An extremely rare astronomical phenomenon is about to take place, as Jupiter and Saturn appear side by side in the sky in the closest alignment seen for nearly 400 years.

The 'great conjunction' between the two planets occurs every 20 years, but it often happens during daylight and is impossible to see with the naked eye. In other cases the planets do not come very close together.

This year's great conjunction will be the closest since 1623, the year Shakespeare's collected works were first published.

And although the exact alignment does not take place until Monday, the best time to see it will be this afternoon, according to a leading Devon astronomer.

The chairman of Sidmouth's Norman Lockyer Observatory, David Strange, said:

"This event is so dependent on favourable weather.

"Unfortunately the forecast for the date of the exact conjunction on Monday is very poor, so this afternoon looks like offering the best chance."

"Tonight they will be visible in a low power field of view of a small telescope., being closer than the width of the full moon."

The observatory plans to livestream the Great Conjunction on its NLO YouTube page, starting from this afternoon from 4pm onwards if clear.

Exeter University will also offer a livestream.

The Great Conjunction is being seen as hugely significant by astrologers around the world, because Jupiter and Saturn will align in the air sign of Aquarius.

In astrology, the sign is associated with social change, humanitarian efforts, and justice.

The astrology site Forever Conscious predicts: "Along with the world becoming more digital, we may also start to see a rise in community-based living.

"Rather than looking to big organisations or government, we may instead find the support we need from our own community members.

"Creating a sense of community may also feel more important or necessary."

The conjunction will peak on Monday, December 21, at 6.37pm UK time. If the clouds clear it will be visible in Britain from about 4.30pm until 6pm at 15 degrees above the south-west horizon.