Special event for stargazers

Stargazers could be in for a treat tonight, but it all depends on the weather.

Starting at around 6.10pm, the moon will "occult" - pass in front of - the planet Jupiter and its four moons but astronomers urge people to get outside at about 6.05pm to make sure they don't miss it.

The event will be visible with the naked eye from almost everywhere north of Dunedin - provided the sky isn't too cloudy, but for keener amateur astronomers, binoculars or a telescope will give a better view.

It's the first good look at the moon occulting Jupiter for at least half a century because it happens at night when the moon will be relatively high in the sky. "You might have to wait at least 70 years to see it better than this so it's quite a special event," said astronomer Dr Grant Christie.

Starting around 6.10pm, the first of Jupiter's four moons, Ganymede, will be snuffed out by the dark edge of the moon, the right-hand side. Next will be the moon Io and then, at around 6.15pm, Jupiter itself will be occulted.

Because of its size, Jupiter would fade quite slowly over several minutes while the moons would simply "blink" and be gone, Dr Christie said.

The other two Jupiter moons, Europa and Callisto, will disappear behind the dark edge of the moon a few minutes after Jupiter.

It will take about an hour for the giant planet and its moons to pass behind our moon and begin reappearing on the other side, the brighter edge. That will happen from 7.15pm and it will take about two minutes for Jupiter to emerge completely.

But weather forecasters warn cloud is expected to cover most of the country this evening.

Auckland, Waikato and Bay of Plenty will have patchy cloud cover but the sky could be clouded over in Northland and Coromandel Peninsula.

The southwest of the North Island should be mainly fine with high cloud and the forecast was similar over inland eastern parts of the South Island. "It's probably going to be the luck of the draw but some people might be lucky," said forecaster Cameron Coutts.

Depending on visibility, Jupiter will appear as a bright orange-coloured star to the naked eye but through a telescope it appears as a silvery disc with its reddish bands of cloud high in the atmosphere.

The planet is 2000 times further from Earth than the moon, 766,210,574km away as compared to the moon's 386,431km.

"It's by watching a rare event like this that you can appreciate the three-dimensional scale of the solar system," Dr Christie said.