Large in the sky and perfectly round, Monday night’s super blue moon is an attention-getter.
But how rare, really, is it?
It is unusual because it’s both blue and super. According to NASA, about 25% of all full moons are big or “super” moons, but only 3% of full moons are blue moons. Super blue moons happen irregularly; the time between them can be up to 20 years, but on average it’s 10 years. The next super blue moon will occur in January 2037.
While Monday night will be the peak for full moon viewing, it will still be an impressive sight through early Wednesday.
Michelle Nichols, director of public observing at the Adler Planetarium in Chicago, said the astronomical phenomenon of just a blue moon is more a “quirk” of the calendar.
“You’re not going to like this answer: It’s really not all that special,” she cautioned, trying to manage expectations. But, she added, “I don’t want to squash people’s enthusiasm about the moon. Get out there and look at it, because it’s the moon. It’s our closest neighbor in space.”
“Once in a blue moon” denoted an absurdity several hundred years ago and later came to indicate a rarity. A blue moon has two astronomical definitions — which have little to do with color.
Monday night skies will be bright with a seasonal blue moon, meaning it will be the third of four full moons this fall — that’s one more full moon than the usual three. This happens around every 2½ years; the next one will occur May 20, 2027.
The second definition is the monthly blue moon, that is, the second full moon in a calendar month. The next time this will happen will be May 31, 2026.
In addition, a supermoon occurs when the full moon nears Earth along its oval-shaped orbit, and appears larger than usual. This is not a particularly big one, Nichols said, as the full moon on Sept. 17 will be about 2,000 miles closer, and the one on Oct. 17 another 2,000 miles nearer. But there is no consensus on what distance makes it seem big enough to be considered super. Depending on the definition, supermoons happen one to three times a year.
“It’s not anything that you can look up in a science textbook. It’s not, like, agreeing on what an igneous rock is,” she said, referring to some types of rocks found on the lunar surface. “Everybody agrees on what an igneous rock is. People aren’t going to necessarily agree with what a supermoon is.”
The full moon will reach its peak at 1:26 p.m. in Chicago; at that moment it will be exactly opposite the sun in its orbit around the Earth.
“At that point, it’ll be up for folks on the other side of the planet,” she added, “so they would get to see the full moon at the actual time that the full moon is occurring.”
To the naked eye, however, the difference between the full moon at its peak and shortly after is indistinguishable. In Chicago, the moon will rise from the east at 8:05 p.m. Monday. At first, it will take on an orange hue; this is a normal occurrence and has nothing to do with it being a blue moon.
On Sunday afternoon, as the moon came over the horizon, its bright orange light filtered through trees in Grant Park and shone on DNC protesters. It then seemingly shed that color in favor of a bright white as it reached its prime height.
As the lunar surface reflects the brightness of the sun, it scatters blue light at the horizon and takes on the warm end of the color spectrum. When the moon is higher up, it is viewed through less air, and that blue light is scattered less. The same happens with the sun every day: When it rises and sets, it takes on red and orange colors.
The blue moon doesn’t normally look blue except under specific and rare atmospheric conditions that can happen outside a full moon, according to experts. The Adler says that, after the 1883 eruption of the Indonesian volcano known as Krakatoa, the dust and ash filtered some wavelengths of sunlight and made the moon appear blue. Smoke from forest fires can also cause similar tinted effects.
For casual viewers, it also matters how high the moon is in the sky. If it’s low, they might miss it. On Monday night, it will be just under 30 degrees above the horizon — not even a third of the way up.
“If you’re downtown, for example, and you’re among all the buildings, you might not see the moon easily … because it just won’t clear a lot of the buildings that high in the sky,” Nichols said.
At its highest point, the supermoon in September will be just under 40 degrees, and the one in October will be about 55 degrees up.
After exciting months of a total solar eclipse in April, an active year for the sun, brighter Northern Lights and the Perseids meteor shower last Sunday, Nichols said viewers might want to keep looking up this year. There will be a partial lunar eclipse Sept. 17 from 9:12 p.m. to 10:15 p.m., when the moon will pass through a portion of the Earth’s shadow.
“It’ll look like the moon has a little bit of a bite taken out of it,” she said. “The moon will mostly be its normal self. And then you’ll see a little bit of, in this particular case, a little shadow curve on the top part of the moon as you’re looking at it.”
Nichols encouraged Chicagoans to get outside regardless of whether the celestial objects and phenomena they witness are rare or regular occurrences.
“It’s really neat to see an astronomical object with your eyes,” she said. “That, really, is the reason we are here: To get outside and look up. And if it happens to be a little reminder, when these little calendar quirks happen,” she said, referring to the blue moon, “Hey, whatever it takes — go for it.”
