Skywatchers this week will have the chance to witness something that hasn't happened in two years, a second full moon in one month.
August opened with a bright full moon that, while beautiful, created a bit of a headache for hopeful meteor shower viewers. But on Wednesday night, August 30, another full moon will light up the skies in a rare phenomenon called a “super blue moon.”
This special event is the coincidence of two uncommon moon traits: A supermoon, which occurs when the moon appears larger than usual, and a blue moon, or the second full moon in a month. According to NASA, a blue moon only occurs once every two or three years on average—and a blue moon that is also a supermoon is even rarer. While a “super blue moon” can occasionally happen twice within two months, at other times, it could be 20 years before the phenomenon repeats. On average, such an event occurs once per decade, per the agency.
Astronomy lovers get to experience supermoons because the moon’s orbit around Earth isn’t a perfect circle. As a result, the distance between us and the moon varies as it loops around our planet. Roughly three or four times each year, the full moon occurs while the moon is at its closest point to Earth, called the perigee. These supermoons appear 7 percent larger than the average full moon and 14 percent larger than a full moon at its most distant point from Earth, or the apogee.
While Wednesday’s “super blue moon” will appear large, those expecting to see it glow with an unusual color will be disappointed. During a “blue moon,” the lunar surface won’t change its hue—but viewers can still appreciate its rarity. The last blue moon happened in August 2021, and after the one this month, a blue moon won’t return again until May 2026.
Every 29.53 days, the moon undergoes a complete phase cycle, transitioning from a bright full moon to a dark new moon—and back to a full moon again. Since 29.53 days is relatively close to the lengths of the months in our calendar, most months only have one full moon. But because our calendar is based on the Earth’s motion around the sun, not the phases of the moon, the periods don’t match up exactly. As a result, two full moons are sometimes squeezed into one month. (February, with only 28 or 29 days, can never have a blue moon.)
It’s the same reason that, if you get a paycheck every two weeks, you’ll occasionally end up getting three paychecks within one month, since two 14-day pay periods (28 days) don’t match exactly with the lengths of months in the calendar. In essence, that’s what’s happening on Wednesday—but at a cosmic scale.
So, if the moon won’t actually be blue, what’s the story behind the colorful name? Although the term’s origin is frequently cited as a piece of old folklore, that isn’t really the case, as Philip Hiscock, a professor of folklore at Memorial University in Canada, wrote in Sky & Telescope in 2012. Instead, the story of the term blue moon is “a truly modern piece of folklore, masquerading as something old,” Hiscock wrote.
In the early 1900s, in places such as the Maine Farmers’ Almanac, the term “blue moon” was used to refer to a related phenomenon—when four full moons occurred within a given season, instead of the typical three. In these cases, the third full moon was known as “blue.” However, in 1946, amateur astronomer James Hugh Pruett incorrectly interpreted the term in an article he wrote in Sky & Telescope using the meaning we know today. The mistake was repeated several times—notably, in 1980 on the NPR show “StarDate”—and eventually, the new definition stuck, along with a common misattribution to traditional folklore, which “appeals to our modern sensibilities, including our desire to have plausible origins,” Hiscock wrote. Since then, the term has been appropriated for everything from a novel to a butterfly to the widely popular Belgian white-style beer.
“Blue moon” isn’t the only colorful name for our natural satellite: When a second new moon occurs within one month, some people call it a black moon. And any full moon during the month of April is sometimes known as a pink moon.
During a lunar eclipse, however, the moon truly appears red, as the only sunlight that reaches it has been filtered through Earth’s atmosphere, scattering its blue wavelengths away. In another rare event in January 2018, a supermoon and a blue moon overlapped with a lunar eclipse, creating a large-looking, rust-colored moon.
On very rare occasions, the moon actually can appear blue, if extreme wildfires or volcanic eruptions blast enough smoke or ash into Earth’s upper atmosphere. If these particles are roughly one micron wide (one-millionth of a meter), they can interact with light reflecting off the moon, scattering the long-wavelength red light and leaving just the bluer light to pass through to viewers on the ground. This has happened several times in history, such as during the June 1991 eruption of Mount Pinatubo in the Philippines, which caused the moon to take on a blueish-green hue in several places around the world.
Perhaps this accounts for the most commonly used meaning of the term, “once in a blue moon,” which refers to something that happens on a very infrequent basis. Unlike the blue moon that you can see on August 30 and can count on spotting periodically, witnessing a moon that’s actually blue in color will be much more difficult. To do so, you might have to be patient for quite a while—and wait around for a massive volcanic explosion.
Editor’s Note, August 28, 2023: In 2012, we looked at the science and etymology of “blue moons.” The story above has been updated to include information about the upcoming event, and the headline has been changed.