Rare planetary alignments could create a “Christmas star” similar to the one that is said to have led the Three Wise Men to the nativity scene, according to the astronomer.

On Monday, Saturn and Jupiter will come as soon as they have done in centuries, creating what the researcher describes as “an amazing event in the sky.”

While conjugations – where objects appear so close together in the sky – are not unusual, this would be “ exceptionally close ” – the closest since 1623, Professor Michael Burton said.

The director of the tallest astronomical observatory in the British Isles said that the two largest planets in the solar system would be only 0.1 degrees away, one-fifth of the diameter of the full moon, and together they would appear as the brightest object in the sky. .

Professor Burton said: “When it happened it was in the daytime sky so people wouldn’t have seen it because it was going to be so light.

“The last one that was actually in good position to see was 1226, so we’re going to go 800 years back to the last time they could be seen in the dark sky.”

The astronomer at the Armagh Planetarium and Observatory in Northern Ireland said the pairing is one possibility for what has become known as the Christmas star from the story of Jesus’ birth.

He said, “Certainly one of the possibilities is that two of the planets came together.

“The whole question of what a Christmas star might be is a very exciting question in itself.

“Whether it was a coupling, if it was a guilty one, whether it was a supernova. Nobody actually knows what it was.”

He said the projected brightness of the sky on December 21 – which is also the winter solstice – is “one possibility of what was seen 2,000 years ago.”

The advice for avid star-watchers is to look southwest as soon as possible after sunset, with the best time between 4.30pm and 6pm, and while a telescope or binoculars help, the phenomenon will be visible with the naked eye – weather permitting.

“We have to cross our fingers that there will be a clear sky,” said Professor Burton.

The Royal Astronomical Society advised people to find a line of sight without tall buildings or trees blocking the view.

He said, “With the eye, a pair (of planets) will appear as two bright points of light. Binoculars will show these as slightly larger points: viewers may also be able to identify the four largest moons of Jupiter, and possibly Saturn’s largest moon Titan.

“With a small telescope, the two planets will be in the same field of view, which is an extremely rare event, and Jupiter’s tow belts and Saturn’s rings should be clear.”

The Bureau of Meteorology predicted rain for Monday, and said the chance of clear skies is greater in the northern half of the UK.

Even in bad weather, the Royal Astronomical Society said not all was lost because “the two planets appear to be diverging very slowly, and will remain unusually close in the following days.”