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The Science behind Blue Moons

Blue Moon is a term used to describe the third Full Moon of a season that has four Full Moons. It is also commonly used to refer to the second Full Moon in a calendar month.

Close-up image of the Full Moon.

Blue Moons are not really blue.

If the Moon appears blue it is because of a certain size dust particles in the atmosphere.


What Is a Blue Moon?

The phrase once in a Blue Moon is colloquially used to suggest that something is very rare. But just how rare depends on your definition.

Two Definitions

In astronomy, a Blue Moon is defined as either the third Full Moon of an astronomical season with four Full Moons or as the second Full Moon in a calendar month.

Seasonal Blue Moons

There are four astronomical seasons in a year:

When one of the seasons in a year has four Full Moons, instead of the usual three, the third Full Moon is called a Blue Moon, or seasonal Blue Moon.

Monthly Blue Moons

These days, the second Full Moon in a calendar month is also referred to as a Blue Moon, or a monthly Blue Moon. This particular use was popularized due to a miscalculation published in a 1946 article in Sky and Telescope magazine.

How Rare Are Blue Moons?

Blue Moons occur around once every two or three years. In the 1100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons.

Not Really Blue

A Blue Moon does not look blue. However, blue colored Moons do rarely occur when dust or smoke particles in the air are of a specific size. Such particles help create a blue colored Moon by scattering away red light.

Moons which appear red can be caused by other sizes of dust particles or by a total lunar eclipse. These are much more common than Blue Moons.

Why Is it Called Blue Moon?

There are different accounts of why the third Full Moon of a season of four Full Moons is called a Blue Moon.

For instance, the ecclesiastical calendar, which indicates the dates of the Christian fasts and festivals, uses the phases of the Moon to determine the exact dates for holidays like Lent and Easter.

The month of Lent contains the Lenten Moon. The first Full Moon of spring – also known as the Easter Moon or the Paschal Moon – falls a week before Easter. In order to ensure that Lent and Easter coincide with the phases of the Moon, the calendar has termed the third Moon of the season as the Blue Moon.

Another version of this is that since each Full Moon of a normal year already has a given name, for instance, Harvest Moon, the 13th nameless Full Moon in a year was named a Blue Moon. This way the lunations and calendars were aligned to make sure celebrations and customs would still fall during their "proper" times.

Is There a Perfect Calendar?

Did You Know?

About once every 19 years, the month of February does not have a Full Moon, which is known as a Black Moon. The years when this happens also have 2 Full Moons in 2 different months.

Topics: Astronomy, Moon

Recent/Upcoming Blue Moons

2019May 183rd Full Moon in season
2020Oct 312nd Full Moon in a month
2021Aug 223rd Full Moon in season

Blue Moons can vary by time zone. Dates above are based on the local time in Seattle. Change location


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