Monday's rare super blue moon is a confounding statistical marvel
A rare sky spectacle will occur Monday night that (probably) won't happen again for years: A full moon that can be called both a supermoon and blue moon will grace the night skies over the U.S.
Dubbed a "super blue moon," the rare combination occurs when the different cycles of blue and supermoons happen to align on the calendar, said NASA's Noah Petro, Lunar Reconnaissance Orbiter project scientist.
If that's not confusing enough, astronomers also have varying definitions for what counts as a supermoon and what counts as a blue moon. (Just last year a full moon met the criteria for both, by some definitions.)
Here's why Monday's full moon is so special:
What is a supermoon?
When the moon’s orbit brings it closer to Earth than usual, this cosmic combo is called a supermoon. A supermoon occurs when the moon’s orbit is closest to Earth at the same time the moon is full. The term was coined by astrologer Richard Nolle in 1979.
"Different publications use slightly different thresholds for deciding when a full moon is close enough to the Earth to qualify as a supermoon," NASA said. "Because the orbit of the moon is not a perfect circle, the moon is sometimes closer to the Earth than at other times during its orbit."
When the moon reaches its closest point to Earth, about 226,000 miles away, full moons appears larger and brighter than at other times. A supermoon will appear 30% brighter and 14% larger than a typical full moon.
What is a blue moon?
There are two types of blue moons: seasonal and monthly. The one Monday will be of the seasonal variety.
A seasonal blue moon occurs when there are four full moons in a single season (on this occasion, summer). When this happens, the third of the four is considered a blue moon.
The second definition – which arose from a misunderstanding of the original – is the monthly blue moon, referring to the second full moon in a single calendar month.
In either case, blue moons earn their reputation for being rare because of a quirk in the typical rhythm of one full moon per month.
The moon's cycle takes 29.5 days. But, as Missouri State University professor of astrophysics Mike Reed notes, months can have 28, 29, 30 or 31 days.
The two cycles create odd mismatches resulting in one more moon in a season or month than is typical – that's a blue moon.
Months usually have one full moon – unless there's a (monthly) blue moon. And seasons typically have three full moons – unless there's a (seasonal) blue moon.
In addition, Monday's full moon is unlikely to appear blue, however: Blue-colored moons in photos are usually made using special blue camera filters or photo editing apps.
How rare is a super blue moon? When will the next one occur?
"It's tricky to answer, because it depends on your definition of a supermoon and a blue moon," Edward Bloomer, senior astronomy manager at the U.K.'s Royal Observatory Greenwich told USA TODAY via e-mail.
The answer is different depending on whether you're talking about a seasonal blue moon or a monthly blue moon. If you tweak what counts as a supermoon, the answer also changes.
In general, a blue moon happens once every two or three years on average, according to NASA. The time between super blue moons can be as much as 20 years – but in general, 10 years is the average.
So when will the next super blue moon actually occur?
A safe bet is January 2037, according to NASA.
However, depending on what you count as a super blue moon, you may hear of another one rising before then.