Blue Moons have always been the subject of folklore and intrigue, captivating our imaginations with their rare appearance in the night sky. But what exactly is a Blue Moon, and how does it differ from other full moons? Contrary to popular belief, the term “Blue Moon” has nothing to do with its color. Instead, it is all about the timing of the year’s full moons.
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Understanding Blue Moons
The next Blue Moon is set to grace the skies on August 19, 2024. This year, it is the only Blue Moon to be witnessed, and it falls under the category of a “seasonal” Blue Moon. But what does that mean exactly?
There are two types of Blue Moons: seasonal and monthly. A seasonal Blue Moon occurs when it is the third full moon within an astronomical season that consists of four full moons. This phenomenon takes place between the summer solstice and the fall equinox, as explained by NASA. On the other hand, a monthly Blue Moon refers to the second full moon within a single month that features two full moons.
Marking Your Calendar
If you’re eagerly anticipating the next monthly Blue Moon, you’ll have to wait until May 31, 2026. The most recent occurrence of a monthly Blue Moon took place on August 31, 2023. However, it’s worth noting that not everyone may have the chance to witness a Blue Moon, depending on their time zone.
Astronomy Magazine shed light on this peculiar occurrence back in 2004. Astronomers calculate the instant of the full moon using Universal Time (UT), which is equivalent to Greenwich Mean Time. If you reside in a time zone that is considerably distant from Greenwich, England, adjusting the UT time of the full moon to your local time might push the date to the first day of the following month. Consequently, what was once a Blue Moon could lose its status. In July 2004, for instance, the Blue Moon appeared either in July or a month later, coinciding with the second full moon of August.
How Common are Blue Moons?
Given their reputation as uncommon celestial events, you might assume that Blue Moons rarely occur. However, they actually happen approximately every 33 months. Monthly Blue Moons occur when a full moon falls at the beginning of a calendar month, ensuring that there are at least 29.5 days between the two full moons. Both types of Blue Moons occur every two or three years.
Every two and a half years, a 13th full moon graces our calendars. This happens because moon phases repeat every 29.5 days, resulting in 354 days for 12 full moon cycles. Since this period is shorter than a full year, a Blue Moon is bound to occur every two and a half years.
Between the years 1550 and 2650, a total of 408 seasonal Blue Moons and 456 monthly Blue Moons have or will occur. Interestingly, there can be instances where both types appear within the same year. Initially, almanacs referred to the extra full moons as “Blue,” as stated in the Encyclopedia Britannica. However, a magazine’s error in the 1980s, which claimed the extra moon was the second in a calendar month rather than the fourth in a season, led to a new interpretation that eventually gained widespread usage.
Does the Moon Actually Turn Blue?
Despite the name, a Blue Moon does not actually change color. However, specific atmospheric conditions, such as smoke or volcanic ash, can create an optical illusion, causing the moon to appear indigo.
Next time you catch a glimpse of a Blue Moon, remember that its magic lies not in its color but in the rarity and timing of its appearance.