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Spokane, Washington  Est. May 19, 1883

It’s the last blue supermoon until 2037, but clouds may get in the way for viewers in Spokane. Here’s what to know

A yellowish waxing gibbous moon rises south of Spokane on Monday night, as seen from the north side of the city.  (Jesse Tinsley/THE SPOKESMAN-REVIEW)

There’s once in a blue moon, and there’s once in a blue supermoon.

A blue moon is when there’s an extra full moon in a month, said Nicole Moore, chair of the physics department at Gonzaga University. Because a moon phase cycle is 29.5 days, and most months are slightly longer, “over time, the extra days add up and you end up with two of them in a month,” Moore said.

The term supermoon “indicates that the moon is relatively close to the earth in its orbit,” said Moore, who teaches, among other classes, an Intro to Quantum Physics class and a class about Life on Mars at the university.

That translates to the appearance of a bigger and brighter moon.

“The orbit of the moon around the Earth is not a perfect circle, it’s an ellipse, so sometimes it’s a little closer, sometimes it’s a little further away, and when it’s a little closer, it looks a little bit bigger when it’s full, and that’s what a supermoon is,” Moore said. “A blue supermoon is both of those things happening at the same time.”

According to NASA, 3% of full moons are blue moons, while about a quarter of full moons are supermoons.

“It’s fairly rare that they happen to be combining, but it’s basically a coincidence,” Moore said.

Indeed, Earth won’t see another blue supermoon until January 2037. Yet the weather in Spokane may prevent a clear view of the moon in the coming days.

“We think that there will be some clearing tomorrow evening to see the supermoon, but it could be touch-and-go,” said Greg Koch, a meteorologist with the National Weather Service. “We are looking at a 40% sky coverage during the evening hours, so areas from Spokane and points south will have a better shot of seeing the moon.”

Although the moon will be at its biggest and brightest point Wednesday night, the event will still be visible a few days after.

“It’ll probably still look pretty darn full on Thursday. Sensitive people might be able to tell it’s not full, but I personally have a hard time telling the difference between one day off of a full moon and a truly full moon,” Moore said.

Even so, skies aren’t predicted to clear until Friday.

“Thursday, our chances don’t look very good,” Koch said. “There may be some peeks through the overcast, but it’s not likely that we’ll see a whole lot of the moon Thursday night.”

In terms of visibility on Wednesday, Koch recommends viewing the moon shortly after sunset.

“That’s probably about when the moon would be coming up as well,” he said.

Moore agrees.

“It’ll look most impressive when it is just coming over the horizon. There’s this additional effect called the moon illusion that makes the moon look bigger right as it’s, like, hovering over trees or buildings,” she said. “Assuming that the skies are clear, that’s the time that it’s going to look the neatest.”

The moon will rise around 8 p.m. Wednesday.

Moore says people shouldn’t get too upset if the weather prevents them from seeing the moon this week.

“The full moon is always beautiful and fun to observe, so … it’s neat that there are two in August and it’s neat that it’s extra close,” she said. “If we are lucky, that’s fun. If not, it really won’t be that much more fun than any other full moon.”

And Moore pointed out a silver lining to the clouds.

“If it is cloudy, I mean, the rain is doing other good things for our region,” Moore said.

Roberta Simonson's reporting is part of the Teen Journalism Institute, funded by Bank of America with support from the Innovia Foundation.