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How to see the ‘blue supermoon’ Wednesday night

A supermoon sets over Dorchester on March 20, 2019.Craig F. Walker/Globe Staff

Like the saying “once in a blue moon” suggests, blue moons are pretty unusual. But a blue supermoon is a rare treat indeed, and one is arriving Wednesday.

As the second full moon this month, it’s a blue moon, which happens every two to three years. And because this full moon is at or near its closest point to Earth, making it appear especially large and bright in the sky, it’s called a supermoon.

Supermoons aren’t all that rare, happening a few times a year. But blue moons are, so the combination doesn’t come along often. The next super blue moons will occur in a pair, in January and March 2037, according to NASA.

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So Wednesday between 10 p.m. and 1 a.m. is your last chance for a while, even if blue moons don’t actually look blue.

“Blue moons look like any other full moon. However, this Wednesday’s full moon is extra special. In addition to being “blue,” it is also this year’s ‘supermoon,’” Jacqueline McCleary, an assistant professor of physics at Northeastern University, said by email Monday.

A “supermoon” is when the moon looks larger and brighter because it’s closest to Earth in its orbit, McCleary said. It appears 14 percent bigger, like the size difference between a nickel and a quarter, than when the moon is furthest from Earth, according to NASA.

A “blue moon” doesn’t refer to the color of the moon, but a full moon occurring for the second time in a calendar month, said Quinn Sykes, the manager of Boston University’s Coit Observatory.

“It’s something people tend to think of some sort of astronomical importance that doesn’t happen very often,” Sykes said. “True, it doesn’t happen very often, but that’s just an artifact of our calendar.”

Viewers should be able to see the blue supermoon between 10 p.m. and 1 a.m. as long as it’s not raining, McCleary said.

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“A useful rule of thumb is that the full moon reaches its highest point in the sky around midnight,” McCleary said. Wednesday night is expected to be cooler and breezy, but there is a slight chance of rain, according to a National Weather Service forecasters discussion.


Talia Lissauer can be reached at talia.lissauer@globe.com. Follow her on Instgram @_ttphotos.