13 Nov 2016

'Supermoon' promises celestial spectacle

9:07 pm on 13 November 2016

The largest, brightest full moon in nearly seven decades will grace the skies overnight Monday/Tuesday, promising a celestial "supermoon" spectacle.

The moon will come nearer to Earth than at any time since 1948, astronomers said.

Supermoon rising over Cape Palliser, Wairarapa on Sunday.

A pervious supermoon rises over Cape Palliser, in Wairarapa. Photo: Mark Gee Photo: Mark Gee

The closest approach will be at 12.23am on Tuesday (New Zealand time). The moon will pass within 348,400 kilometres of Earth's surface, about 35,400km closer than average, they said.

Space Place at Carter Observatory science curator Dr Claire Bretherton said the moment of the full moon would be 2.52am on Tuesday, but several hours either side would look good.

She said the moon looked biggest when it was low in the sky. Because of the "horizon illusion", it looked bigger when one could compare it to landmarks.

For the best view of the supermoon, Dr Bretherton suggested looking east as the moon rose about 7.40pm on Monday, or - if one preferred a darker sky - just before 9pm on Tuesday.

"Anywhere with a clear view of the sky should have a pretty good view," she said.

There are three supermoons this year. There was one in October and another would come in December, she said.

Christchurch braces for surface flooding

The Christchurch City Council posted on Facebook the supermoon could bring king tides from Sunday night.

In preparation, contractors had put sandbags at the Owles Terrace boat ramp in New Brighton.

"We are expecting some surface flooding of roads in the New Brighton and Avonside areas on Monday, Tuesday and Wednesday.

"Likely roads to flood include New Brighton Road, Owles Terrace and Avonside Drive near Kerrs Reach."

"We will be monitoring the situation, and contractors will be out placing signage around any affected areas, and possibly closing some roads temporarily."

Why is the moon closer to Earth at some times but not others?

The distance between the moon and the Earth varies by about 50,000 kilometres, which creates the effect, according to Dr Brad Tucker from Mount Stromlo Observatory.

"Everyone thinks the orbit of the moon is a perfect circle - well, it's actually slightly elongated," he said.

Supermoons could be as much as 14 percent bigger and 30 percent brighter than a full moon at its furthest orbital point, but it was not always easy to tell the difference.

This was because of two key factors: weather and perception.

Firstly, a 30 percent difference in brightness could easily be masked by clouds or the glare of urban lights, so the sky must be clear for people to see the full effect.

Secondly, one full moon could look much like any other, NASA said.

This was because there were no rulers floating in the sky to measure lunar diameters, so low-hanging moons created "moon illusions", which made the moon appear unnaturally large.