What Is a Blue Moon and When Is the Next One?

Blue moon, as a term, is pretty misleading. It’s neither particularly rare, as suggested by the popular idiom “once in a blue moon,” nor blue in color. So, what does a blue moon signify? Here’s everything you need to know.

A Calendar Blue Moon or a Seasonal One?

The term blue moon has two commonly known definitions. The most popular and widely known definition comes from a man named James Hugh Pruett, who, in March 1946, wrote an article called “Once in a Blue Moon” in the Sky & Telescope magazine. In this article, Pruett essentially dubbed the second full moon in a calendar month as a blue moon. This blue moon is also known as the calendar blue moon. It was popularized in the mid-1980s and got international recognition in the late 1990s.

However, this is not the so-called traditional definition of a blue moon. The “traditional” definition has been derived from the 1937 Maine Farmers’ Almanac, a provincial US magazine. However, as no tradition of it existed before 1937, it’s more of an invented tradition.

This definition calls the third full moon in an astronomical season, having four full moons as a blue moon. It’s also known as a seasonal blue moon. More on the seasonal blue moon later.

The Second Full Moon in a Calendar Month

As mentioned, the calendar blue moon is the widely known version. It occurs because the duration between two calendrical full moons is approx. 29.5 days, which is less than the length of a typical calendar month. As a result, it’s possible for two full moons to fall in a single calendar month, one at the start of the month and the other around the end. The second full moon is called a blue moon.

As February has 28 days (29 days in a leap year), it can never have a blue moon, and the months with 31 days have a higher chance of having one. On average, a blue moon happens every 33 full moons or seven times every 19 years. However, an actually rare phenomenon is having two blue moons in a calendar year, which can happen about four times every hundred years.

A Seasonal Blue Moon

A seasonal blue moon occurs when a season, which typically has three full moons in three months, ends up with four full moons. The third of these four is called a seasonal blue moon. A single astronomical season is defined as the time between a solstice and an equinox, or vice versa. There are two equinoxes and two solstices in a year, and the four seasons, spring, summer, autumn, and winter, fall between these.

The timing of the solstices and equinoxes, combined with the length of the lunar cycle, makes it the most likely for a seasonal blue moon to typically happen around February, May, August, or November (about a month before the equinox or solstice).

Seasonal blue moons are almost as common as calendar blue moons. However, when you throw a seasonal blue moon in the mix, it makes it possible to have three blue moons in a calendar year. This is pretty rare, and the last occurrence of three blue moons in a year was in 1934.

When Is the Next Blue Moon?

The next calendar blue moon will occur on May 31, 2026. The first full moon of May 2026 is on the 1st, allowing the second full moon to sneak into the last day of the month as a blue moon.

However, the next seasonal blue moon is set for May 20, 2027. This is possible because there are four full moons in spring 2027: the first on March 22, the second on April 20, the third on May 20, and the fourth on June 18. So, the third full moon on May 20 is a seasonal blue moon.

If you’re looking forward to two blue moons in a year, you’ll have to wait until 2037, when the first blue moon will happen on January 31 and the second on March 31.

Does the Moon Appear Blue on a Blue Moon?

Unfortunately, the moon doesn’t typically appear blue on a blue moon. It’s pale grey, white, or yellowish, the same as the moon on any other night.

However, under very specific atmospheric conditions, the moon can appear blue on a blue moon or any other night. This happens when tiny particles in the air, like smoke and dust, scatter the red wavelengths of light and make the moon appear blue. It’s a rare phenomenon and has nothing to do with a calendar blue moon or a seasonal one. It doesn’t even have to be a full moon.

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How Can I Watch a Blue Moon?

A blue moon is typically the same as any other full moon, so you don’t need anything special to enjoy it. However, to get the best blue moon-watching experience, it’s ideal to get information about the blue moon date and its weather forecast beforehand. After all, you don’t want to be greeted with cloudy skies.

Full moons are best viewed when they are at their highest point in the sky. You can check a moon phase calendar for the precise time in your location. Additionally, the moon looks excellent when the sky is dark and not filled with light pollution. Find a good spot away from city lights, with a clear view of the sky.

As it’s a full moon, it’s bright and hard to miss with the naked eye. However, you can improve the experience with binoculars or a telescope if you have access to them. These will reveal the moon’s features in greater detail.

If you can’t find a good spot or have a cloudy sky, it’s easy to find live streams online. The Virtual Telescope Project on YouTube is a channel that streams various blue moon occurrences. You can also search YouTube and social media before the next blue moon to find more sources.