Next week's blood moon, a total lunar eclipse, should be "spectacular", says an astronomy expert.
Wednesday's eclipse will also coincide with what is known as a supermoon, when it is at its closest point to Earth.
University of Auckland senior lecturer of Physics Nicholas James Rattenbury said the blood moon was caused by planet alignment and dispersed light.
"When the eclipse is deep, we have the sun, the Earth and the Moon almost pretty much lined up so the Earth's shadow is falling right across the Moon's surface."
With the Moon orbiting closer to Earth, combined with the eclipse, it is expected to seem coloured and brighter.
"You can see the Moon appear to turn red, but the Moon isn't actually turning red. What's happening is that the light from the sun is partly going through the Earth's atmosphere and the blue light gets scattered down to us - that's why the sky appears to be blue - and the red light which is left over, keeps on going through the atmosphere and gets bent onto the surface of the Moon kind of like a lens," Dr Rattenbury said.
While he was aware of the total lunar eclipse being the sole one of 2021, stargazers should be open-minded as to how red the blood moon may appear.
"We shouldn't expect that the Moon looks like a very bright red. For some people, it looks kind of reddish brown or a dull salmony colour, but it's certainly an effect you want to look out for," he said.
University of Canterbury Mt John Observatory director Karen Pollard said providing clear conditions, a good show would be on.
"The altitude is quite high, 60 degrees up from the horizon... it should be quite visible, just depending on what your current weather situation is like."
Pollard said the event should be observable from anywhere the Moon could be seen, but recommended getting away from street lights for prime viewing.
"They're quite harsh white lights, so if you get away from those, then you'll get a better view. If you have a small telescope or a pair of binoculars, you can have a closer look ... so you can see some of the detail."
The total eclipse will start shortly after 11pm, followed by the maximum eclipse about 11:20pm.
RNZ
