A “blood supermoon” will hang over New Zealand skies tonight. Why is it such a big deal?

First off, what is a blood supermoon?

We can think of it as the combination of two lunar spectacles.

Blood moons – also known as total lunar eclipses – occur when the Earth lines up between the Moon and the Sun.

This hides the Moon from sunlight and blocks most of the blue light, with the remaining light refracting on to the Moon’s surface and causing a red glow.

“Red light is often seen during sunset because we view it through the thicker parts of Earth’s atmosphere, and being of a longer wavelength, more red light gets through,” Stardome Observatory expert John Rowe said.

“When the Moon goes into Earth’s shadow, the reddish light passing through Earth’s atmosphere around the limb of Earth refracts onto the Moon, creating a blood-red colour across the lunar surface.

“Unlike solar eclipses, lunar eclipses are completely fine to view with the naked eye.”

A “supermoon”, meanwhile, occurs when the Moon is at the closest point in its orbit around Earth – making it appear about 14 per cent larger than a normal full Moon and around 30 per cent brighter.

How rare is it?

During every 27-day orbit around Earth, the Moon reaches both its perigee, about 363,300km from Earth, and its farthest point, or apogee, about 405,500km from Earth.

Total lunar eclipses are relatively common – the next one after tonight’s is May 15-16, although it won’t be visible from New Zealand.

Supermoons are more common still – one was visible only a month ago.

There are also two super new moons – or when a new moon is at its closest point to Earth – on November 4 and December 5.

But it’s the combination of a total lunar eclipse and a supermoon that’s rare.

The eclipse takes place just a few hours after the Moon reaches perigee, the closest point to Earth on its orbit, making it a “Super Flower Blood Moon”.

Stardome has reported the last “blood supermoon” visible from New Zealand was December, 1982 – but a “blood supermoon” also occurred on January 31, 2018, and could be seen here.

Yet there’s no single, specific and universally-recognised definition of a supermoon, which isn’t a formal astronomical term.

Nasa says a “supermoon” can be used to describe a full Moon that comes within at least 90 per cent of perigee.

TimeandDate.com prefers a definition of 360,000km – and considers that the 2018 event was “almost” a supermoon.

Where will be best to view it?

Much of the country should get a decent look at the event.

The actual period of “totality” – when the Earth lines up between the Moon and the Sun and blocks out the Sun’s light – will be relatively brief, lasting 14 minutes from 11.11pm.

But the eclipse process itself is due to last five hours, beginning at 8.47pm.

Niwa reports the best places to view it are Southland, Otago, West Coast, Canterbury, Tasman, Nelson, Marlborough, Wellington, Manawatu-Whanganui, Taranaki and Waikato.

Niwa forecaster Ben Noll said cloud could likely spoil the view from areas like Bay of Plenty, Gisborne and Hawke’s Bay, cloud around Auckland, Coromandel and Northland would grow more scattered as night falls.

How can I photograph it?

Stardome astronomy educator and astro-photographer Josh Kirkley has offered tips on how to get the best shot of the spectacle.

“Look for clear unobstructed night sky views. The blood supermoon will be visible high in our sky so you should be able to find a good spot wherever you are in New Zealand,” Kirkley said.

“When the Moon goes into Earth’s shadow – just before 9pm – the reddish light passing through Earth’s atmosphere will be refracted on to the Moon, creating a blood-red colour across the lunar surface.”

Kirkley said anyone who wanted real photos would need a DSLR or mirrorless camera, with interchangeable lenses.

“Phone cameras just won’t cut it for a nice clear shot,” he said.

“Many standard cameras come with 18-50mm lenses, but I’d recommend lenses closer to the 300-600mm mark to get crisp close-up shots of the Moon.”

The higher the number, he said, the better.

“Ideally, you’ll also need a tripod, as night photography with handheld cameras will inevitably end up blurry.”

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