What Is Blue Moon And When is the next Blue Moon?

What Is Blue Moon?

Blue moon, the second full moon in a calendar month. The period from one full moon to another is about 29 1/2 days, so when two occur in the same month, the first of these full moons are always on the first or second day of the month. February, which has only 28 days (29 days in leap years), can never have a blue moon. Months with 31 days—January, March, May, July, August, October, and December—have a much better chance of hosting a blue moon because of their length. On average, a blue moon occurs once every 33 months or full moons. 41 times per century, or about seven times every 19 years. An even rarer event is when two blue moons happen in the same calendar year, which happens about four times per century.

The Moon is not actually blue, and as long ago as the 16th century the expression “the Moon is blue” meant something that was not possible. However, greenish- and bluish-tinted Moons are sometimes observed just before sunrise or just after sunset when large quantities of dust or smoke particles are concentrated high in the atmosphere, filtering out the longer wavelengths of colors such as red and yellow. For two years following the eruption of the Krakatoa volcano in Indonesia in 1883, people around the world reported seeing strangely colored sunsets and a Moon that appeared blue. With this possible, but uncommon, occurrence, “once in a blue moon” came to mean rare rather than impossible.

When is the next Blue Moon?

The next Blue Moon will come on October 31, 2020. It’ll be called a Blue Moon because it’ll be the second of two full moons to occur in a single calendar month. The last Blue Moon by this definition of the term happened on March 31, 2018.
The first full moon of that month, by the way, will fall on October 1-2, 2020. It’ll be the Northern Hemisphere’s Harvest Moon.
There’s another definition for Blue Moon. It can be the third of four full moons in a single season, with a season being between a solstice and equinox. The next seasonal Blue Moon will be August 22, 2021.

In recent years, people have been using the name Blue Moon for these two different sorts of moons. Second of two full moons in a calendar month, or third of four full moons in a single season.
Someday, you might see a true blue-colored moon in the sky. Blue-colored moons are rare – aren’t necessarily full – and happen when Earth’s atmosphere contains dust or smoke particles of a certain size, slightly wider than 900 nanometers. Particles of this size are very efficient at scattering red light. When these particles are present in our air, and the moon shines through them, the moon may appear blue in color.

Meanwhile, the full moon you’ll see on October 31, 2020, likely won’t look blue. But – as the second full moon in a month – many will call it a Blue Moon.

Let’s talk about seasonal Blue Moons first. It’s an older definition for Blue Moons, stemming from old skylore. A year has 12 months, of course. A month – or “moonth” – has a length more or less based on a single orbit of the moon around Earth. What we call a season – winter, spring, summer, fall – typically lasts three months, and typically has three full moons.
If a season has four full moons, then the third full moon may be called a Blue Moon, according to the old Maine Farmer’s Almanac. There was a Blue Moon by this definition on November 21, 2010. Another on August 20-21, 2013, another on May 21, 2016, and another on May 18, 2019
The next will be on August 22, 2021.

Next Dates
2020 31 October
2021 22 August
2023 31 August
2026 31 May
2028 31 December

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