The world will see a rare blue supermoon this week, with stargazers getting a glimpse Monday night into Tuesday morning.
The moon will appear about a seventh bigger and brighter on Monday night as a blue moon and supermoon coincide.
What is a supermoon?
The Moon moves around the Earth in an orbit that is not perfectly circular, so there is a single point in its orbit that is closest to the Earth as well as a point that is farthest away.
The moon usually sits about 384,000 km from the Earth, but on Monday evening will be 23,000 km closer – almost twice the Earth’s diameter.
When the moon moves within that closest point while it is full or new, it appears largest and brightest and a supermoon occurs.
What is a blue moon?
A blue moon occurs every two to three years, when there are two full moons within a single calendar month or four full moons within a season.
However, the moon will not 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?
According to Nasa, the supermoon will appear full for three days from Sunday morning until early Wednesday morning.
The moon will reach its closest point to Earth at 04:26 AEST on Tuesday (2:26 PM EDT on Monday), but Australian stargazers should get their best view on Monday evening.
The moon is expected to rise around 5pm and become fully visible over the horizon from around 6pm across Australia’s capital cities.
“At dusk it should actually appear the largest because of the way 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 at the horizon today at dusk.”
Monday night will also be a good opportunity to see the moon sit next to Saturn, according to University of Queensland astrophysicist Benjamin Pope.
“It’s going to be a really fun night for it if you have a small telescope or binoculars. You should be able to see both Saturn’s rings and all the beautiful detail of the lunar surface,” he said.
For those who miss the moon on Monday, Sydney Observatory suggests looking 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 night than Tuesday, so stargazers should get out early, according to the Bureau of Meteorology.
“The earlier in the night you come out, the more likely you are to have some clearer skies over you,” BoM senior meteorologist Miriam Bradbury said.
Cities on Australia’s east coast will have mostly clear, dry conditions on Monday night, but will face increasing clouds overnight. Melbourne is expected to see chances of showers increasing until Tuesday morning.
Adelaide and Perth are unlikely to get a good view, with clouds and possible showers Monday night into Tuesday.
Darwin, meanwhile, is forecast for a clear, dry Monday evening.
When will the next blue supermoon be?
Supermoons occur three or four times a year and the next ones should fall on September 18, October 17 and November 16. September’s supermoon will be even closer to Earth, 27,000 km away.
But blue moons are much rarer, so there won’t be another one until May 2026. And since 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 are not scientific, meaning definitions for both terms are contested, adding to the volatility of estimates for when the next blue supermoon might be.
