Jupiter and Saturn are set to align in a conjuction for the first time in 800 years to create a “Christmas Star”.

The planets will line up on December 21 and appear to form a double planet.

The Great Conjunction, also nicknamed the “Christmas Star” or “Star of Bethlehem”, hasn't been seen for 800 years.

According to Forbes, a “once-in-a-lifetime” sighting of this proportion won’t occur again until 2080 and then sometime after 2400.

What is the Christmas Star?

Jupiter and Saturn will venture close to one another and appear to be colliding later this month.

The gas giantswill be closer in the night sky over Christmas than they have been for centuries.

But NASA said in reality, they'll actually be hundreds of millions of miles apart.

News Shopper: The celestial sight should — local weather permitting — be visible from anywhere on the EarthThe celestial sight should — local weather permitting — be visible from anywhere on the Earth

The conjunction is said to have last been seen in the 13th Century and it reportedly won't happen again until around 2080.

Speaking to Forbes magazine, astronomers have said that such an alignment is "rather rare" to observe.

Patrick Hartigan, astronomer at Rice University in Texas, said: "Alignments between these two planets are rather rare, occurring once every 20 years or so.

"This conjunction is exceptionally rare because of how close the planets will appear to be to one another."

He added: "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."

When will it be visible?

The "Christmas Star" is set to be visible to stargazers later this month on the evening of December 21.

It's been reported that stargazers should expect the celestial event to present itself just after sunset.

It can be observed from anywhere on Earth where skies are clear.

It means that those in the UK should look for the phenomenon from 3.53pm onwards.

The "star" will appear very low on the horizon just after sunset, with binoculars or a telescope necessary.

Professor Hartigan noted that the planetary duo will appear low in the western sky around sunset each evening — and should be bright enough to be viewed during amid the twilight sky.

"The further north a viewer is, the less time they’ll have to catch a glimpse of the conjunction before the planets sink below the horizon," Professor Hartigan explained.

"By the time skies are fully dark in Houston, for example, the conjunction will be just 9 degrees above the horizon,' he added. 

"Viewing that would be manageable if the weather cooperates and you have an unobstructed view to the southwest.'"