A rare super blue moon on Monday is a fascinating statistical anomaly.
How rare is it? That depends on who you ask. But one thing is certain: Monday's full moon will be something special.
On Monday night, a rare celestial event will unfold: a full moon that qualifies as both a supermoon and a blue moon will light up the skies over the U.S. This unusual occurrence, known as a "super blue moon," happens when the cycles of blue moons and supermoons align on the calendar, explained Noah Petro, Lunar Reconnaissance Orbiter project scientist at NASA.
To add to the complexity, astronomers differ on what exactly constitutes a supermoon and a blue moon. (In fact, just last year, a full moon met the criteria for both, depending on the definition.)
Here’s why Monday’s full moon is so special:
What is a supermoon?
A supermoon occurs when the moon’s orbit brings it closer to Earth than usual while it’s full. This cosmic alignment results in a larger and brighter moon in the night sky. The term "supermoon" was coined by astrologer Richard Nolle in 1979.
"Different publications have slightly varying criteria for determining when a full moon is close enough to Earth to be considered a supermoon," according to NASA. "Since the moon's orbit is not a perfect circle, its distance from Earth fluctuates during its orbit."
When the moon reaches its closest point to Earth, about 226,000 miles away, it appears larger and brighter than usual. A supermoon can appear 30% brighter and 14% larger than an average full moon.
What is a blue moon?
There are two types of blue moons: seasonal and monthly. Monday’s blue moon falls into the seasonal category.
A seasonal blue moon occurs when there are four full moons in a single season—like this summer. In such cases, the third of the four full moons is designated as a blue moon.
The second definition, which originated from a misinterpretation of the original, is the monthly blue moon. This refers to the second full moon within a single calendar month.
In both cases, blue moons are considered rare due to the unusual occurrence of having more than one full moon in a month, disrupting the typical pattern of one full moon per month.
The moon's cycle lasts 29.5 days, but as Missouri State University astrophysics professor Mike Reed points out, months can have 28, 29, 30, or 31 days.
These differing cycles occasionally result in an extra full moon within a season or month, giving us a blue moon.
Normally, months have one full moon—unless there's a monthly blue moon. Similarly, seasons usually have three full moons—unless there's a seasonal blue moon.
Despite the name, Monday's full moon is unlikely to appear blue. Blue-tinted moons seen in photos are typically achieved with special blue camera filters or through photo editing.
How rare is a super blue moon? When can we expect the next one?
"It’s a tough question to answer since it depends on how you define a supermoon and a blue moon," Edward Bloomer, senior astronomy manager at the U.K.'s Royal Observatory Greenwich, explained in an email to USA TODAY.
The frequency of a super blue moon varies depending on whether you're referring to a seasonal or monthly blue moon. Adjusting the criteria for what qualifies as a supermoon also impacts the answer.
On average, a blue moon occurs every two to three years, according to NASA. The gap between super blue moons can be as long as 20 years, though 10 years is more typical.
So, when is the next super blue moon?
A safe estimate is January 2037, according to NASA.
However, depending on your definition of a super blue moon, you might hear of another one appearing before then.
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