The term “blue moon” is famous in folklore, often regarded as a rare and mysterious phenomenon. In fact, a blue moon does not mean the moon actually turns blue; it is related to the number of times the moon appears in a year. Specifically, a blue moon occurs approximately every 33 months, but this phenomenon has no direct connection to the moon’s color. A blue moon occurs because there are 13 full moons in a year, while normally there are only 12 months in a year. Therefore, an extra full moon appears approximately every 33 months, which is what is called a blue moon. Although a blue moon sounds very romantic and mysterious, it is actually an astronomical term used to describe this specific time interval.
The Next “Blue Moon” is May 31, 2026.
This will be a monthly “blue moon”; specifically: there are two types of blue moons: seasonal blue moons and monthly blue moons. According to NASA, a seasonal blue moon is “the third full moon in an astronomical season with four full moons”, occurring between the summer solstice and the autumnal equinox. A monthly blue moon, which is what most people think of when they hear the term, is the second full moon in a single calendar month.
The Next Seasonal Blue Moon is May 20, 2027.
Incidentally, people living in specific time zones may not be able to experience a “blue moon”. Astronomy magazine explains: “Astronomers use Universal Time (UT) to calculate the moment of a full moon, which for the purposes of this article is equivalent to Greenwich Mean Time. If you happen to live in a time zone far from Greenwich, UK, adjusting the universal time of the full moon to local time may be enough to advance the date to the first day of the new moon, meaning it is no longer a ‘blue moon’. For example: ‘Depending on your time zone, the July 2004 ‘blue moon’ may have occurred in July or a month later, as the second full moon in August.'”
How Rare Are They?
This idiom is usually used to describe extremely rare events that will happen in the distant future or actually never happen at all. They occur approximately every 33 months, and this situation has specific examples in astronomy.
The blue moon phenomenon is a typical example. A blue moon does not mean the moon actually turns blue; it refers to the occurrence of two full moons in the same lunar month. A lunar month, which is the time interval from one new moon to the next, is usually 29.5 days. This means that if a full moon occurs at the beginning of a month, the moon will be full again at the end of the month, thus forming a so-called blue moon. Due to the length of the lunar month, it takes the moon about 354 days to complete 12 full moon cycles. This is about 11 days less than a solar year, so a blue moon phenomenon occurs approximately every two and a half years. This rare astronomical event is a vivid example of the rare events described by the idiom.
Between 1550 and 2650, there will be 408 seasonal “blue moons” and 456 monthly “blue moons”. Seasonal “blue moons” and monthly “blue moons” can occur in the same year. Almanacs originally referred to the extra “blue moon” as a “blue moon”. According to the Encyclopedia Britannica, a mistake in a magazine “mistook the extra moon for the second moon in a calendar month rather than the fourth in a season, [giving] it a new meaning that eventually came into widespread use in the 1980s.”
Does the Moon Turn Blue?
No, although during a blue moon, the moon does not actually turn blue. Certain atmospheric conditions, such as the presence of smoke or volcanic ash, can sometimes give the moon an indigo appearance. This does not mean that a full moon affected by such atmospheric phenomena truly becomes a so-called blue moon.