The world will see a rare blue supermoon this week, with stargazers getting a glimpse on Monday night into Tuesday morning.
The moon will appear about a seventh bigger and brighter on Monday evening as a blue moon and super moon coincide.
What is a supermoon?
The moon travels around Earth in an orbit that is not quite circular, so there is a single point in its orbit that is closest to Earth as well as a point that is furthest away.
The moon normally sits about 384,000km from Earth but will be 23,000km closer on Monday night – almost double Earth’s diameter.
When the moon passes within that closest point while it is full or new, it appears biggest and brightest and a supermoon occurs.
What is a blue moon?
A blue moon happens every two to three years when there are two full moons within a single calendar month. But a blue moon is also defined as the third full moon in an astronomical season when there are four full moons, with the astronomical seasons beginning and ending at the equinoxes and solstices.
However, the moon won’t appear blue. In fact, it will appear more red or yellow at dusk thanks to the light refracting around the atmosphere at the horizon.
When and where can you see the blue supermoon?
The supermoon will appear full for three days, according to Nasa, from Sunday morning through early Wednesday morning.
The moon will reach its closest point to Earth at 4.26am AEST on Tuesday (2.26pm EDT on Monday), but Australian stargazers should get their best view on Monday evening.
The moon is forecast to rise at about 5pm and become fully visible over the horizon from about 6pm across Australia’s capital cities.
“At dusk, it should actually look largest because of the way that light is reflected by the atmosphere,” said Macquarie University astronomer Prof Richard de Grijs.
“If people want to see a larger than usual moon, look today at dusk towards the horizon.”
Monday evening will also be a good opportunity to see the moon sitting alongside Saturn, according to University of Queensland astrophysicist Benjamin Pope.
“It’s going to be a really nice night for it if you’ve got a small telescope or binoculars. You should be able to see both Saturn’s rings and all of the beautiful detail of the lunar surface,” he said.
For those who miss the moon on Monday, Sydney Observatory suggests having a look on Tuesday night, while the moon is still closer than usual.
Will the weather be clear?
Most of Australia will have clearer skies on Monday evening than on Tuesday, so stargazers should get out early, according to the Bureau of Meteorology.
“The earlier in the night you get out, the better you’re more likely to have some clearer skies overhead,” BoM senior meteorologist Miriam Bradbury said.
Cities on Australia’s east coast will have mostly clear, dry conditions on Monday evening, but face increasing clouds overnight. Melbourne is expected to see chances of showers increasing into Tuesday morning.
Adelaide and Perth are unlikely to get a good view, with clouds and possible showers on Monday evening into Tuesday.
Darwin, meanwhile, is forecast to have a clear, dry Monday night.
When will the next blue supermoon be?
Supermoons happen three or four times a year and the next ones should fall on 18 September, 17 October and 16 November. September’s supermoon will be even closer to Earth, sitting 27,000km away.
But blue moons are much rarer, so there won’t be another one until May 2026. And as there won’t be a supermoon that month, it will be at least three years before another blue supermoon arrives.
The terms blue moon and supermoon aren’t scientific, meaning definitions for both terms are disputed, adding to the variability of estimates for when the next blue super moon could be.
