The sky — astronomy

Blue Moon: What the Term Really Means

A Blue Moon is usually the second full moon in one calendar month. An older seasonal definition calls it the third full moon in a season that has four. The name is a calendar term, not a colour description. The Moon only looks bluish under rare conditions, such as smoke or volcanic ash in the air.

The phrase “Blue Moon” sounds like a colour, but in astronomy it is mostly a way of counting full moons on the calendar. Here are the two definitions, where the phrase comes from, and when the Moon can actually look bluish.

Two definitions: monthly and seasonal

A monthly Blue Moon is a second full moon within the same calendar month. This is the most common modern definition. One example is 31 August 2023, when the month’s second full moon was also a supermoon (a full moon that occurs when the Moon is relatively close to Earth; see Supermoon).

A seasonal Blue Moon is the older definition. An astronomical season, running from an equinox to a solstice or the other way round, normally holds three full moons. Sometimes it holds four, and in that case the third of the four is called a Blue Moon.

Both definitions are about counting. Neither says anything about the Moon’s colour, and neither describes a physically special kind of full moon.

Why it happens: simple arithmetic

The cycle of lunar phases, the synodic month, lasts about 29.53 days on average (more in Moon Phases). Most calendar months are longer, with 30 or 31 days. So sometimes a full moon falls at the very start of a month and the next one still fits at the end.

Twelve lunar cycles add up to about 354 days, while a year has about 365. The gap of roughly 11 days builds up until an “extra” full moon appears. That is why a monthly Blue Moon comes around every two to three years on average, though the intervals are not perfectly regular.

February is too short to hold two full moons. Sometimes it has none at all, and then the neighbouring months can have two each.

The moment of full moon is the same worldwide, but its calendar date depends on the time zone. In some places a Blue Moon may therefore fall on the neighbouring date. The next monthly Blue Moon falls on 31 May 2026.

Where the phrase comes from

The English expression “once in a blue moon” means “very rarely”. It is older than the astronomical meaning and was originally just an image for something unusual.

The seasonal definition goes back to an American publication, the Maine Farmer’s Almanac, which used a seasonal count to name full moons. According to the usual account, a 1946 article in Sky & Telescope magazine misread that rule as “the second full moon in a month”. The monthly meaning then spread over the following decades and is now the best known.

Can the Moon look blue?

Usually not. The Moon is greyish and reflects sunlight. Low on the horizon it can look yellowish or orange, because the atmosphere scatters more of the blue light away and lets more red through.

In rare cases the Moon does take on a bluish or greenish tint. The cause is airborne particles of a particular size, around one micrometre, which scatter red light more strongly than blue. Such particles appear after powerful volcanic eruptions or during large forest fires. After the 1883 eruption of Krakatoa, unusually coloured Suns and Moons were reported around the world. These events have nothing to do with the calendar: a Blue Moon by count is no bluer than any other full moon, and a bluish Moon can appear on a night that is not a Blue Moon at all.

The Blue Moon and the calendar

The Blue Moon is a reminder that our calendar and the lunar cycle are not in step. The solar year sets the seasons, our calendar months are conventional units, and the lunar month is another, shorter measure of time.

It is also a good excuse to look up. Every full moon is worth watching, especially on a clear night away from city lights. To learn more about our satellite, read The Moon.

In short

• Monthly Blue Moon: a second full moon in a calendar month.
• Seasonal Blue Moon: the third full moon in a season with four.
• The name is a calendar term, not a colour description.
• A bluish tint only occurs when unusual particles are present in the atmosphere.

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