May’s blue – A true blue moon will take over the sky this weekend: the second full moon in a single month. It peaks at 4:45 a.m. ET on Sunday, features a micromoon, and reaches its fullest night across regions on May 30 and May 31.
When the weekend ends and Sunday begins, the night sky will deliver one of those rare calendar quirks the phrase “once in a blue moon” was built for: a blue moon—defined the plain, straightforward way—as the second full moon to appear in a single month.
This isn’t a moon that turns a new color for the sake of a catchy name. Earth’s satellite will show up its usual silvery white or golden hue. What makes it “blue” is the timing, not the pigment.
The moon cycle runs through eight lunar phases, and it averages 29.5 days—just shy of the length of a typical month. That mismatch is why, occasionally, a calendar month can hold two full moons. The second one has a name: a calendrical blue moon.
Blue moons don’t show up often. They come around seven times every 19 years, roughly every two to three years. And sometimes, the pattern even slips in a second way: a single year can feature two blue moons. In 2018, January and March both had two full moons, while February lacked one. The next time this will happen is in 2037, according to The Old Farmer’s Almanac.
There’s also a different kind of blue moon—seasonal. That version arrives when four full moons appear within a three-month season, with the third full moon labeled the blue moon. The next seasonal blue moon will rise on May 20, 2027.
The moon “turning blue” is mostly a story the sky refuses to repeat. The phrase “the moon is blue” dates back to the 1500s and was used to describe something thought to be impossible. where the term “blue moon” comes from. according to Brittanica. The first documentation of the phrase as we know it today traces to the 1937 Maine Farmer’s Almanac. noted by Dr. Pamela Gay, senior scientist at the Planetary Science Institute.
Still, there is one exception—rare and atmospheric. The moon can appear blue when Earth’s atmosphere contains smoke or dust particles that are a hair wider than 900 nanometers. according to Brittanica. In 1883, people reported seeing the moon turn blue after the eruption of Indonesia’s Krakatoa volcano.
This weekend’s event has its own twist, and it’s measurable. May’s blue moon will be fullest, or peak, at 4:45 a.m. ET on Sunday before the sun rises. It also comes with a micromoon, meaning it will be near its farthest point from Earth in its orbit, according to EarthSky.
Because the moon’s orbit isn’t a perfect circle. its apparent size shifts by about 10% at different points in its path. Gay said the closest point is called perigee and the farthest is the apogee. A full moon around the time of apogee becomes a micromoon—and the upcoming event will be this year’s most distant full micromoon.
Numbers make it plain: Sunday’s 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), according to NASA. But that difference won’t jump out to the unaided eye.
“If you’re used to comparing the moon to known things in your landscape — or holding your thumb up at arm’s distance to compare it to your thumb — you can see that 10% difference. But to the average person you’re not going to notice the difference at all,” Gay said.
If you’re comparing it to the last calendar blue moon. 2023 had a very different orbit character: it was no micromoon. Instead. it was the opposite kind of coincidence—a supermoon. when the full moon coincides with perigee. Earth’s closest approach of the moon in its orbit. At perigee, the moon is about 226,000 miles (363,300 kilometers) away from Earth, according to NASA.
For viewing plans, timing shifts across the globe. For the Americas, Africa and Europe, the moon will appear fullest the night of May 30. Australia, New Zealand and Asia will find the brightest full moon on the night of May 31.
“When that moon is rising. sometime between 5:30 and 6:30 in your time zone. and the sky is still this beautiful blue. there’s not a lot of weird settings to fuss with. your cellphone will take care of all the details for you. ” Gay said. “This time of year is really kind of an awesome time to get those photos of the full moon.”.
The sky won’t look like a “once in a blue moon” miracle that changes color on command. But the rarity is real—and for a few nights, the calendar and the lunar clock line up just right.
May blue moon micromoon full moon May 2026 lunar phases 4:45 a.m. ET EarthSky Pamela Gay NASA moon distance