What is a blue moon?
4 min read
The phrase "once in a blue moon" is often used to describe something rare, but what does a Blue Moon really mean? Many people assume it refers to the Moon appearing blue, but that’s not quite accurate. The term actually has an astronomical meaning, and it’s related to the lunar calendar. In this article, we’ll explore what a Blue Moon is, how it occurs, and whether the Moon ever actually turns blue.
Understanding the Blue Moon
A Blue Moon is an extra full moon that appears within a specific time period. There are two definitions for a Blue Moon:
Monthly Blue Moon – This occurs when there are two full moons in a single calendar month. The second full moon is called a Blue Moon.
Seasonal Blue Moon – This happens when there are four full moons in one season (between a solstice and an equinox). The third full moon in that season is called a Blue Moon.
The Monthly Blue Moon
The monthly Blue Moon is the more commonly known definition. Since the lunar cycle is about 29.5 days long, most months only have one full moon. However, because our calendar months vary in length, sometimes a second full moon can fit into the same month. When this happens, the second full moon is called a Blue Moon. This usually happens every 2.5 to 3 years.
For example, if a full moon occurs on August 1st, and another full moon happens on August 31st, then the second one is a Blue Moon.
The Seasonal Blue Moon
The seasonal definition is a bit older. Normally, each season has three full moons. But because the lunar cycle doesn’t perfectly match the solar year, sometimes a season ends up with four full moons instead of three. In that case, the third full moon is labeled a Blue Moon. This prevents the traditional names for the last full moon of the season from being disrupted.
Why Does a Blue Moon Happen?
A Blue Moon occurs because our solar calendar (365 days) and the lunar cycle (29.5 days) don’t match up perfectly. Over time, the difference adds up, leading to an extra full moon every few years. Since our calendar is man-made, the Blue Moon is simply a result of how we count days.
Frequency of a Blue Moon
A monthly Blue Moon occurs about every 2.5 years.
A seasonal Blue Moon happens about every 2 to 3 years.
A double Blue Moon (two Blue Moons in a single year) is much rarer and happens about once every 19 years.
Does the Moon Actually Turn Blue?
Despite its name, a Blue Moon does not usually appear blue. The term “Blue Moon” comes from old English sayings and is unrelated to the Moon’s actual color.
However, there have been rare cases where the Moon did appear blue. This happens due to atmospheric conditions, such as volcanic eruptions or large forest fires. Particles in the air can scatter red light and make the Moon appear bluish. Some notable events that caused this effect include:
1883 – Krakatoa Eruption: The volcanic ash from the Krakatoa eruption in Indonesia made the Moon appear blue worldwide for several months.
1950s – Canadian Wildfires: Smoke from massive forest fires caused blue-colored moons in some regions.
1991 – Mount Pinatubo Eruption: Similar to Krakatoa, this volcanic eruption created blue-tinged moons in some areas.
The Science Behind a Blue-Tinted Moon
For the Moon to actually appear blue, the air must have particles about 1 micron wide, which scatter red light while allowing blue light to pass through. This effect is rare and unpredictable.
The History of the Blue Moon Definition
The term "Blue Moon" has an interesting history. The seasonal Blue Moon definition dates back to The Maine Farmers’ Almanac in the 19th century. However, the monthly Blue Moon definition became popular due to a misunderstanding in a 1946 issue of Sky & Telescope magazine. The error was later repeated on a 1980 radio broadcast, making the "second full moon in a month" meaning widely accepted.
Blue Moons and Folklore
Throughout history, the Moon has been surrounded by myths and folklore. A Blue Moon has sometimes been seen as a sign of:
Change and transformation
Unusual or unpredictable events
A time of heightened emotions
Some cultures believed a Blue Moon had spiritual significance, influencing human behavior and natural events.
When Is the Next Blue Moon?
If you're interested in seeing the next Blue Moon, check a lunar calendar. The next occurrences are:
Monthly Blue Moon – August 30, 2023
Seasonal Blue Moon – August 19, 2024
For more upcoming Blue Moons, refer to sources like NASA, the Farmers’ Almanac, or the Sky & Telescope website.
Conclusion
A Blue Moon is a fascinating celestial event that occurs due to the mismatch between the lunar cycle and our calendar. Whether it’s a monthly Blue Moon (second full moon in a month) or a seasonal Blue Moon (third of four full moons in a season), this event happens every few years. While the Moon doesn’t actually turn blue, atmospheric conditions can sometimes create a rare blue-colored Moon.
So next time you hear the phrase "once in a Blue Moon," you’ll know exactly what it means!
External Sources:
NASA - Understanding the Lunar Cycle: https://moon.nasa.gov/
Farmers’ Almanac - Blue Moon Explanation: https://www.farmersalmanac.com/
Sky & Telescope - The History of the Blue Moon: https://skyandtelescope.org /