Stargazers worldwide should be ready because very soon they will have another chance to observe one of the moon’s rarest calendar events, called the Blue Moon, which will appear in the sky on May 31, 2026.
During this event the moon will not be blue in color. According to NASA, the name is actually given simply for the unusual synchronization of the full moon.
A blue moon can be defined in two different ways. The first is a seasonal blue moon, the third full moon in a given season if there are four full moons in that season.
The second, and more functional today, is a month in which there happens to be two full moons; so a monthly blue moon. This upcoming Blue Moon in May 2026 refers to the second definition mentioned here, which is more functional today: a month with two full moons.
Why do blue moons happen?
The moon’s cycle in the sky repeats in about 29.5 days. Calculated in a cycle of 12, it repeats in 354 days. This is shorter than the calendar year. This cyclical difference means there is an extra full moon every two to three years. This moon, which does not fit the traditional naming system, is called a Blue Moon.
According to NASA, February can never have a blue moon because it is too short. On rare occasions, February can miss a full moon completely. In this situation it is called a Black Moon. In general, having a blue moon occurs when a month has four full moons.
The last Blue Moon was on August 19, 2024. After the Blue Moon of May 2026, there will be another few years of waiting, as nature designed.
Can the moon really turn blue?
Yes, but only under exceptional atmospheric conditions. According to NASA, volcanic eruptions of large magnitude, such as the eruptions of Krakatoa in 1883 and Mount Pinatubo in 1991, spewed out enormous amounts of fine particles that filled the atmosphere with particles that preferentially scattered red light, giving Earth’s moon a blue or blue-green appearance.
In most cases, however, a Blue Moon remains just a poetic expression of a rare but predictable phenomenon.
