'Very rare to find': Why Friday night's 'blood moon' isn't really a blood moon, but likely to be just as cool

The weather's looking good for Friday night's lunar eclipse, a 'blood moon' that isn't technically a blood moon at all.

And that's not the only thing that's got astronomers excited - lasting several hours, it will also be the longest eclipse any of them will likely see in their entire careers. Or any of us amateurs too, if the skies stay clear. 

"If you can see the moon, you can see this," astronomer Rob Davison of Auckland's Stardome Observatory told The AM Show on Friday. 

"It will start just after the moon rises, about 8:20pm. As it rises higher and higher in the sky the shadow will move across the face. Just after 10pm is when it will be at its peak. That will be the best time if you want to catch some of that red colour."

Usually the red colouring that gives the blood moon its name only happens during a total eclipse

"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," Stardome observatory expert John Rowe told The AM Show earlier this year, ahead of the May eclipse. 

During a total eclipse, this is what gives the moon the 'blood' colour. But partial eclipses are usually much less colourful - the part of the moon that's not in the Earth's shadow ends up shining so bright, the colour is washed out.

But this partial eclipse will see 97 percent of the moon's face in shadow, perhaps enough to make it a blood moon.

"You have to be very, very, very close to totality to have a chance of seeing some red colour," said Davison.

The length of the show is because the moon is at apogee - a fancy way of saying it's slightly further away than usual, so it takes a bit longer to make its way across the sky, spending more time in shadow. 

The eclipse earlier this year in May was New Zealand's first 'blood supermoon' in 40 years - a total lunar eclipse at perigee, when the moon is slightly closer to us than usual. 

"A supermoon happens very often - it's very normal. We will have two or three - depending on your definition - supermoons in the middle of next year. The moon will be full and it will be closer to us than average," said Davison. 

'A blood moon is really just another term for a total lunar eclipse. The reason the one we're having tonight is special is because it could be a blood moon, but still technically a partial eclipse. That tightrope is very rare to find." 

Rob Davison.
Rob Davison. Photo credit: The AM Show

Davison said he's crossing his fingers the weather holds. 

"It's a beautiful morning, it was lovely yesterday, so if it can just keep it up for the rest of the day, I'll be very happy." 

If 97 percent coverage isn't good enough for you, the next total lunar eclipse visible from New Zealand will be on November 8, 2022 - at one minute to midnight