up at the night sky on Dec. 21, you will see something humans haven’t seen since the Middle Ages. Jupiter and Saturn will appear in the night sky separated by only one-tenth of 1 degree. For viewers on Earth, it will appear the two planets are separated by the width of a dime held at arm’s length, according to NASA.

Astronomers call the event a great conjunction: From Earth’s perspective, Jupiter and Saturn will sit next to each other, though they are still separated by about 450 million miles.

The two planets will reach their nearest alignment in the early afternoon of Dec. 21 in North America. They will still be visible to the naked eye in the early evening, weather permitting, according to EarthSky. They should appear in the western sky just above the sunset with Jupiter, the solar system’s largest planet, appearing larger and brighter while Saturn transits above.

Jupiter catches up to Saturn in our sky by circling the Sun faster. While Saturn takes nearly 30 years to make one full orbit, Jupiter makes its circuit in just under 12. Every 19.6 years, Jupiter laps Saturn and stargazers see the two giant planets travel the night sky together. Because the Earth, Jupiter, and Saturn all occupy slightly different orbital planes, each of the periodic great conjunctions are slightly different.

“It’s like teenagers at a high school dance: They’re getting closer and closer together,” Harvard-Smithsonian Center for Astrophysics spokeswoman Amy Oliver told the Boston Globe. “It’s been a year of watching this, of them getting closer, and now they’re going to have a close slow dance.”

In 1623, Jupiter and Saturn would have appeared even closer from Earth. But because the conjunction occurred just 13 degrees away from the Sun, likely no one saw it. The great conjunction of 1226 was the last time stargazers got to see so close a passing.

“Most adult people have never seen a conjunction like this, and they won’t have an opportunity to see this again,” Oliver said.

The occurrence of this great conjunction at the end of December has led some in the media to dub the event the Christmas star or the star of Bethlehem. The gospel of Matthew tells of a star that led magi from the East to Jerusalem to look for Jesus after his birth. The 17th-century astronomer Johannes Kepler, who was a Christian, postulated the magi could have seen the triple conjunction of Jupiter and Saturn in the year 7 B.C., when the passing of the two planets was visible in the night sky three separate times over the course of several months.

Astronomer John Mosley wrote that Kepler did not claim the conjunction was the actual star of Bethlehem, but he theorized the two could have somehow been connected. Astronomers and Biblical scholars have also said the star could have been a nova, a comet, or a purely supernatural phenomenon.