The Moon Blue moon
Blue moon
A name that is not about colour
A calendar coincidence with nothing to do with colour: one full Moon more than usual.
- Common definition
- The second full Moon in a calendar month
- Older definition
- The third of four full Moons in one season
- How often
- About once every 2.7 years
01 / Overview
Meet Blue moon
A blue moon usually means the second full Moon in a calendar month. Because the synodic month is a day or two shorter than most months, the coincidence turns up about every 2.7 years on average. The older definition comes from farmers’ almanacs: when a season holds four full Moons, the third of them is the blue moon.
Where the two definitions came from
The seasonal definition is the older one. The monthly definition began as a 1946 magazine article misreading that almanac, then stuck because it spread so widely. Both are calendar accidents and say nothing about the Moon itself.
A Moon that really does look blue
The Moon does occasionally look blue. When the air holds particles close to a micrometre across — after the 1883 eruption of Krakatoa, say, or a large forest fire — red light is scattered preferentially and the Moon takes on a bluish cast. It has nothing whatever to do with the name.
02 / Key data
Core parameters
- Common definition
- The second full Moon in a calendar month
- Older definition
- The third of four full Moons in one season
- How often
- About once every 2.7 years
- Relation to colour
- None at all
- Recurrence
- Once every two or three years
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- In the sky a blue moon looks exactly like any other full Moon.
- February can never hold a blue moon under the monthly definition.
- Photographing a truly blue Moon takes volcanic ash or smoke, not a calendar.
04 / Sources