A Blue Moon is a rare occurrence of an additional full moon within a specific time frame. There are two main definitions:
- Monthly Blue Moon: This is the second full moon in a single calendar month that has two full moons. Since the lunar cycle is about 29.5 days, months with 31 days can sometimes fit two full moons. The first full moon usually occurs on the 1st or 2nd day of the month, and the second on the 30th or 31st day. February cannot have a Blue Moon because it is too short. This type of Blue Moon happens roughly once every 2 to 3 years, about seven times every 19 years
- Seasonal Blue Moon: This is the third full moon in an astronomical season that has four full moons instead of the usual three. Astronomical seasons are defined by solstices and equinoxes. This older definition comes from the Maine Farmer’s Almanac and relates to the timing of church festivals and holidays
The Moon does not actually turn blue during a Blue Moon. The phrase "once in a blue moon" means something rare or unusual. Occasionally, the Moon can appear bluish due to atmospheric conditions like volcanic ash or smoke particles filtering light, but this is unrelated to the calendrical Blue Moon
. The next Monthly Blue Moon will occur on May 31, 2026, and the next Seasonal Blue Moon on May 20, 2027
. In summary, a Blue Moon is a second full moon in a month or a third full moon in a season with four full moons, both considered rare but predictable astronomical events. The term originated from a misinterpretation in the 20th century but has become widely accepted today
