Jupiter and Saturn, the two largest planets in the Solar System, were stationed in nearly the same spot in the night sky on Monday, creating a bright spectacle.
The positions of the two planets align once every 20 years, according to NASA. While conjunctions happen several times during the course of a typical lifetime, a conjunction of this magnitude is quite rare. The last time the two planets were this close to each other in the night sky was 400 years ago, but no one was able to see it as that conjunction occurred during the day.
The last time a conjunction of his magnitude happened at night was 800 years ago.
So what causes the conjunction?
“You can imagine the solar system to be a racetrack, with each of the planets as a runner in their own lane and the Earth toward the center of the stadium,” said Henry Throop, astronomer in the Planetary Science Division at NASA Headquarters in Washington. “From our vantage point, we’ll be able to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21.”
The timing of the conjunction adds to the spectacle. Monday also marks the winter solstice.
Being four days before Christmas, some are dubbing the event as “The Christmas Star,” although the conjunction is of planets and not stars. Scientists have long speculated whether the Star of Bethlehem was the 7 BC conjunction of the two planets.
“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” said Throop. “The date of the conjunction is determined by the positions of Jupiter, Saturn, and the Earth in their paths around the Sun, while the date of the solstice is determined by the tilt of Earth’s axis. The solstice is the longest night of the year, so this rare coincidence will give people a great chance to go outside and see the solar system.”
While the two planets will have the appearance of being very close to each other, in reality, they will be hundreds of millions of miles apart.
For the next few days, Saturn and Jupiter will remain very close to each other in the night sky. The best time to view the planets will be roughly one hour after sunset in the southwest sky. Jupiter will appear to be brighter than Saturn as it's closer and larger.