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Winter will conclude with an astronomical double act on Wednesday and Thursday as a rare blue supermoon illuminates the night sky.
For the first time since 2009, a supermoon and a blue moon will coincide. The full moon rising on Wednesday is the second supermoon this month, and it’ll appear bigger and brighter than normal.
The moon in Sydney, one day after clouds obscured a supermoon in November 2016. Credit: Janie Barrett.
When is the best time to see the blue supermoon in Australia?
The peak of the blue supermoon will occur on Wednesday night, but the phenomenon will still be unfolding on Thursday night as well.
Keen observers should keep their eyes on the horizon just after sunset to experience the full effect of the moon looking larger, Professor of Astrophysics at Macquarie University Richard de Grijs said.
“There are trees and buildings and all that, so you have a scale to compare the moon with if it’s close to the horizon,” de Grijs said. “If it’s up higher in the sky, you’ve got nothing to compare it with.
“Secondly, when it’s close to the horizon, the light reflecting off the moon coming towards you is refracted by the atmosphere. Because of that refraction, the moon starts to look a little bit bigger than it actually is.”
What’s a supermoon?
A supermoon occurs when a full moon swells at the same time the moon is at its closest point to Earth.
The moon doesn’t have a perfectly circular orbit. Its path around our planet is more of an ellipse, or oval, meaning that sometimes the moon is physically further away from Earth and sometimes it’s closer.
A super blood moon rises off Sydney’s Manly Beach in 2018.Credit: Getty
At its furthest distance from Earth, the moon swings to a distance of 405,500 kilometres, a point known as the apogee. The moon’s closest point to Earth is the perigee, about 363,300 kilometres from Earth, or about 42,000 kilometres closer than the apogee.
When a full moon coincides with the moon arriving at the perigee, it’s known as a supermoon.
A full moon at the perigee is 14 per cent closer and 30 per cent brighter than a full moon farthest from Earth, or micromoon.
There are about three or four supermoons in a normal year; this month’s will be the closest and brightest supermoon.
The term supermoon came from the less than scientific realm of astrology, not astronomy. If zodiac signs aren’t your thing, you can use the supermoon’s technical name: the perigee syzygy.
What’s a blue moon?
The blue moon might be most notorious for the famous phrase used to describe an infrequent event.
But it’s also a colloquial term used when two full moons rise during the same month; the second full moon in a calendar month is called the blue moon. Professor de Grijs explains they come around once every two or three years because the lunar cycle is 29.5 days.
A lunar eclipse can drastically alter the colour of the moon, unlike a blue moon.Credit: Getty Images
“There’s no statistical or scientific meaning whatsoever. It’s just the fact that the moon goes around the Earth 29.5 days. So once, every so often, we have two full moons during a calendar month.”
The moon can blush “blood” red during a lunar eclipse, but it doesn’t actually turn blue during a blue moon.
Although, in the two years following the catastrophic eruption of the Indonesian volcano Krakatoa in 1883, people from the time wrote the moon had a blue tinge due to the volcanic ash that had belched into the atmosphere.
The next blue supermoons come in 2037, when the phenomenon will light up the skies in both January and March.
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