Blue Moons

A Blue Moon is a relatively rare event when two full moons occur within a single Gregorian calendar month. Because the lunar cycle (synodic month) is about 29.53 days and the average calendar month is about 30.44 days, the small difference occasionally allows a 2nd full moon to fit into the same month. This happens about once every 2.7 years, on average.

Despite the name, the Moon does not actually appear blue during a Blue Moon. The name's origin is uncertain — one theory traces it to the 1888 eruption of Krakatoa, which put so many particles in the atmosphere that the Moon literally appeared blue for over a year. The phrase "once in a blue moon" came to mean "very rarely" — though astronomically, a blue moon happens every 2-3 years, much more often than the phrase suggests.

On rare occasions, the Moon can appear with a bluish tint — but only when the atmosphere contains dust or smoke particles of a very specific size (around 1 micron, typically from volcanic eruptions or large forest fires). This is unrelated to the calendrical Blue Moon.

In Vedic tradition, every Purnima (full moon) is considered auspicious — the full moon represents completeness, fulfillment, and the luminous aspect of the mind (Chandra is the devata of mind and emotions). A Blue Moon, being a 2nd Purnima in the same solar month, carries the same significance as any Purnima: fasting, Satyanarayana Katha, Lakshmi Puja, and other Purnima observances can be performed.

Some modern neo-pagan and Wiccan traditions hold that a Blue Moon is exceptionally powerful for spells and rituals, given its rarity. While not a Vedic belief, this reflects the universal human tendency to mark rare celestial events as sacred. Many contemporary Hindus use a Blue Moon as an opportunity for extra japa (mantra recitation), dana (charity), or pilgrimage.

Several full moons throughout the year have culturally significant names in different traditions — Wolf Moon (January), Pink Moon (April), Strawberry Moon (June), Buck Moon (July), Sturgeon Moon (August), Harvest Moon (closest to September equinox), Hunter's Moon (October), Beaver Moon (November), Cold Moon (December). A blue moon can apply to any of these when the 2nd full moon in a month carries one of these names.

The phrase implies extreme rarity, but astronomically, blue moons happen about 7 times every 19 years — averaging one every 2.7 years. February is the only month that can skip a full moon entirely (because it has only 28-29 days and the synodic month is 29.53 days), which forces both January and March to have 2 full moons that year — producing two blue moons in one calendar year, a relatively rare occurrence (about once every 19 years).

The next double-blue-moon year will be calculated automatically by the search on this page. Watch for two consecutive entries with the same year — that's a double-blue-moon year.