Skywatchers are in for a unique celestial event as a rare Blue Micromoon appeared on Sunday May 31, 2026. This unusual full moon combines two lunar phenomena in a single night.

It is both a “blue moon,” meaning the second full moon within a calendar month, and a “micromoon,” occurring when the Moon is near its farthest point from Earth.

The result is the smallest-looking full moon of the year, though most observers may not notice the difference without careful comparison.

According to data from NASA Goddard Space Flight Center SKYCAL, the Moon reached its full phase at 08:45 UTC on May 31. For much of North America, the Moon appeared full as it rose Saturday evening and remained visible throughout the night.

What Is a Blue Moon?

Despite its name, a blue moon does not appear blue in color.

The term “blue moon” is commonly used to describe the second full moon occurring within a single calendar month. Because the lunar cycle lasts approximately 29.5 days, months that begin with a full moon can occasionally contain a second one before the month ends.

This year, the first full moon occurred on May 1. The timing of the lunar cycle allowed another full moon to occur on May 31, creating the rare calendar event.

NASA explains that the modern definition became widely known after a 1946 article published in Sky & Telescope magazine. An older definition referred to the third full moon in a season containing four full moons.

Learn more through NASA Moon Information.

Why This Full Moon Is Called a Blue Micromoon

The more unusual aspect of this event is that it occurs near the Moon’s apogee—the point in its orbit farthest from Earth.

The Moon does not travel around Earth in a perfect circle. Instead, it follows an elliptical orbit.

At perigee, its closest approach, the Moon sits about 226,000 miles from Earth. At apogee, that distance increases to roughly 252,000 miles.

Because this full moon occurs near apogee, it appears slightly smaller in the sky than average.

Astronomers sometimes refer to such an event as a “micromoon,” the opposite of a supermoon.

NASA notes that the apparent size difference between the largest and smallest full moons of a year can be as much as 14 percent.

The Science Behind the Size Difference

While a 14 percent difference may sound dramatic, the variation is surprisingly difficult to detect with the naked eye.

Most people observing the Moon on May 31 would likely see what appears to be a normal full moon.

Experienced astronomers and astrophotographers often compare photographs taken under identical conditions to measure the difference accurately.

The Moon's changing apparent size is simply a consequence of orbital mechanics.

As Earth’s natural satellite moves closer and farther away during its monthly orbit, its apparent diameter changes accordingly.

More information about lunar science can be found on NASA's Solar System Exploration: Moon page.

The Traditional Flower Blue MicroMoon

In addition to being a blue moon and micromoon, this lunar event is also traditionally known as the Flower Moon.

The name comes from seasonal traditions associated with springtime blooms across the Northern Hemisphere.

Many Native American communities, as well as early colonial settlers, used names tied to seasonal changes to identify full moons throughout the year.

The Flower Moon reflects the abundance of flowers that emerge during late spring.

Information about traditional moon names can be found through The Old Farmer’s Almanac.

Best Ways to View the Blue Micromoon

Astronomers recommend finding a location with a clear eastern horizon around sunset for the best viewing experience.

Interestingly, the Moon often appears largest when it is low in the sky due to a psychological phenomenon known as the Moon Illusion.

This optical illusion causes the Moon to look larger near the horizon than when it is high overhead, even though its actual size remains unchanged.

Ironically, the illusion may make this year’s smallest full moon appear larger than it really is.

How Rare Is a Blue Micromoon?

Blue moons occur approximately once every two-and-a-half to three years.

Micromoons occur whenever a full moon coincides with the Moon’s distant orbital position, but the overlap of both events is less common.

While not extraordinarily rare, a blue micromoon is unusual enough to attract the attention of astronomers and skywatchers alike.

The next blue moon under the calendar definition is expected in December 2028, although whether it will also qualify as a micromoon depends on the Moon’s orbital position at that time.

A Cosmic Event

Ultimately, the Blue Micromoon is not the result of a mysterious celestrial event but rather a predictable outcome of the Moon’s orbit and the way humans organize time through calendars.

Still, the event offers an excellent opportunity to step outside, look up, and appreciate the celestial dynamics unfolding above Earth every month.

Even if the Moon appears much the same as any other full moon, understanding the science behind it adds a new layer of wonder to one of the night sky’s most familiar sights.

Relevant Links

• NASA Moon Information
• NASA Solar System Exploration: Moon
• NASA Goddard SKYCAL Database
• The Old Farmer’s Almanac