What exactly is the 'blood supermoon' and how is it different from a full moon?

Late on Wednesday night, for the first time in 40 years New Zealanders will get to see a 'blood supermoon', weather permitting.

To astronomers, it's technically a full lunar eclipse at the perigee of the moon's orbit around the Earth. But what does that mean in English?

Let's break it down.


The term 'supermoon' has gained in popularity in recent years, describing a time when a full moon coincides with our only natural satellite's closest approach to Earth.

"The moon goes around the Earth in an ellipse, not a circle, so sometimes it's closer to the Earth than at other times, and this is one of those times," University of Auckland physics lecturer Nick Rattenbury told Newshub.

An ellipse is an oval. The distance from the Earth to the moon varies considerably - at its closest, the perigee, it's just 363,396km away; and at its apogee, it's more than 405,000km away.

Three or four times a year the moon is at its perigee at the same time it's full.

"It's fun that we have a full moon at a time that it's also close to the Earth, and this is an occasion which people call a 'supermoon'," said Dr Rattenbury. "It appears a little bit larger and somewhat brighter in the night sky."

About 30 percent brighter and 14 percent larger than when it's at the apogee, to be precise.

'Full moon'

You might think you know what a full moon is - when you can see all of it. But what exactly is happening there?

A full moon happens when the moon is on the opposite side of the Earth to the sun. The entire side of the moon facing the Earth reflects light back to us, so the moon appears 'full'.

When the moon is on the same side of the Earth as the sun, it appears to disappear because it's not reflecting any light towards us at all. This is what we call a 'new moon'. In between new and full moons, only part of the moon's surface reflects light to us, waxing as it gets closer to being full and waning as it disappears from view.

'Blood moon'

A 'blood moon' is simply a fun name for a total lunar eclipse. While we get 12 or 13 full moons a year, two or three times a year the moon passes through the Earth's shadow.

"The shadow goes about a million kilometres out into space, so the moon is passing through the centre of that shadow," Stardome observatory expert John Rowe told The AM Show.

But why 'blood'? For the same reason the sky is blue and sunsets are red, it turns out.

"When everything is almost exactly lined up at the deepest part of the lunar eclipse, we have an interesting phenomenon where light from the sun... some of that light is going through Earth's atmosphere," explained Dr Rattenbury.

"Molecules in Earth's atmosphere scatter the sunlight, and that scattering process is most effective for blue light - that's why we see the sky as blue on Earth... The red light, the leftover light, keeps on going through the atmosphere."

A blood full moon rises above in Nakhon Sawan province, north of Bangkok, in 2019..
A blood full moon rises above in Nakhon Sawan province, north of Bangkok, in 2019. Photo credit: Getty

Some of that red light gets refracted or "bent" off its original path, like sunlight passing through a lens, and ends up on the lunar surface. Mixed in with the shadow, Dr Rattenbury says it can end up casting the moon in anything from "a slight reddish hue, to a brownish-red, through to kind of a salmony pink tinge".

It happens because red light has a longer wavelength than blue, so isn't as easily scattered - we see the effects of this every sunrise and sunset, the sky turning red as the sun's light has to pass through more of the atmosphere before reaching our eyes.

When and where can I see it?

Not every full moon is a blood moon because the moon's orbit isn't perfectly aligned with the Earth and sun - it's about 5 percent off. While it passes through the Earth's shadow a few times a year, each time it can only be seen from certain parts of the world - in this case, only New Zealand, Papua New Guinea, the Pacific nations, the eastern half of Australia and Antarctica will get the full show.

In New Zealand, the moon will start to enter the edges of the Earth's shadow from 8:47pm. It will start to turn red from 9:44pm, and totality - when it's completely red - will kick in at 11:11pm, and last about 14 minutes.

"The South Island's a good place evidently, pretty clear around most of the South Island," said Rowe. "The North Island, looking fairly good in the west. East coast might have a fair bit of cloud."

While Auckland was looking pretty clear Wednesday morning, and the forecast ahead is fine, Rowe said he doesn't like to predict what the super city's weather might do.

WeatherWatch said most of New Zealand is looking good, except maybe the eastern areas.

"Our IBM supercomputer modelling shows many parts of NZ have clear skies on the way," said head forecaster Philip Duncan.

"But it won't be ideal for everyone, with cloudiest weather likely in the eastern North Island due to the offshore low".

He said WeatherWatch's sister site ruralweather.co.nz has regularly updated cloud cover information, if you're weighing up whether it's worth staying up late for the rare show.

Unlike most other astral phenomena, such as solar eclipses, you won't need any special equipment to watch the blood supermoon.

"You could have binoculars to give you a better view, but you absolutely don't need to," said Dr Rattenbury. "A good thing would be to get away from the bright lights of the city."

When is the next one?

If you miss this one, the next lunar eclipse visible from New Zealand will take place at 11:16pm on November 8, 2022. Sadly it will not be a supermoon.