The rarity of events and celebrations such as milestone birthdays and cosmic happenings makes them special.
The phrase “once in a blue moon” exists because it is not an everyday occurrence. This year, May has not one but two full moons lighting up the night sky, the second of which is going down tonight, in the early morning hours of Sunday, May 31.
Illumination reaches its peak at 4:45 a.m. ET.
But this is also considered a micromoon. Here’s everything you need to know before you stargaze.
What is a blue moon?
This has nothing to do with color. There are two ways to define a blue moon. The first is when a second full moon occurs in the same calendar month. This happens about once every 30 months. Tonight is the night for May 2026, as the first full moon took place on May 1.
The second definition deals in quarters. A typical astronomical season contains three full moons. Every two to three years, a fourth sneaks in, causing the third full moon to be considered a blue moon.
When will the next blue moon occur?
If you miss this one, you will have to wait until May 20, 2027, for a seasonal blue moon, according to Royal Museums Greenwich.
The next calendar blue moon won’t happen again until December 2028.
Can 2 blue moons happen in the same year?
Metaphorical lightning—or in this case, two blue moons—can strike twice in the same year, but it is rare.
According to The Old Farmer’s Almanac, in 2018, both January and March had a blue moon. February had no full moons. This won’t happen again until 2037.
What is a micromoon?
Although this blue moon is considered “micro,” the naked eye won’t be able to tell much difference in size. According to National Geographic, it will only look about 10% to 15% smaller in diameter and a bit dimmer.
The natural satellite orbits the Earth in an elliptical pattern, making it appear bigger and brighter or smaller and dimmer, depending on where it is on that journey.
A micromoon occurs around apogee, a fancy term for when the moon is at the farthest point away from Earth.