Blue Moon Calculator
Find the next Blue Moon — the second full moon in a single calendar month.
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How It Works
The tool scans upcoming full moons to detect when two fall in the same calendar month. Blue Moons occur roughly every 2.5 years due to the mismatch between the lunar cycle (29.53 days) and calendar months.
Formula
Blue Moon = 2nd full moon in a calendar month (occurs ~every 2.5 years)
Frequently Asked Questions
How rare is a Blue Moon?
A Blue Moon (by the popular definition of the second full moon in a month) occurs approximately every 2.5 to 3 years.
Why is it called a Blue Moon?
The phrase "once in a blue moon" predates the astronomical definition. The modern definition was popularized in 1946 in Sky & Telescope magazine.