Jupiter and Saturn will come into near alignment this month in a rare event that will make them appear as one, like a “Christmas star” in the night sky.
The planets' orbits are pulling them closer and on December 21 they will appear to be just a tenth of a degree apart – about "the thickness of a dime held at arm's length," Nasa said.
The event, called a “great conjunction,” takes place about every 20 years.
But their orbits have not aligned in the sky as closely as this since 1623, a few years after Galileo built his first telescope.
And that year, the two planets were just 13 degrees away from the Sun, making them almost impossible to view from Earth.
The last visible encounter when they were so close would have been in the Middle Ages, in the year 1226.
And they will not appear as near again until 2080, meaning few people alive today will ever witness it twice.
"You'd have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky," Patrick Hartigan, an astronomer at Rice University, told Forbes.
“On the evening of closest approach on December 21, they will look like a double planet, separated by only 1/5th the diameter of the full moon.”
The planets may appear close on December 21, the day of the Winter Solstice, but they will still be separated by a distance of about 400 million miles.
Their apparent proximity comes from the fact their orbital paths will cross, making them appear to meet in Earth’s sky, said Thabet Al Qaissieh, who runs Al Sadeem Observatory in Al Wathba.
"It will be right after sunset and you probably will have an hour or two at most to see them in the west of the sky," he told The National.
He said the planets would be visible with the naked eye, so there was no need to search for them using a telescope.
“They will appear like two very close stars from our perspective. It will look quite different.
“It will definitely be worth checking out.”
The Christmas Star is a key feature of the nativity story, which tells the tale of Jesus' birth.
In the Biblical story, the Star of Bethlehem led three wise men to baby Jesus.
Astronomers have long suspected the star was the result of an astronomical event such as a supernova, or even a conjunction between Jupiter, Saturn and Mars.
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