NEWCASTLE locals will be wishing upon on a star that the sky is clear enough to get a glimpse of the rare blue supermoon tonight.
Aussies only have two chances to see it, and if both nights are cloudy, it's a six-year wait for the next one in 2029.
The Bureau of Meteorology (BOM) has forecast the possibility of a thunderstorm in Newcastle later this afternoon or evening, which could be severe.
It was sunny on Wednesday morning but the clouds were expected to move in later in the afternoon, according to the BOM.
If the stormy skies hold off, Newcastle residents should be able to glimpse the big and bright blue supermoon.
So, what is the phenomenon and when is the best time to see it?
What is a blue supermoon?
The blue supermoon is a combination of two things, Australian National University astrophysicist Dr Brad Tucker explained.
"It's one of those combinations that isn't once in a lifetime, but it is a special treat," he said.
"What I like about it is it shows just how ever-changing the Earth, the moon and the sun is - we don't realise every day it's happening above us."
The 'blue' moon doesn't mean it will actually appear blue, rather it refers to the phenomenon of two full moons occurring in the same calendar month.
"The lunar cycle takes about 29.5 days so generally we get one full moon a month," Dr Tucker said.
"Except, if we get one at the beginning of the month we can sometimes get a second full moon - it is rare to get that."
A blue moon occurs every few years, Dr Tucker said, and it's where the saying "once in a blue moon" comes from, to refer to something happening very rarely.
The second factor is the supermoon, scientifically known as perigee-syzygy, which is more common with a couple each year.
As the moon orbits Earth the distance between them varies by about 40,000km.
"So when the moon is closer to the Earth it can therefore look a bit bigger and a bit brighter in the sky," Dr Tucker said.
The best time to see the rare blue supermoon
The moon will be completely full about 11.45am on August 31, so while it falls during the day it means it will "appear full" on two nights.
Dr Tucker said the best time to view the blue supermoon is around sunset on August 30 and 31 as it rises in the east.
"The moon looks biggest as it's rising in the evening sky," he said.
Full moons always rise around sunset and distortion from the atmosphere, as well as things to compare it to such as trees and houses make it look bigger, he explained.
"That's the time when it has the coolest effect," he said.
