Blue Moon Meaning: Seasonal and Calendar Definitions
“Blue moon” commonly means either the second full moon in one calendar month or the third full moon in an astronomical season containing four. Both describe calendar placement rather than color, and the monthly meaning grew from a twentieth-century reinterpretation of the older seasonal convention.
Timeline
- Seasonal convention: An extra full moon in a four-full-moon season is handled by naming the third one the blue moon, preserving the usual name for the season’s final full moon.
- 1946 reinterpretation: A Sky & Telescope article interpreted the term as the second full moon in a calendar month.
- Modern usage: The monthly definition became widespread while the seasonal definition remained in use, leaving two accepted calendar meanings.
A blue moon is usually a label for the timing of a full moon, not its appearance. Two definitions are now common. A monthly blue moon is the second full moon within one civil calendar month. A seasonal blue moon is the third full moon in an astronomical season that contains four full moons rather than the usual three. A forecast or headline should say which definition it uses because the same full moon need not qualify under both. [1][2][3]
The monthly case is possible because the cycle from one full moon to the next is about 29.5 days, shorter than most calendar months. If a full moon occurs early enough in a 30- or 31-day month, another can occur before the month ends. This arrangement recurs roughly every two to three years, although the exact interval varies. February is too short to contain two full moons, and in some time zones a full moon near midnight can fall on a different calendar date or month. [1][2][4]
The seasonal definition uses the intervals from equinox to solstice and solstice to equinox. Most such seasons hold three full moons, but occasionally one holds four. In the traditional naming system described by the Library of Congress, the third—not the fourth—was called the blue moon so that the customary name for the season’s final full moon remained in its expected position. Thus “extra moon” does not automatically mean “last moon of the season.” [1][2][3]
The two-definition puzzle has a documented modern history. The Library of Congress explains that a 1946 Sky & Telescope article misinterpreted an earlier discussion of the Maine Farmers’ Almanac and called the second full moon in a month a blue moon. Radio and later popular references spread that easier calendar rule. Usage can outgrow its mistaken origin: NASA now explains both the monthly and seasonal definitions, while noting that the monthly meaning is widely used. [1][2][3]
Neither calendar definition normally changes the Moon’s color. The full moon reflects sunlight as usual and may look yellow or orange near the horizon because its light passes through more atmosphere. On rare occasions, particles of a suitable size from smoke, dust or volcanic material can scatter red wavelengths and make the Moon appear bluish. That optical event is unrelated to whether the date satisfies a monthly or seasonal blue-moon rule. [1][2][3]
“Blue moon” is also separate from “supermoon.” Supermoon is an informal label for a full moon occurring near the Moon’s closest orbital distance to Earth, so it concerns distance and apparent size or brightness. A full moon can be blue by a calendar definition, super by a distance convention, both, or neither. Different publications also set slightly different supermoon thresholds, which is another reason to avoid treating a combined nickname as a formal astronomical category. [2][3]
To verify an advertised blue moon, check the exact full-moon instants from a reliable ephemeris, choose the location’s civil time zone, and apply one stated rule. For the monthly meaning, count full moons falling between the first and last local dates of the month. For the seasonal meaning, count them between the relevant equinox and solstice boundaries and select the third when there are four. This method explains apparent disagreements without claiming that one observer saw a different lunar phase. [1][2][4]
Sources
- Library of Congress — What Is a Blue Moon?
- NASA Science — Super Blue Moons: Your Questions Answered
- NASA Science — August 2021 Brings Rare Seasonal Blue Moon
- NASA Scientific Visualization Studio — Halloween Blue Moon