May's Blue Moon Peaks This Weekend: What to Know

This weekend, stargazers will witness a rare celestial event: the second full moon to occur within a single calendar month, commonly known as a blue moon. This phenomenon, a calendrical blue moon, happens approximately seven times every 19 years, or roughly every two to three years. This particular blue moon will peak at 4:45 a.m. ET on Sunday, May 31, before the sun rises.

The moon’s orbit around Earth is not a perfect circle, lasting about 29.5 days. This slight discrepancy with the typical 30- or 31-day month occasionally leads to two full moons within one calendar month. The second full moon of such a month is dubbed the blue moon. In rare instances, a single year can even host two blue moons, as happened in 2018 when both January and March had two full moons, leaving February without one. The next occurrence of two blue moons in one year is anticipated in 2037.

Understanding Blue Moon Types

Beyond the calendrical blue moon, there is also a seasonal blue moon. This occurs when a season contains four full moons instead of the usual three; the third full moon in that sequence is designated the blue moon. The next seasonal blue moon is slated for May 20, 2027. Despite its evocative name, the moon will not appear blue. Its color will remain its typical silvery white or golden hue. The phrase itself dates back to the 1500s, initially describing something considered impossible, according to Brittanica. The term was first documented in the 1937 Maine Farmer’s Almanac and evolved to describe this specific lunar event.

While generally silvery white, the moon can, on rare occasions, appear blue. This happens when Earth's atmosphere contains smoke or dust particles larger than 900 nanometers. A notable historical instance was in 1883, following the eruption of Indonesia's Krakatoa volcano, when observers reported the moon appearing blue, as detailed by Brittanica.

Adding another layer to this weekend's event, the May blue moon is also classified as a micromoon. This designation means it will be near its farthest point from Earth in its elliptical orbit, known as apogee. While the moon's distance varies by about 10% throughout its orbit, the difference in apparent size is generally not noticeable to the unaided eye, according to Dr. Pamela Gay, senior scientist at the Planetary Science Institute. This upcoming micromoon will be the most distant full micromoon of 2026, situated approximately 252,334 miles (406,093 kilometers) from Earth, compared to the average distance of about 238,855 miles (384,400 kilometers), according to NASA. In contrast, the last calendar blue moon in 2023 was a supermoon, occurring at perigee, its closest approach to Earth.

For viewers in the Americas, Africa, and Europe, the moon will appear fullest on the night of May 30. Those in Australia, New Zealand, and Asia will experience the brightest full moon on the night of May 31. Dr. Gay suggests that the period when the moon is rising, typically between 5:30 and 6:30 in the local time zone, offers a beautiful setting for capturing photographs of the full moon with a typical cellphone camera.