Mark your calendars! On May 31, skywatchers will be treated to a rare celestial sight: a Micro Blue Moon. While it may not be as spectacular as a total lunar eclipse or as bright as a Supermoon, this subtle lunar event offers a wonderful opportunity to observe the Moon from a unique perspective—and to learn about the mechanics of its orbit.
What Is a Blue Moon?
There are two types of Blue Moons:
A calendar-based Blue Moon is the second Full Moon in a calendar month, when a month contains two Full Moons. This type of Blue Moon occurs about once every 2–3 years.
A seasonal Blue Moon is the third Full Moon in a season with four Full Moons. A season typically has three Full Moons, but when a fourth occurs, the third is designated a Blue Moon.
Does a Blue Moon Actually Look Blue?
No, a Blue Moon looks like any other Full Moon in the night sky. The term “Blue Moon” refers to the timing of the Moon in the calendar, not its color.
However, the Moon can occasionally appear bluish due to unusual conditions in Earth’s atmosphere. Large volcanic eruptions or intense wildfires can fill the air with tiny particles that scatter red light, giving the Moon a blue tint.
Despite the name, this rare effect is unrelated to an actual Blue Moon and can happen during any lunar phase.
What is a Micromoon?
What makes May’s Blue Moon especially interesting is that it coincides with apogee—the point in the Moon’s orbit where it is farthest from Earth.
Because the Moon’s orbit is slightly elliptical (not a perfect circle), its distance from Earth changes throughout the month. When a Full Moon occurs at apogee, it appears:
- Slightly smaller in size than an average Full Moon
- A bit dimmer than average
A Full Moon at a distance of 405,500 km or more (~251,900 miles) is considered a Micromoon. This occurs about 2 to 3 times each year. Although the difference is not extremely dramatic to most people’s eyes, some observers may notice the subtle change in size and brightness compared to a typical Full Moon. The Full Flower Moon of May 1 was also at apogee and labeled a Micromoon.
How Does a Micromoon Compare to a Supermoon?
Most people are familiar with the term Supermoon, as it often appears in news feeds when it happens. A Supermoon occurs when a Full Moon happens near perigee—its closest point to Earth. The Super Full Moon appears:
- Slightly larger in size, about 14% bigger than a Micromoon.
- Brighter than average, about 30% brighter than a Micromoon.
A Micromoon is the opposite of a Supermoon. One appears larger and brighter because it is closer to Earth, while the other appears smaller and dimmer because it is farther away.
Observing the Micro Blue Moon
With the Naked Eye
- On May 31, look towards the east around sunset and watch the Full Moon rise.
- It may look like a “normal” moonrise, but remember, you are seeing it at one of its more distant points from Earth.
With Binoculars or a Telescope
Even though Full Moons are not ideal for viewing fine surface details (due to the direct sunlight washing out shadows), you can still enjoy:
- Bright ray systems (like those around Tycho crater)
- Large maria—the dark areas also known as “seas.”
For the best views of the Micro Blue Moon, plan to observe near moonrise or moonset (when the Moon is lower on the horizon and appears larger due to the Moon illusion.
Astrophotography Comparison Idea
Take photos of the Micro Blue Moon on May 31, the Full Moon on July 29, and the Supermoon on November 24 or December 23. Try to keep your camera settings similar each time to see how their apparent sizes compare.
- Include foreground elements such as trees, mountains, landmarks, or rooftops to add scale and make the composition more interesting.
Make it a fun challenge.
More Than Just a Moon
While a Micro Blue Moon may not grab headlines like a Super Blue Moon, it reminds us that even familiar objects like the Moon are constantly changing and reflect the evolving relationship between Earth and its natural satellite. The Micro Blue Moon on May 31 shows it has a story to tell, too. Whether you are a casual observer or a seasoned astronomer, take a moment to step outside, look up, and enjoy this celestial event.
Clear skies!