Skywatchers around the world have a chance to see a rare blue moon this Sunday, an extra full moon that appears only once every two to three years.
Despite its name, the moon does not appear blue. A blue moon is the second full moon to occur within a single calendar month, a result of the mismatch between the lunar cycle and the calendar. The moon takes about 29.5 days to complete its cycle, allowing two full moons to occasionally fall within the same month.
According to EarthSky, calendar blue moons occur about seven times every 19 years. The event inspired the phrase “once in a blue moon,” which is commonly used to describe something rare.
Why blue moons happen
Astronomers also recognize a seasonal blue moon, which occurs when four full moons appear within a single astronomical season instead of the usual three. In that case, the third full moon is designated as the blue moon.
The next seasonal blue moon is expected on May 20, 2027. The next year to feature two calendar blue moons will be 2037, according to The Old Farmer’s Almanac.
The moon is not actually blue
While blue moons are not actually blue, the phrase has a long history. Britannica notes that the expression “the moon is blue” was used as early as the 16th century to describe something impossible or highly unlikely. The term later became associated with the lunar event known today.
The only Blue Moon we'll see all year is coming on May 31st! This is a rare celestial event that occurs when a second full moon rises in a single calendar month. Since that typically runs 29.5 days, we get an occasional second full moon in the same month! This happens every… pic.twitter.com/8P4obzY5qy
— Museum of Science (@museumofscience) May 31, 2026
In rare cases, the moon can appear blue because of conditions in Earth’s atmosphere. Scientists say smoke, dust, or volcanic ash particles of a specific size can scatter light in a way that gives the moon a bluish tint.
One of the best-known examples followed the 1883 eruption of Indonesia’s Krakatoa volcano, when observers in several regions reported seeing a blue-colored moon.
This year’s blue moon will be a micromoon
This month’s event also comes with an additional distinction. The blue moon will be a micromoon, meaning it will occur near the moon’s farthest point from Earth, known as apogee. According to NASA, the moon will be about 252,334 miles (406,093 kilometers) from Earth, compared with its average distance of about 238,855 miles (384,400 kilometers).
The previous calendar blue moon in 2023 was a supermoon, which occurs when a full moon coincides with perigee, the closest point in the moon’s orbit to Earth.
When and where to watch
The blue moon will reach peak illumination at 4:45 a.m. ET on Sunday.
Astronomers say the best views and photographs are often captured shortly after moonrise, when the full moon appears low on the horizon against the fading evening sky.
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