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‘Once in a super blue moon’: those who study the skies say a blue moon is not as rare as many think

| Thursday, August 31, 2023

Last night was the first time the sky saw a blue moon since 2021, with a second occurrence happening tonight, August 31. But, what exactly is a blue moon?

Contrary to the name, a blue moon is no different in color than any other night – it is not blue at all.

Valeria Bautista Misakova
Photo taken at 10 p.m. at the Notre Dame Observatory under a 32 mm telescope Wednesday night.

Valeria Bautista Misakova, a sophomore studying physics, explains blue moons are not as uncommon and rare as one may think.

“A blue moon just means that you have a full moon twice a month,” Bautista Misakova said. “The second full moon of that month is called the ‘blue moon’. But what is rare is that it’s a super blue moon, meaning that it’s going to be at its full brightness and about 30% brighter than most nights.” 

According to Bautista Misakova, multiple factors played into Wednesday night’s full moon: its status as a blue moon, as well as what’s called a supermoon.

Professor Lauren Weiss, of Notre Dame’s Department of Physics and Astronomy, explains that the term ‘blue moon’ is largely a product of our cultural calendar system. In reality, nothing different is happening from an astrophysical standpoint.

“The time it takes for the moon to go from one full moon to the next is 29 and a half days,” Weiss said. “So, as you can imagine, if you’ve got a full moon right at the beginning of the month, then you can just squeeze in another full moon right at the end of the month — any month except for February.”

Only about 3% of full moons are blue moons, meaning they occur roughly every two and a half to three years. However, as Bautista Misakova explained, this week’s blue moon is also a “supermoon.” Weiss said that this term just refers to the moon’s location in its orbit around Earth.

“The shape of the Moon’s orbit is what we call an ellipse, which means that in part of its orbit, it’s a little bit closer to Earth,” Weiss said.

“You can get a full moon at any point along the moon’s orbit around the Earth … a supermoon is just a name for when that full moon is happening near its closest part to Earth (the perigee).”

About 25% of all full moons are supermoons. According to NASA, a supermoon can appear about 14% larger and 30% brighter than a ‘micro-moon’ (a full moon occurring at the furthest point from Earth in the rotational ellipse). NASA compares this to the difference between a quarter and a nickel. 

It’s time to debunk the idiom — “once in a blue moon” is really not all that rare.

“If you think about that, every few years or so there’s going to be a blue moon,” Weiss said.

“So ‘once in a blue moon’ means, like, once every three years. You know, once in every college education, or whatever. Or maybe, once per oil change of your car!” 

Weiss agreed that perhaps a more appropriate phrase might be “once in a super blue moon.” The true rareness of this week’s blue moon is rather its position as a supermoon. According to NASA, the next super blue moon will not grace the skies until January 2037.

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