Where to watch this weekend's astronomy extravaganza

It's not every weekend a celestial event comes along, but this weekend Kiwis get to see three at the same time. 

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The south of New Zealand will witness an extremely rare total lunar eclipse known as a 'selenelion'. That's when a lunar eclipse - or 'blood moon', as the media has taken to calling them in recent years - happens so early in the morning it's visible at the same time as the sun.

This event will coincide with Mars appearing up to 10 times brighter in the sky, alongside five of our solar system's planets being visible all at once across the horizon. Mars and the Earth will be closest this year on July 31/August 1.

Each of these events will be visible from New Zealand this weekend, according to astronomy educator Josh Kirkley from Auckland's Stardome Observatory. But only some Kiwis will be lucky enough to witness all of them. 

Image of a Blood Moon.
Image of a Blood Moon. Photo credit: Facebook / Aaron Duckett

Here's how to see them

To see Mars this weekend, it doesn't really matter where you are, because the red planet is going to be quite high in the sky, Mr Kirkley told Newshub. You won't need any special type of viewing conditions for that. But it pays to be somewhere high. 

As for the eclipse, you will need to have an unobstructed view of the western horizon, he said. In Auckland, for example, you could head up any of the mountains including Mt Eden or One Tree Hill, so long as you can see the western horizon without anything in your way. 

But the selenelion will only be visible if you're quite far down in the South Island, says Mr Kirkley. In Invercargill, there is five minutes between sunrise at 8:12am and moonset shortly thereafter, with the Moon entirely within Earth's shadow. 

In Dunedin, the interval is reduced to four minutes and in Christchurch three minutes. Whereas from Whanganui to Wellington, there is only a slim chance of seeing the totally-eclipsed Moon as the Sun peeks above the horizon. 

"With Mars, planetary oppositions happen about every two-and-a-half years. As for eclipses, they're also not that rare, so again, every couple of years," said Mr Kirkley. 

"The last lunar eclipse seen in New Zealand was in January so this is our second of the year. But in saying that, the next one isn't expected until 2021."

The selenelion event is special, Mr Kirkley said, because you have to be quite low to see both the Sun and the Moon at the same time. Seeing them both at the same time isn't that special, but seeing the Sun and the Moon being eclipsed by Earth at the same time is what's special.  

A selenelion happens when the eclipsed Moon can be seen on one horizon, whilst the rising Sun can also be observed near the opposite horizon, explains Dr Duncan Steel from the Centre for Space Science Technology in Otago. 

This might seem impossible - as a typical lunar eclipse happens when the Sun, Earth and Moon are in a straight line - so if the Moon is above the horizon then the Sun must be below it. 

But the selenelion is possible due to the bending of the rays of light caused by Earth's atmosphere, says Dr Steel. This enables you to see both the eclipsed Moon and the Sun at the same time, so long as you're in the right place. 

All these exciting events are happening at once, including a planetary alignment. All five naked-eye planets are visible at the moment - including Mercury, Venus, Mars, Jupiter and Saturn - in a line across the sky. It's not very often you see all five planets at the same time. 

"This will only be visible for another day or two. To see this you need to be quite high because Mercury is quite low on the horizon," said Mr Kirkley.