The Mechanics of the Monthly Blue Moon
The term Blue Moon has less to do with color and more to do with the friction between our Gregorian calendar and the lunar cycle. While a standard calendar month lasts 30 or 31 days (with the exception of February), the moon’s phases complete a full cycle every 29.5 days. This mismatch creates a surplus of lunar days over the course of a year.
Every two to three years, this timing allows a 13th full moon to squeeze into a single calendar year. In May 2026, the first full moon—the Flower Moon—rose on May 1. Because the lunar cycle is shorter than the month, a second full moon arrives just before the month concludes. According to Forbes, this specific occurrence on May 31 is classified as a monthly Blue Moon.
It is a quirk of timing that has entered our lexicon as a metaphor for rarity. USA Today notes that the expression “once in a blue moon,” reflects the uncommon nature of the event, though its linguistic roots trace back to the 16th century when the phrase was used to describe something seemingly impossible.
Why This Event is an Apogee Micromoon
Not all full moons are created equal. The moon does not orbit Earth in a perfect circle, but rather in an elliptical path. This means the distance between the two bodies is constantly shifting due to gravitational interactions with the Sun and other planets.
When a full moon occurs at its closest point to Earth, it is a Perigee Supermoon, appearing larger and brighter. This May 31 event is the opposite. As The Weather Network explains, this is an Apogee Micromoon, occurring when the moon reaches its farthest distance from Earth in its orbit.
The distance is significant: the moon may be roughly 251,000 miles away from our planet. Because of this extreme distance, the lunar disk will appear smaller and dimmer than a typical full moon. This stands in sharp contrast to the Super Blue Moon of August 19, 2024, which was characterized by its proximity and brilliance.
Viewing Windows and Atmospheric Effects
Despite the “Blue” moniker, observers should not expect a blue-tinted sky. The moon will actually appear orange when it first crests the horizon. This is the result of Rayleigh scattering, where Earth’s atmosphere filters out shorter blue wavelengths of light and allows longer red and orange wavelengths to pass through more directly.
For those wanting the best view, the peak occurs early Sunday morning, May 31, at 4:45 a.m. ET. However, the most visually striking moment will be during dusk on Saturday, May 30. Skywatchers are encouraged to find an unobstructed southeastern horizon at moonrise to witness the orange glow.
While the naked eye is sufficient, binoculars can provide a more detailed view of the lunar surface.
Broader Celestial Alignments
The Blue Moon is the centerpiece of a busier May sky. Early-rising observers may spot Mars and Saturn in the eastern sky by waking 45 to 60 minutes before sunrise. Similarly, Venus and Jupiter are currently inching closer together, with the optimal viewing window occurring 30 to 45 minutes after sunset, according to FOX Weather.
The moon’s orbital path also brings it into proximity with several bright stars this week. On May 26, the waxing gibbous moon appeared near the blue-white star Spica in the constellation Virgo. By May 31, Space.com reports that the moon will meet the red supergiant Antares in the constellation Scorpio.
This lunar activity serves as a backdrop to long-term exploration goals. The moon’s south pole, visible during various phases, remains a primary target for the planned 2028 Artemis 4 moon mission.
It is important to distinguish between the two types of Blue Moons, as they follow different astronomical rules. The monthly (or calendrical) Blue Moon is the one occurring this May. The other variety is the seasonal Blue Moon, which is defined as the third full moon in an astronomical season that contains four full moons.
These two types of events are typically offset by a year or more. While the monthly Blue Moon is happening now, a seasonal Blue Moon—the Blue Flower Moon—is expected on May 20, 2027.
The timeline for these occurrences is infrequent:
- May 31, 2026: Monthly Blue Moon (Apogee Micromoon)
- May 20, 2027: Seasonal Blue Moon
- December 31, 2028: Next Monthly Blue Moon
This cycle reinforces the variability of the lunar orbit. While the monthly Blue Moon occurs on average once every 30 months, the rare alignment of both a monthly and seasonal Blue Moon in a single year is an even more elusive event, with one such instance projected for 2048.
