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What Is a Blue Moon?
A Blue Moon is a calendar label, not a separate astronomical phase. In modern common usage it is the second Full Moon within one calendar month.
Because the lunar cycle is shorter than most calendar months, two Full Moons occasionally fit in one month. An older seasonal definition calls the third Full Moon in a season containing four the Blue Moon.
Two definitions in use
The widely used monthly definition is simple: when a calendar month contains two Full Moons, the second is a Blue Moon. The seasonal definition applies when an astronomical season contains four Full Moons; the third is labeled blue so traditional names for the last Full Moon of the season stay aligned.
Neither definition changes the Moon’s geometry. It is still an ordinary Full Moon, and the exact instant can be calculated in the usual way.
Timezone can change the label
A Full Moon is one worldwide instant, but local calendars are not. If the event occurs near the start or end of a month, it may fall on different local dates in different time zones. One region can therefore have a monthly Blue Moon while another does not. UTC-based lists should say explicitly that they use UTC.
Does it look blue?
Usually, no. The name does not describe color. Unusual particles in the atmosphere can sometimes scatter light so the Moon appears bluish, but that rare optical effect is unrelated to whether the Full Moon is first or second in a month.
Questions, answered
Frequently asked questions
How often is a Blue Moon?
The familiar phrase ‘once in a blue moon’ fits: monthly Blue Moons occur only every few years, though the interval is irregular.
Is a Blue Moon a supermoon?
The labels describe different things. A Full Moon can be both if it meets a chosen supermoon distance definition and a Blue Moon calendar definition.