What Is a Blue Moon, And When Is The Next One?

Star Walk
4 min readApr 10, 2023
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On August 31, 2023, at 01:35 GMT, the second Full Moon in a calendar month will occur. Although such Full Moon is widely known as a Blue Moon, this naming is the result of a mistake. Read on to learn the true meaning of a Blue Moon.

What is a Blue Moon?

The origin of the name is still unclear. It has been around for a long time; some even trace it back to 400 years ago, when it meant something that could never happen (hence the English expression “once in a Blue Moon”).

A Blue Moon can be seasonal and calendrical. Let’s take a more detailed look at each definition.

Seasonal Blue Moon

A seasonal Blue Moon is the third Full Moon in an astronomical season with four Full Moons. As a rule, each season (defined by solstices and equinoxes) has three months and three Full Moons, but sometimes it has the fourth Full Moon.

How often does a seasonal Blue Moon occur?

Seasonal Blue Moons are relatively rare. We normally have 12 Full Moons in an astronomical year, as a lunar cycle lasts approximately one month. However, its exact duration is 29.5 days, which means it takes 354 days to complete 12 lunar cycles. This falls short of the 365/366 days in a year. So, every 2.5 to 3 years, we experience an “extra” Full Moon in a season.

When is the next seasonal Blue Moon?

- August 19, 2024, 18:26 GMT

- May 20, 2027, 10:59 GMT

- August 24, 2029, 01:51 GMT

Origins of a seasonal Blue Moon

The curious thing about a seasonal Blue Moon is that it’s always the third Full Moon in an astronomical season of four Full Moons. But why not the fourth?

The possible explanation refers to the Christian ecclesiastical calendar. In calculating the dates for Lent and Easter, Catholic clergy identified the Lenten Moon (the final Full Moon of winter) and Easter Moon (the first Full Moon of spring). The third Full Moon of the season arrived too early and upset the arrangement of Church holidays. It was marked a “betrayal Moon” to ensure that Lent and Easter corresponded with the right Full Moons and that other festivals would still fall on their “proper” dates.

According to the other version, the tradition dates back to the 19th century. The now-defunct Maine Farmer’s Almanac calculated and listed all of the Full Moons in a year, along with their traditional names that had to correspond with the particular months. So the authors had to use a special name for the “extra” Moon to make things work.

If you want to learn more about the traditional names given to the Full Moon by the Native Americans and other peoples, see our colorful Full Moon Calendar.

Calendrical Blue Moon

The majority of us are familiar with a сalendrical Blue Moon, which is the second Full Moon in a calendar month with two Full Moons. This is the most popular and best-known definition. However, it is a result of a mistake made by amateur astronomer James Hugh Pruett. Back in 1946, he misinterpreted the seasonal definition while writing an article for the Sky & Telescope magazine. The wrong definition has gone viral in the media and has become even more popular than the original one.

How often does a calendrical Blue Moon occur?

Calendrical Blue Moons may occur more often than seasonal ones. However, they are not the same worldwide, as the exact Full Moon dates depend on the time zones. For example, China, Japan, Korea, Australia, New Zealand, and many other countries (observing the time zones that are at least 8 hours ahead of Greenwich) will experience a calendrical Blue Moon on January 30 and then on March 30, 2029, while the rest of the world will only see one calendrical Blue Moon on December 31, 2028.

When is the next calendrical Blue Moon?

- August 31, 2023, 01:35 GMT

- May 31, 2026, 08:45 GMT

- December 31, 2028, 16:48 GMT

- September 30, 2031, 18:58 GMT

Note that the exact date of a Full Moon may differ depending on your time zone. Use the Moon tab in the Sky Tonight calendar to see which month has two Full Moons according to your local time.

When is the next Blue Moon?

The next calendrical Blue Moon will occur on August 31, 2023, at 01:35 GMT. This Full Moon will also be the year’s biggest and brightest, as our natural satellite will come especially close to the Earth and become a Supermoon.

The next seasonal Blue Moon will occur on August 19, 2024, at 18:26 GMT.

Is a Blue Moon actually blue?

Although it is called “blue,” and many pictures show the Moon in bluish tones, the actual Moon has nothing to do with this color. In the sky, it looks pale gray or white.

Sometimes, the Moon can look blue, but this phenomenon cannot be predicted and mainly depends on atmospheric conditions. For instance, in 1883, after the volcano Krakatoa erupted, the Moon appeared bluish because of all the dust in the air.

The lunar surface actually has gray, light-blue, brown, and yellow patches. Can you tell why it appears red, orange, or white when observed from the Earth? Pass our quiz about the colors of the Moon and check yourself!

Bottom line

A Blue Moon is a particular name for the third of four Full Moons in an astronomical season or the second Full Moon in a calendar month. It is not a specific astronomical phenomenon but rather the result of a mismatch between the calendar year and the lunar one, just like a Black Moon. For an astronomer, a Blue Moon is not any different from some other Full Moon. Check the upcoming Full Moon dates in Sky Tonight or in the dedicated article.

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