How Northlanders can see Monday's 400-year celestial reunion of Saturn and Jupiter

Jupiter (left) and Saturn will be the nearest they've been in almost 400 years on Monday. Photo / Supplied

The two largest planets in our solar system will reunite for the first time in almost 400 years on Monday in an event named 'The Great Conjunction'.

Jupiter and Saturn, the fifth and sixth planets from the sun respectively, will converge on Monday and appear as one object around sunset for the first time since 1623. While the two planets come close to each other every 20 years, this year will be the first since famed astronomer Galileo was alive that they will intersect.

To spot the rare event, look west towards the setting sun. Then, by using the distance between your outstretched index and little fingers, measure one and a half hand spans to the north (right) and one hand span up and the planets should be clearly seen.

Keen stargazers have been tracking the two planets for weeks as they have inched closer together. For many, this year's sighting will be their only chance as the next projected conjunction will be in 2080.

"To sit on top of each other, that's particularly rare," Planetarium North director Peter Felhofer said.

"For the first time in living memory, people will be able to look through a telescope and see so much of the solar system in one peek."

The high-powered telescopes at the Whangārei observatory will be of great use on Saturday. Photo / Tania Whyte

The event is so significant, the Whangārei observatory is opening its doors to the public on Saturday for people to examine the two planets through high-powered telescopes and binoculars.

People are encouraged to visit the observatory at 500 State Highway 14 in Maunu from 9pm to view the spectacle as well as the Matariki star cluster and The Orion Nebula later in the evening. Admission is free with a donation advised.

Felhofer, who had been tracking the planets' progress closely in recent weeks, said it was a great opportunity to view the wonders of Saturn's rings and Jupiter's moons.

"The fact that these are two planets that are both visually stunning just means you're going to have so much in the field of view to absorb.

"To actually see it in a natural setting is very unusual."

The observatory is situated along State Highway 14 in Maunu. Photo / Tania Whyte

Felhofer said it was a momentous occasion for astrophotographers as it would be the first time one image of two planets could be taken.

This year's conjunction is especially rare as it will be the first time since 1226 that the two planets' intersection is visible. In 1623, the glare of the sun made it difficult to see.

At their closest, Jupiter and Saturn will be about 720 million kilometres from each other - almost 2000 times the distance between the Earth and the Moon.

MetService meteorologist Tahlia Crabtree was optimistic the weather on Saturday evening in Northland would suit stargazing but warned of high cloud cover on Monday evening which could obscure the planets.