A rare “close encounter” of the solar system’s two largest planets Dec. 21 will be a chance to see one of the theories to explain the biblical “Christmas Star.”
Jupiter and Saturn will come so close together – visually – in the western sky that they will be basically one bright point of light. Because Saturn is five times farther from the Earth than Jupiter, they won’t really be that close but will appear that way. The two planets align every two decades, but astronomers say you’d have to go back to hundreds of years for a conjunction this close.
“The last time they were this close was about 400 years ago in 1623,” says David Weigel, director of the INTUITIVE Planetarium at the U.S. Space & Rocket Center in Huntsville, Ala. That’s the time of early astronomers like Gallileo and Johannes Kepler, Weigel said.
“Kepler was the first person who really understood how orbital mechanics worked and these objects traced elliptical paths through space,” Weigel said. “Once he was able to figure out the equations that governed this, he was able to trace time back a little bit and try to understand what things were like in the past.”
Kepler’s mathematics indicated a triple conjunction – Jupiter, Saturn and the Sun – in 7 B.C. Some astronomers theorize that the Star of Bethlehem in the biblical story may have been a rare triple conjunction of Jupiter, Saturn and Venus. Others point to a supernova around 5 B.C. as a possible explanation.
In the northern hemisphere this December, the planets will be low in the western sky at the time conjunction. They will be special to see any night on the week of Christmas, but they will be closest on the 21st. Weigel advises having “as clear a view as possible” of the western sky right at sunset.
“I think the neatest thing to do would be to watch them throughout the month of December to see them get closer and closer,” adds Mitzi Adams, assistant manager of the Heliophysics and Planetary Science Branch at NASA’s Marshall Space Flight Center, “then after the 21st , to watch them move farther and farther apart.”
“People really get excited about some things that are not a huge deal, and they don’t get excited about things that are a huge deal,” Weigel said, “and I think this is in the middle.... It is brighter than normal, a neat thing astronomically (and) not going to be close to this bright until 2080. In most people’s lifetimes, it’s not going to get any better than this with Jupiter and Saturn coming close together.”
“This is exciting, this is rare, but it’s not like the brightest thing anyone has seen in the sky,” Weigel said. “It’s like airplane bright.”