A rare blue ‘micromoon’ is rising at the end of May
The moon won't turn blue—but you will be able to see two unusual celestial events in one night.
May’s full moon will come with a rare double designation: It’s both a blue moon and a micromoon. While the moon won’t actually appear blue, the combination highlights an unusual overlap between the lunar cycle and the moon’s orbit around Earth.
The full moon reaches peak illumination at 8:45 a.m. UTC on May 31, offering skywatchers a chance to catch one of the year’s more unusual lunar events. The moon may appear slightly smaller and dimmer than a typical full moon, though binoculars or photography can help reveal the subtle differences.
What is a blue moon?
“A ‘blue moon’ doesn’t refer to color. It’s a calendrical term,” says Seth McGowan, president of the Adirondack Sky Center in Tupper Lake, New York.
There are actually two accepted definitions of a blue moon. The most widely known is a monthly blue moon, which is a second full moon in a calendar month. May’s event falls under this monthly definition.
The secondary definition is a seasonal blue moon, which refers to the third full moon in an astronomical season that contains four full moons instead of the usual three. “This definition is older and comes from traditional almanac usage,” says McGowan.
Despite the phrase “once in a blue moon” implying extreme rarity, blue moons occur fairly regularly. Because the lunar cycle lasts roughly 29.5 days—which doesn’t align perfectly with calendar months—both monthly and seasonal blue moons happen every two to three years.
(Earth always has more moons than you think.)
So, why call it a blue moon if the moon doesn’t actually turn blue? The term likely evolved from older expressions used to describe something rare or absurd. One possible origin traces back to the medieval English phrase “the moon is blue,” which referred to something absurd or unlikely.
There have been rare cases where the moon literally appeared blue-ish. After major volcanic eruptions or large wildfires, smoke and ash particles in Earth’s atmosphere can scatter red light and allow more blue light to pass through. Following the 1883 eruption of Krakatoa, for example, observers around the world reported seeing blue-tinged moons.
What is a micromoon?
The second half of this month’s lunar pairing comes from the moon’s elliptical orbit around Earth. Because the moon does not travel in a perfect circle, its distance from Earth changes slightly throughout each orbit.
A micromoon occurs when a full moon coincides with apogee, the point at which the moon is farthest from Earth. By contrast, a full moon at perigee, the moon’s closest point to Earth in its orbit, is what’s known as a “supermoon.”
(The moon is even older than we thought.)
Given the extra distance, a micromoon appears smaller and dimmer compared to a “normal” full moon. The difference, however, is very subtle. Compared to a supermoon, a micromoon can appear roughly 10 to 15 percent smaller in apparent diameter. “Most casual observers wouldn’t notice the difference without a side-by-side comparison, but careful observers or photographers can detect it,” says McGowan.
How to see the May blue micromoon
The best time to observe the micromoon is after moonrise on May 30 or in the early morning hours of May 31, depending on your location. If there are no clouds, look toward the eastern horizon to catch the moon. No telescope is necessary to look at the moon, though binoculars can help reveal details of the lunar surface, including craters and the dark volcanic plains known as maria.
(How to take stellar photographs of the night sky.)
Photographers hoping to capture the subtle size difference may want to compare images of this moon with photos taken during a supermoon—the next one is on December 24, 2026.
Even without dramatic visual changes, the rare pairing offers a good excuse to spend a few moments looking up at one of the sky’s most familiar sights.
