This weekend’s sky will feature a blue moon, the term for the second full moon in a single calendar month. Although the name suggests a colorful change, the moon will look normal to the naked eye, appearing silvery white or golden like any other full moon. The event is relatively uncommon, occurring about every two to three years, or roughly seven times in 19 years, because the lunar cycle lasts about 29.5 days and does not fit neatly into the length of a typical month.
A blue moon can mean different things. The most familiar version is the calendrical blue moon, which is the second full moon in a month. Another type is the seasonal blue moon, which is the third full moon in a season that contains four full moons instead of the usual three. The article notes that the next seasonal blue moon is expected on May 20, 2027. It also explains that some years can contain two blue moons, as happened in 2018, when January and March each had two full moons while February had none. The next time that will occur is 2037.
The expression “blue moon” has older roots. The phrase “the moon is blue” dates back to the 1500s and was used to describe something believed to be impossible. Over time, the meaning shifted, and the term became associated with this rare lunar timing. The 1937 Maine Farmer’s Almanac is credited with the first documented use of the phrase in the modern astronomical sense, helping establish the term as it is used today.
Despite the name, the moon rarely turns blue in color. That can happen only under unusual atmospheric conditions, such as when smoke or dust particles are present in the atmosphere at a certain size. A famous example occurred in 1883 after the eruption of Krakatoa, when observers reported a blue moon because of volcanic particles in the air.
This month’s blue moon will also be a micromoon, meaning it occurs when the full moon is near apogee, the farthest point in its orbit from Earth. Because the moon’s orbit is not a perfect circle, its apparent size changes by about 10% over the course of a month. At apogee, the moon can appear slightly smaller, though the difference is usually too subtle for most people to notice without careful comparison. The article says Sunday’s moon will be about 252,334 miles from Earth, farther than the average lunar distance of about 238,855 miles.
The moon will reach peak fullness at 4:45 a.m. ET on Sunday, before sunrise. In the Americas, Africa and Europe, the moon will appear fullest on the night of May 30, while observers in Australia, New Zealand and Asia will see the brightest full moon on the night of May 31. Experts say the best time for viewing and photography is around moonrise, when the sky is still blue and the moon is low on the horizon.
