New Zealand stargazers will be able to witness a once-in-a-lifetime celestial event.

It has been hundreds of years since Jupiter and Saturn appeared this close together - as viewed from Earth - and it will be a long time before it happens again.

It's known by some as the 'Bethlehem Star'.

Otago Museum director and astronomer Dr Ian Griffin told Morning Report tonight, the planets Jupiter and Saturn will get remarkably close.

"They are going to be so close together, you won't be able to separate them ... you'll need binoculars and a telescope to split them, and this is something called a conjunction."

Jupiter and Saturn come together roughly every 20 years, "that's nothing particularly special," he says, but "to get this close together in the sky is incredibly rare".

Griffin says some astronomers' calculations suggest the two planets had been seen this close back in 1226.

"And you're not going to see them this close together until the year 2080."

To spot the planets, he explains: "Go out tonight, find the moon, and then look down to the left and you should see a bright star and that is Jupiter and Saturn.

"Then if you point your telescope or even a pair of binoculars at that bright star you'll see there are two planets - Jupiter and Saturn with its wonderful rings.

"And you if look really carefully you'll see ... four of Jupiter's moons and one of Saturn's. It will be seven planetary bodies all in that one field of view."

It is called the 'Star of Bethlehem' because "in 7BC there was a conjunction between Jupiter and Saturn, and back then the 'wise men' were astrologers and this would have had some significance to them" and it was mentioned in the Bible as well.

Griffin says anywhere with clear skies tonight will make for perfect viewing, ideally looking towards the southwest.

"They disappear by 11.15pm, so you need to be out just after sunset - [until] about an hour and half later."

Griffin will be going to the Mt John Observatory in Tekapo to view the conjunction of the planets.

-RNZ