Blue Moon 2028: Meaning and Dates

Origin of the name

A second full moon in one calendar month. It happens about every two and a half years and has nothing to do with color.

The monthly definition became popular after a 1946 Sky and Telescope article misread the Maine Farmers' Almanac, which used an older seasonal rule: the third full moon in a season that has four. Both definitions are in use today. The phrase "once in a blue moon" predates both and simply means rarely.

Viewing tips

Because the calendar month decides it, a Blue Moon in one time zone can be an ordinary full moon in another when the full moon falls within hours of midnight on the first or last day of the month.

Lore and folklore

The phrase once in a blue moon is older than any astronomical definition and originally meant something absurd or never. The modern rule, a second full moon in one calendar month, spread from a 1946 magazine article and a 1980s radio programme. Genuinely blue-looking moons have been seen after huge volcanic eruptions and forest fires, when smoke particles scatter red light.

Trivia

• Monthly Blue Moons happen about every 2.7 years.
• The older seasonal rule calls the third full moon of a season with four the Blue Moon.
• After Krakatoa erupted in 1883 the Moon appeared blue or green for nearly two years.