Extremely rare 'selenelion' to be seen in South Island skies this weekend

  • 26/07/2018

The south of New Zealand will witness an extremely rare celestial event this weekend. 

A few minutes after 8am on Saturday, South Islanders can observe a total lunar eclipse where the Sun and Moon are both visible, which is known as a 'selenelion'. 

While you mightn't have heard of it, a selenelion is a must-see occurrence. The areas of our planet from which it can be experienced are very limited, because the total lunar eclipse must be on-going at the time of moonset/sunrise. 

The first record of any such event dates from 1666.

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. 

Where can you witness it?

The best chance of witnessing the eclipse this weekend is if you're in the South Island. 

Dr Steel explains that in Auckland the eclipse is still partial, as the Moon disappears below the horizon, while from Whanganui to Wellington, there is only a slim chance of seeing the totally-eclipsed Moon as the Sun peeks above the horizon. 

The South Island is the best place to see it. 

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. 

What will it look like?

If you're lucky enough to catch a glimpse of the eclipse, the Moon will appear much fainter than it usually does, and will look a bit darker with a reddish tinge, Dr Steel says. 

The reason for this is sunlight leaking through Earth's atmosphere and reaching the Moon, with red light standing a better chance of doing so than other colours. 

Dr Steel says the event is "quite rare". Although there is a lunar eclipse visible from New Zealand every few years, very few people have witnessed a selenelion. 

"Being located in the right place to see a celestial peculiarity like a selenelion is much less frequent, and very few people have witnessed one," he says.