What Does "Blue Moon" Mean in 2024
The term "blue moon" has two standard definitions used by astronomers and calendars. The most common definition is the second full moon in a single calendar month. The older definition, from the Maine Farmer's Almanac, is the third full moon in an astronomical season that has four full moons instead of the usual three. Neither definition makes the moon physically appear blue in color. The next calendrical blue moon by the second-definition rule occurs in August 2024, and the next seasonal blue moon by the older rule occurs in August 2024 as well, according to timeanddate.com blue moon definition and dates.
Calendrical blue moons happen because the lunar cycle is about 29.5 days, which is shorter than most calendar months. This mismatch causes a full moon to occasionally fall on the first and last days of a month. The seasonal rule exists because a solar year contains 12 full moons, but 13 full moons occur in some years, leaving one season with four full moons. The phrase "once in a blue moon" now means a rare event, but the actual frequency of a calendrical blue moon is about every 2 to 3 years, as noted by NASA NASA moon phases guide.
When Does the Moon Actually Look Blue
Atmospheric Conditions That Cause a Blue Moon
A physically blue moon is rare and happens when particles in Earth's atmosphere are close to 1 micron in size. This size scatters red light and allows blue light to pass through, a process called Mie scattering. Events like large volcanic eruptions, wildfire smoke, or dust storms can create these conditions. For example, the 1883 eruption of Krakatoa and the 1983 eruption of El Chichón produced blue moons and blue suns for months afterward, as documented by the U.S. Geological Survey USGS FAQ on blue moon.
More recent events have also produced blue-tinted moons. Large boreal wildfires in Canada in 1950 and in British Columbia in 2023 sent smoke plumes into the stratosphere, leading to reports of blue and lavender moons in affected regions. The moon can also appear blue near the horizon due to Rayleigh scattering, but this is a common optical effect and not a true blue moon. The frequency of a true blue moon from volcanic or wildfire particles is irregular and depends on the scale and altitude of the particles in the atmosphere.
How Often Does a Blue Moon Happen
Frequency of Calendrical Blue Moons
Calendrical blue moons occur roughly every 2.7 years on average. This is because the extra 10.8 days in a solar year compared to 12 lunar months accumulate until a second full moon fits inside one month. The next calendrical blue moon will be in May 2026, followed by December 2026, March 2028, and October 2028, based on calculations from timeanddate.com blue moon calendar.
Seasonal blue moons are slightly more frequent in some decades because the timing of equinoxes and solstices shifts relative to the calendar. The last seasonal blue moon was on August 19, 2024, and the next one is expected on May 31,