May’s night sky is bringing a rare lunar event: a Blue Moon. A Blue Moon occurs when two full moons fall within a single calendar month. This Blue Moon will occur on the night of May 31.
What is a Blue Moon?
"Because the Moon takes 29.5 days to go through a complete cycle of phases, there are times when more than one Full Moon falls within the same calendar month,” shared Nick Anderson, Astronomer at the Cleveland Museum of Natural History. “This is the modern definition of a Blue Moon, and these events are somewhat uncommon, occurring only about once every 2 to 3 years. Don’t expect the Moon to appear blue in color though. That requires certain atmospheric conditions stemming from forest fires or volcanic eruptions.”
The term “Blue Moon” is purely calendar-based, though the Moon can occasionally appear reddish, orange, or yellow depending on atmospheric conditions near the horizon. True, blue-colored Moons occur much less often.
The rarity of the event is also where the phrase “once in a blue Moon” comes from—a popular saying used to describe something uncommon or infrequent. While Blue Moons are not extraordinarily rare, they are infrequent enough to be a memorable sight for skywatchers.
What Makes This Full Moon Special?
This particular Blue Moon is considered a micromoon, meaning the Moon will be at the farthest point in its orbit around Earth. Because the Moon follows a slightly elliptical orbit rather than a perfect circle, its distance from Earth changes throughout the month. During a micromoon, the Moon can appear slightly smaller and dimmer than an average full Moon, though the difference is subtle to the naked eye.
Even though the full moon may look completely illuminated, we are actually seeing sunlight reflected off the Moon’s rocky surface. A full moon occurs when Earth is positioned almost directly between the Sun and Moon, allowing the side facing Earth to be fully lit. The Moon itself does not produce any light of its own.
Why Does the Moon Look Different in the Sky?
Full Moons have long been used to track time and seasonal changes across many cultures. Traditionally, each full Moon throughout the year carries its own name tied to nature and seasonal events. The first full Moon of May is often called the Flower Moon because it arrives during the height of spring blooms in many parts of North America.
Although full Moons occur every month, each one can appear slightly different depending on its position in the sky and atmospheric conditions on Earth. When the Moon is low on the horizon, it can appear larger or more golden in color because its light is traveling through more of Earth’s atmosphere. This effect, sometimes called the “Moon illusion,” is a trick of human perception rather than an actual change in the Moon’s size.
How to Observe the Blue Moon
Scientists continue to study the Moon only because it influences tides and ecosystems on Earth, but also because it provides clues about the early history of our solar system. The Moon’s surface preserves billions of years of impact craters and geological history that have largely been erased on Earth by weather, water, and plate tectonics. Recent missions like Artemis II have helped us expand our knowledge about the Moon, especially on the side facing away from Earth.
For skywatchers, the Blue Moon is an easy event to observe without any special equipment. The Moon will appear brightest as it rises in the eastern sky shortly after sunset. Binoculars or a small telescope can help reveal darker lava plains, bright crater rays, and other details across the lunar surface.
Events like a Blue Moon are reminders that even familiar objects in our night sky are constantly moving in predictable—but sometimes surprising—patterns shaped by gravity, orbital motion, and the ongoing relationship between Earth, the Moon, and the Sun. For those interested in learning more about the Moon, planets, and other celestial events, the Nathan and Fannye Shafran Planetarium offers immersive programs and experiences that bring the science of the night sky a little closer to home.
