JUPITER and Saturn are set to closely align in the night’s sky to form a ‘Christmas Star’, for the first time in 800 years.

The two planets will line up to create a bright light visible from Earth on Monday 21 December - the same date as the winter solstice.

The celestial event is known as the Great Conjunction by astronomers and has also been nicknamed a Christmas Star or Star of Bethlehem.

What is the Christmas Star?

In the bible, the Christmas Star was what the wise men used as a guide to help them find the birthplace of Jesus Christ.

What the actual bright light was has been debated for many years with one possible explanation being a rare conjunction of multiple planets.

In more recent times the Christmas Star, or Star of Bethlehem, is often the final decorative piece placed on the tops of Christmas trees.

The Great Conjunction has also taken the nickname of Christmas Star and Star of Bethlehem due to the time of the year this alignment occurs.

What is the Great Conjunction?

It’s a rare phenomenon which sees Jupiter and Saturn appear to align in the night’s sky.

The two Gas Giants are rarely seen this close, with experts predicting they will be separated by less than the apparent diameter of the moon.

Though they will look aligned the two planets will be hundreds of millions of miles apart.

When was the last Great Conjunction?

The orbits of the two Gas Giants and Earth means there is an alignment every 20 years, which makes Jupiter and Saturn appear closer in the night’s sky.

The last time they appeared this close was way back in the 13th Century and NASA doesn’t predict another alignment this close for another 60 years.

Patrick Hartigan, an astronomer at Rice University in America, said in an interview with Forbes that it was an “exceptionally rare” event.

And added the last time the planets were aligned this close was on 4 March 1226.

Where will I be able to see the Christmas Star?

The Christmas Star will be visible from Earth, with the help of a clear night’s sky.

The two planets will move into closer alignment from Thursday 17 December for roughly a week, with the closest point being Monday 21 December.

Read more: When is the shortest day of the year 2020? Date of the winter solstice and how many hours of daylight there are

Stargazers can expect to see the phenomenon low in the western sky shortly after sunset, meaning people in the UK should be on the lookout from 3.54pm.

It is bright enough to be seen without the use of telescopes or binoculars, yet astronomers say several of the planets’ satellites will be visible with viewing aides.

How far away are Saturn and Jupiter from Earth?

Jupiter's distance from Earth is constantly changing due to the different orbits of the two planets. It is 365 million miles away at its closest point and 601 million miles away at its furthest.

It's a similar story with Saturn's orbit, which has its own course and doesn't run parallel with Earth's, meaning the distance varies.

Even at its closest point Saturn is still 746 million miles away and just over a billion miles away from Earth at its furthest point.

There are 403.3 million miles between Jupiter and Saturn.

How big are Saturn and Jupiter?

Jupiter, the fifth planet from the Sun, is the largest in the Solar System. It is 11 times wider than Earth, with a radius of 43,440 miles.

Saturn is sixth in line in the order of planets from the Sun and is the second largest. It is nine times the width of Earth, with a radius of 36,183 miles.

Earth has a radius of 3,960 miles. The radius is a straight line from the centre to the circumference of a circle.