Kiwi stargazers will be in for an end-of-year treat when Jupiter and Saturn appear close together in the night sky on Monday night.
It's been nearly 400 years since these two planets aligned as closely as they will night, and to the unaided eye, they will appear to almost touch. At their closest, they'll be just 0.1 degrees apart, which is just one-fifth of the diameter of the moon. This extra-close conjunction won't be matched again until March 2080.
The two planets' positions are aligned about once every 20 years, but 1623 was the last time Jupiter and Saturn passed this close to each other in the sky, and the first time in nearly 800 years since the alignment happened at night.
How to see Jupiter and Saturn
The rare sighting can be seen every night until the end of the year, weather permitting, and can be viewed with your naked eye. However, Otago Museum director and astronomer Dr Ian Griffin told RNZ's Morning Report you'll need binoculars and a telescope to split the planets since they're so close. But they'll still be easy enough to find.
"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," he says.
"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 if you 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."
But they disappear by 11:15pm, so you need to be out just after sunset until about an hour and a half later, he says.
What is a Great Conjunction?
Astronomers use the word 'conjunction' to describe planets appearing very close to each other in the sky when they line up with Earth in their respective orbits. But because these occur relatively rarely, conjunctions of Saturn and Jupiter are called a 'Great Conjunction'.
NASA astronomer Henry Throop says to imagine the solar system as a racetrack with each planet running in their own lane with Earth near the centre of the stadium.
"From our vantage point, we'll be able to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21."
From Earth, the two planets will appear incredibly close, but they're actually hundreds of millions of kilometres apart in space.
"Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits," Throop says.
"The date of the conjunction is determined by the positions of Jupiter, Saturn, and the Earth in their paths around the Sun."
Auckland's Stardome Observatory says Jupiter and Saturn will gradually sink towards the horizon as December progresses and will reappear on the opposite side of the sky, before sunrise, later in February.
